Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0022857 transmembrane transporter activity	IBA GO_REF:0000033	ACCEPT	Summary: SPNS1 is a transmembrane transporter, and the broad term is accurate for the currently supported lysosomal lysophospholipid export activity. Reason: Retain the broad transporter term rather than replacing it with GO:0051978, whose sodium-coupled label does not match the proton-gradient-dependent SPNS1 evidence. A proton-specific lysophospholipid symporter term is requested below. Supporting Evidence: PMID:36161949 the major facilitator superfamily protein Spns1 that is ubiquitously expressed in all tissues as a proton-dependent lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) transporter PMID:39739806 Spns1 mediates the rate-limiting efflux of lysophospholipids from the lysosome to the cytosol
GO:0051977 lysophospholipid transport	IBA GO_REF:0000033	ACCEPT	Summary: This is a core SPNS1 biological process. SPNS1 exports lysosomal LPC and LPE to the cytosol, and loss of SPNS1 causes lysosomal accumulation of these lysophospholipids. Reason: The process accurately captures the conserved cargo-level biology, even though a more precise lysosomal export term would be preferable. Supporting Evidence: PMID:36161949 Spns1 deficiency in cells, zebrafish embryos, and mouse liver resulted in lysosomal accumulation of LPC and LPE species PMID:37075117 leads to intralysosomal accumulation of lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE).
GO:0033700 phospholipid efflux	IBA GO_REF:0000033	ACCEPT	Summary: SPNS1 mediates efflux of lysosomal lysophospholipids that derive from lysosomal phospholipid degradation. These products are exported to the cytosol for re-acylation and broader lipid salvage. Reason: Although the transported substrates are lysophospholipids rather than intact phospholipids, the term is a reasonable process-level description of lysosomal phospholipid catabolite efflux and salvage. Supporting Evidence: PMID:36161949 Flux analysis using stable isotope-labeled phospholipid apolipoprotein E nanodiscs targeted to lysosomes showed that LPC was transported out of lysosomes in an Spns1-dependent manner PMID:40608416 SPNS1 is a lysosomal transporter that mediates the salvage of lysoglycerophospholipids
GO:0016020 membrane	IBA GO_REF:0000033	MODIFY	Summary: SPNS1 is a multi-pass membrane transporter, but generic membrane is too broad. The experimentally and physiologically relevant active location is the lysosomal membrane. Reason: Replace the generic membrane term with the specific lysosomal membrane localization. Proposed replacements: lysosomal membrane Supporting Evidence: file:human/SPNS1/SPNS1-uniprot.txt SUBCELLULAR LOCATION: Lysosome membrane PMID:39739806 While Spns1 primarily localizes to the lysosomal membrane to export its substrates from lysosomal lumen under physiological conditions
GO:0005743 mitochondrial inner membrane	IEA GO_REF:0000044	KEEP AS NON CORE	Summary: Early HSpin1 work and UniProt note occasional mitochondrial localization, but later biochemical and structural work establishes lysosomal membrane transport as the dominant SPNS1 function. Reason: Keep as a possible non-core/overexpression-associated localization rather than treating it as a defining location for SPNS1 activity. Supporting Evidence: file:human/SPNS1/SPNS1-uniprot.txt Mitochondrion inner membrane {ECO:0000269\|PubMed:12815463}; Multi-pass membrane protein {ECO:0000269\|PubMed:12815463}. Note=Ocassionally localizes to mitochondria.
GO:0005765 lysosomal membrane	IEA GO_REF:0000044	ACCEPT	Summary: Lysosomal membrane is the core active location for SPNS1. Transport assays, lysosome lipidomics, human disease work, and structural analyses all interpret SPNS1 as a lysosomal membrane lysophospholipid transporter. Reason: Core localization for the lysosomal efflux function. Supporting Evidence: file:human/SPNS1/SPNS1-uniprot.txt SUBCELLULAR LOCATION: Lysosome membrane PMID:40608416 SPNS1, a ubiquitously expressed lysosomal transmembrane protein belonging to the major facilitator superfamily
GO:0006869 lipid transport	IEA GO_REF:0000117	MODIFY	Summary: SPNS1 is involved in lipid transport, but the reviewed literature defines the more specific process as lysosomal lysophospholipid transport and phospholipid-catabolite efflux. Reason: The broad lipid transport term should be replaced by more informative lysophospholipid transport and phospholipid efflux terms. Proposed replacements: lysophospholipid transport phospholipid efflux Supporting Evidence: PMID:36161949 Our findings identify a phospholipid salvage pathway from lysosomes to the cytosol that is dependent on Spns1
GO:0022857 transmembrane transporter activity	IEA GO_REF:0000002	ACCEPT	Summary: The InterPro-derived transporter annotation is broad but accurate for SPNS1 lysosomal lysophospholipid export activity. Reason: Retain the broad transporter term rather than replacing it with GO:0051978, whose sodium-coupled label does not match the proton-gradient-dependent SPNS1 evidence. A proton-specific lysophospholipid symporter term is requested below. Supporting Evidence: PMID:39739806 Our results reveal molecular insights into lysosomal LPC transport and the proton-sensing mechanism by Spns1.
GO:0055085 transmembrane transport	IEA GO_REF:0000002	MODIFY	Summary: SPNS1 mediates transmembrane movement, but the specific process is lysosomal lysophospholipid export/salvage. Reason: Replace the broad transmembrane transport term with the specific lysophospholipid transport process. Proposed replacements: lysophospholipid transport Supporting Evidence: PMID:37075117 SPNS1 functions to export LPC species from the lysosomal lumen to the cytosol for reacylation to PC
GO:0005515 protein binding	IPI PMID:12815463 HSpin1, a transmembrane protein interacting with Bcl-2/Bcl-x...	MARK AS OVER ANNOTATED	Summary: The early HSpin1 study reported interaction with BCL2 and BCL2L1, but GO:0005515 is uninformative and does not describe SPNS1's core molecular function. Reason: Avoid protein binding as a retained functional annotation. The interaction may be valid as interaction data, but it should not define SPNS1 function. Supporting Evidence: PMID:12815463 HSpin1 bound to Bcl-2 and apoptosis regulator Bcl-X (Bcl-xL)
GO:0005764 lysosome	IEA GO_REF:0000107	ACCEPT	Summary: SPNS1 acts in lysosomes as a lysophospholipid exporter. This orthology transfer is consistent with direct human and model-organism evidence. Reason: Core active cellular location. Supporting Evidence: PMID:36161949 The lysosome is central to the degradation of proteins, carbohydrates, and lipids and their salvage back to the cytosol for reutilization. PMID:40608416 SPNS1 is a lysosomal transporter that mediates the salvage of lysoglycerophospholipids
GO:0051977 lysophospholipid transport	IEA GO_REF:0000107	ACCEPT	Summary: Orthology-based lysophospholipid transport is directly supported by human SPNS1 transport assays, lysosome flux experiments, and disease variant studies. Reason: Core biological process. Supporting Evidence: PMID:36161949 Taken together, these data indicate that endogenous Spns1 mediates the transport of LPCs and LPEs out of lysosomes PMID:40608416 lysophospholipids transported by SPNS1 into the cytosol quantitatively contributed to triglyceride synthesis
GO:0005764 lysosome	IDA PMID:36161949 Spns1 is a lysophospholipid transporter mediating lysosomal ...	ACCEPT	Summary: The 2022 PNAS study directly investigated SPNS1 function in lysosomes and isolated lysosome-enriched fractions showing substrate accumulation in SPNS1-deficient cells. Reason: Core active location. Supporting Evidence: PMID:36161949 Lysosomes isolated from [14C]-choline-labeled Spns1 KO cells showed accumulation of [14C]-LPC
GO:0051977 lysophospholipid transport	IDA PMID:36161949 Spns1 is a lysophospholipid transporter mediating lysosomal ...	ACCEPT	Summary: This is one of the central conclusions of the 2022 PNAS study. SPNS1 transports LPC and LPE out of lysosomes and enables their reuse in phospholipid pools. Reason: Core biological process supported by direct assays and flux data. Supporting Evidence: PMID:36161949 data from our flux analysis support the conclusion that LPCs are transported out of lysosomes by Spns1 and re-acylated into PC pools
GO:0051978 lysophospholipid:sodium symporter activity	IDA PMID:36161949 Spns1 is a lysophospholipid transporter mediating lysosomal ...	MODIFY	Summary: This term captures lysophospholipid transport, but its sodium-symporter label is mechanistically incorrect for SPNS1 because SPNS1 is proton-gradient dependent. Reason: Replace with the broad transmembrane transporter activity term pending a proton-specific lysophospholipid symporter term, which is requested below. Proposed replacements: transmembrane transporter activity Supporting Evidence: PMID:36161949 The uptake of [14C]-LPC-oleate by Spns1 was concentration dependent and saturable, with a pH optimum between pH 5.0 and 6.0 PMID:39739806 we identify a five-residue network that is crucial for proton-sensing by Spns1
GO:0005764 lysosome	IDA PMID:25365221 Spastic paraplegia proteins spastizin and spatacsin mediate ...	ACCEPT	Summary: The seeded original reference is an autophagic lysosome reformation paper focused on spastizin and spatacsin rather than direct SPNS1 evidence. Nevertheless, lysosomal localization of SPNS1 is strongly supported by later direct SPNS1 studies and UniProt. Reason: Accept the term because it is correct, while noting that PMID:25365221 is not the best primary evidence for SPNS1 itself. Supporting Evidence: file:human/SPNS1/SPNS1-uniprot.txt SUBCELLULAR LOCATION: Lysosome membrane PMID:39739806 Spns1 is located on the lysosomal membrane and mediates lysophospholipid efflux depending on the proton gradient
GO:0005765 lysosomal membrane	HDA PMID:17897319 Integral and associated lysosomal membrane proteins.	ACCEPT	Summary: High-throughput lysosomal membrane proteomics is consistent with the established SPNS1 lysosomal membrane localization, but the direct transporter papers are stronger support. Reason: Accurate cellular component annotation consistent with core function. Supporting Evidence: PMID:17897319 We searched for novel proteins in lysosomal membranes, tentatively participating in molecular transport across the membrane file:human/SPNS1/SPNS1-uniprot.txt SUBCELLULAR LOCATION: Lysosome membrane
GO:0007041 lysosomal transport	IDA PMID:36161949 Spns1 is a lysophospholipid transporter mediating lysosomal ...	NEW	Summary: Proposed new annotation from the Proteostasis Network context. SPNS1 exports lysosomal degradation products across the lysosomal membrane, placing its specific lysophospholipid salvage activity within the broader process of lysosomal transport. Reason: GOA already captures lysophospholipid transport and phospholipid efflux, but not the broader lysosomal-transport context highlighted by the PN entry. This should be added conservatively as a broad process annotation; a more precise lysosomal lysophospholipid export term would be better. Supporting Evidence: PMID:36161949 Our findings identify a phospholipid salvage pathway from lysosomes to the cytosol that is dependent on Spns1 PMID:37075117 SPNS1 functions to export LPC species from the lysosomal lumen to the cytosol for reacylation to PC file:human/SPNS1/SPNS1-notes.md The Proteostasis Network places SPNS1 under `Autophagy-Lysosome Pathway\|Autophagic lysosome reformation\|Efflux of autophagy products`.

Core Functions

Proton-gradient-dependent export of lysophospholipids from the lysosomal lumen to the cytosol. SPNS1 transports LPC and LPE, and also transports LPG and lysoplasmalogen species in reported assays. This lysosomal efflux enables phospholipid salvage through downstream re-acylation and supports lipid homeostasis under nutrient stress.

Molecular Function:

transmembrane transporter activity

Directly Involved In:

lysophospholipid transport phospholipid efflux lysosomal transport

Cellular Locations:

lysosomal membrane lysosome

Supporting Evidence:

PMID:36161949
major facilitator superfamily protein Spns1 that is ubiquitously expressed in all tissues as a proton-dependent lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) transporter
PMID:37075117
SPNS1 functions to export LPC species from the lysosomal lumen to the cytosol for reacylation to PC
PMID:39739806
Our results reveal molecular insights into lysosomal LPC transport and the proton-sensing mechanism by Spns1.
PMID:40608416
lysophospholipids transported by SPNS1 into the cytosol quantitatively contributed to triglyceride synthesis

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

GO_REF:0000107

Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

PMID:12815463

HSpin1, a transmembrane protein interacting with Bcl-2/Bcl-xL, induces a caspase-independent autophagic cell death.

PMID:17897319

Integral and associated lysosomal membrane proteins.

PMID:25365221

Spastic paraplegia proteins spastizin and spatacsin mediate autophagic lysosome reformation.

PMID:36161949

Spns1 is a lysophospholipid transporter mediating lysosomal phospholipid salvage.

PMID:37075117

An SPNS1-dependent lysosomal lipid transport pathway that enables cell survival under choline limitation.

PMID:39739806

Molecular basis of Spns1-mediated lysophospholipid transport from the lysosome.

PMID:40608416

SPNS1 variants cause multiorgan disease and implicate lysophospholipid transport as critical for mTOR-regulated lipid homeostasis.

file:human/SPNS1/SPNS1-uniprot.txt

UniProt record for human SPNS1

file:human/SPNS1/SPNS1-notes.md

SPNS1 curation notes

file:projects/PROTEOSTASIS/mappings/autophagy_lysosome_pathway.yaml

Proteostasis Network autophagy-lysosome pathway mappings

Suggested Experiments

Experiment: Reconstitute purified human SPNS1 into proteoliposomes with controlled proton and sodium gradients and measure transport of LPC, LPE, LPG, and lysoplasmalogen substrates.

Hypothesis: SPNS1 is a lysophospholipid:proton symporter rather than a sodium-coupled lysophospholipid transporter.

Type: transport_reconstitution

Experiment: Use autophagy-induced lysosomal cargo flux assays in SPNS1 knockout and rescue cells to measure export of autolysosome-derived lysophospholipids after starvation and recovery.

Hypothesis: SPNS1 mediates lysosomal export of autophagy-derived lysophospholipid catabolites during lysosome recovery and lipid salvage.

Type: lipidomics/genetic_rescue

Experiment: Compare wild-type SPNS1, disease variants, and transport-site mutants for lysosomal LPC/LPE accumulation, cholesterol egress, triglyceride synthesis, and autophagy markers under mTOR inhibition.

Hypothesis: Disease variants cause partial loss of lysosomal lysophospholipid export that secondarily disrupts cholesterol egress and nutrient-stress lipid storage.

Type: disease_variant_functional_assay

Deep Research

Falcon

(SPNS1-deep-research-falcon.md)

Comprehensive Research Report: SPNS1 (Protein Spinster Homolog 1) Falcon

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Comprehensive Research Report: SPNS1 (Protein Spinster Homolog 1)

Gene Identity and Protein Family

SPNS1 (UniProt: Q9H2V7) encodes protein spinster homolog 1 in Homo sapiens, a member of the major facilitator superfamily (MFS) and specifically the SLC63/Spinster family (he2022spns1isa pages 1-2, chen2025molecularbasisof pages 1-2). The protein contains the characteristic 12-transmembrane helix (TM) architecture organized into two pseudosymmetric six-helix domains (N-domain: TM1-6; C-domain: TM7-12), consistent with the canonical MFS fold (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 3-4). SPNS1 belongs to the Spinster subfamily (TC 2.A.1.49), which includes three mammalian homologs: SPNS1, SPNS2, and SPNS3 (ha2024lackofspns1 pages 1-2, he2022spns1isa pages 2-3).

Primary Function and Substrate Specificity

Lysosomal Lysophospholipid Transporter

SPNS1 functions as a proton-dependent lysosomal transporter that mediates the export of lysophospholipids from the lysosomal lumen to the cytosol (he2022spns1isa pages 1-2, he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 1-5, chen2025molecularbasisof pages 1-2). The protein exhibits promiscuous substrate specificity for multiple lysophospholipid species, demonstrating a preference for zwitterionic lysolipids over anionic phospholipids.

Confirmed Substrates:
The most extensively validated substrates are lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE), demonstrated through multiple independent lines of evidence (he2022spns1isa pages 1-2, he2022spns1isa pages 2-3). Cell-based transport assays using radiolabeled LPC showed robust, saturable uptake by SPNS1 with pH dependence (optimal activity at pH 5.0-6.0) (he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 9-12). Stable isotope tracing experiments using deuterated PC (d9-PC 36:2) delivered to lysosomes via apolipoprotein E-complexed nanodiscs demonstrated that SPNS1 deficiency causes 5-6 fold accumulation of LPC in lysosome-enriched fractions and markedly reduces the conversion of lysosome-derived LPC into cellular PC pools (he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12).

SPNS1 also transports lysoplasmalogens and lysophosphatidylglycerol (LPG), as evidenced by both specificity assays and accumulation of these species in SPNS1-deficient cells and tissues (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4). Lysophosphatidylinositol (LPI) accumulates in SPNS1-deficient models, though direct transport activity remains less firmly established (ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4).

Non-substrates:
SPNS1 does not transport anionic lysophospholipids such as lysophosphatidylserine (LPS) and lysophosphatidic acid (LPA) (he2022spns1isa pages 2-3). Importantly, SPNS1 does not transport sphingosine-1-phosphate (S1P) or lyso-sphingomyelin, distinguishing it from its paralog SPNS2, which functions as an S1P exporter at the plasma membrane (he2022spns1isa pages 2-3, chen2025molecularbasisof pages 2-3).

Sphingosine Transport

The role of SPNS1 in sphingosine transport is strongly implicated but requires further direct validation. Genetic ablation of Spns1 in mice and cells causes dramatic accumulation of sphingosine in lysosomes, with levels comparable to those observed in NPC1-deficient models (ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4, ha2024lackofspns1 pages 9-11). Biochemical assays demonstrate that SPNS1 is required for sphingosine release from lysosomes (ha2024lackofspns1 pages 1-2). However, cell-surface transport assays using exogenous sphingosine did not detect direct transport by SPNS1, likely due to the rapid non-specific uptake of sphingosine by cells that confounds such measurements (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 9-11). The protonation of sphingosine at acidic lysosomal pH prevents passive diffusion across membranes, supporting the requirement for a transporter (ha2024lackofspns1 pages 9-11).

Substrate	Transport Activity (Yes/No/Putative)	Evidence Type	Key References
Lysophosphatidylcholine (LPC)	Yes	Direct cell-surface uptake assays with radiolabeled LPC; lysosome isotope-tracing showing lysosomal LPC accumulation in SPNS1-KO cells and reduced recycling into PC; cryo-EM structure of human SPNS1 bound to LPC; mutagenesis and MD simulations defining LPC-binding/proton-sensing residues (he2022spns1isa pages 2-3, he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12, chen2025molecularbasisof pages 2-3, chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6)	He 2022; Scharenberg 2022; Chen 2025 (he2022spns1isa pages 2-3, he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12, chen2025molecularbasisof pages 2-3, chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6)
Lysophosphatidylethanolamine (LPE)	Yes	Lipid uptake/lipidomics specificity assays showing transport of zwitterionic lysolipids including LPE; KO lipidomics showing lysosomal LPE accumulation; genetic/physiological interpretation consistent with export from lysosomes (he2022spns1isa pages 2-3, he2022spns1isa pages 3-5, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)	He 2022; Ha 2024 (he2022spns1isa pages 2-3, he2022spns1isa pages 3-5, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)
Lysoplasmalogens	Yes	Lipidomics specificity assays reported transport of lyso-plasmalogen; in vivo/cellular metabolomics showed accumulation of lysoplasmalogens after SPNS1 loss, supporting SPNS1-dependent export (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)	He 2022; Ha 2024 (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)
Lysophosphatidylglycerol (LPG)	Yes	Specificity assays detected transport of LPG despite generally narrower preference for zwitterionic lysolipids; metabolomics of SPNS1 deficiency also showed lysosomal/organ accumulation of LPG (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)	He 2022; Ha 2024 (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)
Lysophosphatidylinositol (LPI)	Putative	Not detected in He 2022 cell-surface specificity assay, but accumulated in SPNS1-deficient mouse/cell metabolomics, suggesting either direct low-efficiency transport, context-dependent transport, or secondary accumulation; direct transport remains unresolved (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)	He 2022; Ha 2024 (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4)
Lysophosphatidylserine (LPS)	No	Specificity assay did not detect transport of anionic LPS by SPNS1 (he2022spns1isa pages 2-3)	He 2022 (he2022spns1isa pages 2-3)
Lysophosphatidic acid (LPA)	No	Specificity assay did not detect transport of anionic LPA by SPNS1 (he2022spns1isa pages 2-3)	He 2022 (he2022spns1isa pages 2-3)
Sphingosine	Putative	He 2022 did not detect direct uptake in the cell-surface assay, arguing against direct transport in that system; Ha 2024 found strong indirect evidence that SPNS1 is required for sphingosine release from lysosomes and that SPNS1 loss causes sphingosine accumulation, but direct transport remains to be established (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 9-11)	He 2022; Ha 2024 (he2022spns1isa pages 2-3, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 9-11)
Sphingosine-1-phosphate (S1P)	No	Specificity assay showed S1P was not transported by SPNS1, distinguishing SPNS1 from paralog SPNS2; structural comparisons also emphasize different substrate selectivity (he2022spns1isa pages 2-3, chen2025molecularbasisof pages 2-3)	He 2022; Chen 2025 (he2022spns1isa pages 2-3, chen2025molecularbasisof pages 2-3)
Lyso-sphingomyelin	No	Specificity assay found no transport of lyso-SM by SPNS1 (he2022spns1isa pages 2-3)	He 2022 (he2022spns1isa pages 2-3)

Table: This table summarizes the current evidence for which lysolipids and related metabolites are transported by human SPNS1. It distinguishes direct experimental support from indirect or unresolved evidence, which is useful for functional annotation of this lysosomal transporter.

Subcellular Localization

SPNS1 localizes primarily to late endosomes and lysosomes, as demonstrated by immunofluorescence colocalization with LAMP1 (lysosomal marker) and Rab7 (late endosome marker), but not with Rab5 (early endosome marker) (ha2024lackofspns1 pages 2-4). The protein is also detected at or near the plasma membrane, suggesting it may traffic through the plasma membrane before entering the endocytic pathway to reach lysosomes (ha2024lackofspns1 pages 2-4, chen2025molecularbasisof pages 2-3). This trafficking pattern is common for lysosomal transmembrane proteins and has been exploited experimentally through overexpression systems that cause SPNS1 to accumulate at the plasma membrane, enabling cell-surface transport assays to measure activity in the presence of controlled pH gradients (scharenberg2022alysosomallipid pages 9-12, he2022spns1isa pages 2-3).

Molecular Mechanism of Transport

Structural Basis

The molecular mechanism of SPNS1-mediated lysophospholipid transport has been elucidated through cryo-electron microscopy (cryo-EM) structural determination at 3.2 Å resolution (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 3-4). The structure captured SPNS1 in an outward-facing (lumen-facing) conformation with LPC bound in a lateral opening formed primarily between transmembrane helices TM5 and TM8 (chen2025molecularbasisof pages 3-4). In this conformation, the choline headgroup of LPC is positioned within the luminal cavity, while the acyl tail extends into the luminal leaflet of the membrane bilayer (chen2025molecularbasisof pages 3-4).

Key substrate-binding residues identified through structural analysis and validated by mutagenesis include: Y203, G200, L201, and I204 on TM5, and S322, G326, L327, and C330 on TM8 (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6). Mutations introducing steric hindrance at the lateral opening (G200L, G326L) or disrupting key interactions (Y203A) dramatically reduced LPC transport activity (chen2025molecularbasisof pages 4-6). Earlier functional studies also implicated conserved residues R76, E164, and H427 in substrate recognition, with alanine substitutions abolishing or significantly reducing transport (he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 9-12).

Molecular dynamics (MD) simulations over 200 nanoseconds tracked the entry of LPC into the binding site, showing that the phosphocholine headgroup enters the central cavity while the fatty acyl tail gradually inserts between TM5 and TM8 (chen2025molecularbasisof pages 4-6). This lateral entry mechanism resembles that observed in Mfsd2a, a related MFS transporter that mediates sodium-dependent LPC transport at the plasma membrane (chen2025molecularbasisof pages 3-4).

Proton-Coupling Mechanism

SPNS1 employs a sophisticated proton-sensing network rather than relying on a single acidic residue (chen2025molecularbasisof pages 4-6). The critical residue D94 forms part of a hydrophilic network with E87, S97, R300, and S442 that blocks the luminal access in the inward-facing state (chen2025molecularbasisof pages 4-6). Mutagenesis studies demonstrated that D94A mutation abolished transport activity, while the D94N variant partially rescued function, supporting a model in which protonation of D94 disrupts the salt bridge with R300 and hydrogen bonds with S442, triggering conformational changes that open the luminal cavity (chen2025molecularbasisof pages 4-6). Mutation of R300A also nearly abolished transport, highlighting the importance of this interaction network (chen2025molecularbasisof pages 4-6).

This proton-sensing mechanism was validated through chimeric experiments: transferring the E87/S97 residues from SPNS1 to the corresponding positions (Q130/A140) in SPNS2 converted the normally pH-independent SPNS2 into a pH-dependent transporter (chen2025molecularbasisof pages 4-6). Conversely, introducing SPNS2-like residues into SPNS1 (E87Q/S97A) rendered SPNS1 active at neutral pH (chen2025molecularbasisof pages 4-6).

Transport Cycle

SPNS1 likely operates through a proton-coupled alternating access mechanism characteristic of MFS transporters (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6). The cryo-EM structure represents the outward-facing state with substrate bound, while AlphaFold prediction provides a model of the inward-facing (cytosol-open) state (chen2025molecularbasisof pages 3-4). During the transport cycle, LPC enters laterally from the lysosomal membrane through the TM5/TM8 cleft, moves into the central cavity upon protonation-triggered conformational changes, and is released toward the cytosolic side through a rocker-switch motion that alternates the accessibility of the central cavity between luminal and cytosolic sides (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6).

Feature Category	Description	Experimental Evidence	Key References
Protein family/topology	SPNS1 is a lysosomal member of the major facilitator superfamily (MFS), in the SLC63/Spinster family, with the canonical 12-transmembrane-helix architecture divided into N- and C-terminal six-helix domains. Cryo-EM and AlphaFold support alternating inward- and outward-facing conformations typical of MFS transporters.	Human SPNS1 cryo-EM structure resolved in an outward/lumen-facing state; structural comparison with AlphaFold inward-facing model and homologs; prior functional classification as orphan lysosomal MFS transporter. (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 2-3, chen2025molecularbasisof pages 3-4)	Chen 2025 (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 2-3, chen2025molecularbasisof pages 3-4)
Subcellular localization	SPNS1 localizes primarily to late endosomes and lysosomes, colocalizing with LAMP1 and Rab7, and may transiently pass through the plasma membrane before endocytic trafficking to lysosomes.	Immunostaining in HEK293/CHO cells showed colocalization with LAMP1 and Rab7 but not Rab5; overexpression studies also detected some plasma membrane signal, which enabled cell-surface transport assays. (ha2024lackofspns1 pages 2-4, chen2025molecularbasisof pages 2-3)	Ha 2024; Chen 2025 (ha2024lackofspns1 pages 2-4, chen2025molecularbasisof pages 2-3)
Transported substrates/function	SPNS1 functions as a proton-dependent lysosomal exporter of lysophospholipids, especially LPC and LPE, with evidence also supporting transport of lysoplasmalogens and LPG; sphingosine export is strongly implicated but remains less directly established than LPC transport.	Radiolabeled LPC uptake, lipidomics, isotope tracing from lysosome-targeted PC, and KO accumulation of lysosomal lysolipids; SPNS1 loss causes marked lysosomal buildup of LPC/LPE and related lysolipids. (he2022spns1isa pages 2-3, he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 9-11)	He 2022; Scharenberg 2022; Ha 2024 (he2022spns1isa pages 2-3, he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 9-11)
Substrate binding site residues	LPC binds in a luminal/lateral opening formed mainly between TM5 and TM8. Key residues implicated in substrate recognition or transport include Y203, G200, L201, I204 on TM5 and S322, G326, L327, C330 on TM8; earlier functional work also implicated R76, E164, and H427.	Cryo-EM with LPC-bound SPNS1 plus mutagenesis and MD simulations identified the TM5/TM8 cleft as the binding/translocation site; cell-based transport assays showed loss of function with mutations such as G200L, Y203A, G326L; earlier alanine mutagenesis reduced uptake with R76, E164, H427 substitutions. (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6, he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 9-12)	Chen 2025; He 2022; Scharenberg 2022 (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6, he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 9-12)
Proton-sensing mechanism	SPNS1 uses a luminal proton-sensing network rather than a single acidic residue. D94 is central, with E87, S97, R300, and S442 forming a hydrophilic/salt-bridge network that couples low pH to conformational change and transport.	Structure-guided mutagenesis showed reduced transport with D94A, D211A, and especially R300A; D94N partially rescued activity, supporting protonation-dependent function. Engineering analogous residues into SPNS2 conferred low-pH dependence. (chen2025molecularbasisof pages 4-6)	Chen 2025 (chen2025molecularbasisof pages 4-6)
Transport mechanism	Current model is proton-coupled alternating access/rocker-switch transport. LPC enters laterally from the luminal leaflet through the TM5/TM8 cleft, moves into the central cavity, and is then released toward the cytosolic side for salvage/reacylation.	Cryo-EM captured the lumen-facing LPC-bound state; MD simulations tracked LPC entry and movement; isotope-tracing in cells showed lysosome-derived LPC is exported and re-esterified into PC in the ER. (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6, he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12)	Chen 2025; He 2022; Scharenberg 2022 (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6, he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12)
pH dependence	SPNS1 transport is strongly enhanced at acidic pH, consistent with lysosomal lumen conditions, with reported optimal uptake around pH 5.0-6.0.	Cell-surface uptake assays using overexpressed SPNS1 showed saturable radiolabeled LPC uptake with clear low-pH dependence; mutational analysis connected this dependence to the proton-sensing network. (he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 9-12, chen2025molecularbasisof pages 4-6)	He 2022; Scharenberg 2022; Chen 2025 (he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 9-12, chen2025molecularbasisof pages 4-6)
Comparison to SPNS2	SPNS1 and SPNS2 are close homologs, but SPNS1 is lysosomal and proton-coupled, whereas SPNS2 is a plasma-membrane S1P exporter functioning mainly as a facilitated-diffusion uniporter. SPNS1 does not transport S1P in the reported specificity assays.	Structural comparison showed similar overall MFS fold but different cavity features and pH coupling; SPNS2 engineering experiments demonstrated that adding SPNS1-like proton-sensing residues can render SPNS2 pH dependent. Specificity assays found SPNS1 transports LPC/LPE rather than S1P. (chen2025molecularbasisof pages 2-3, he2022spns1isa pages 2-3, chen2025molecularbasisof pages 4-6)	Chen 2025; He 2022 (chen2025molecularbasisof pages 2-3, he2022spns1isa pages 2-3, chen2025molecularbasisof pages 4-6)
Comparison to Mfsd2a	SPNS1 resembles Mfsd2a in using a TM5/TM8-associated cleft to engage LPC-like substrates, but Mfsd2a is a plasma-membrane sodium-dependent LPC transporter, whereas SPNS1 is a lysosomal proton-coupled exporter.	Structural superposition indicated similar LPC entry geometry and hydrophobic translocation path; mechanistically both handle amphiphilic lysolipids, but ion coupling and cellular localization differ. (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6)	Chen 2025 (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6)
Physiologic consequence of transport	SPNS1-mediated export feeds lysosomal phospholipid salvage: exported LPC is reacylated by LPCATs in the ER to regenerate PC, supporting membrane lipid homeostasis and survival under choline limitation. Loss of SPNS1 causes lysosomal storage phenotypes and organ pathology.	Stable-isotope tracing with lysosome-directed PC nanodiscs showed reduced conversion of lysosome-derived LPC into cellular PC in SPNS1-KO cells; KO mouse and cell models accumulated lysolipids and developed developmental, hepatic, and neurologic defects. (he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12, ha2024lackofspns1 pages 1-2, ichimura2024lossofspns1 pages 1-5)	He 2022; Scharenberg 2022; Ha 2024 (he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12, ha2024lackofspns1 pages 1-2, ichimura2024lossofspns1 pages 1-5)

Table: This table summarizes the main structural and mechanistic properties of human SPNS1 as a lysosomal lysophospholipid transporter. It integrates localization, substrate recognition, proton coupling, transport mechanism, and comparisons with closely related transporters.

Biochemical Pathways and Physiological Roles

Lysosomal Phospholipid Salvage Pathway

SPNS1 mediates a critical lysosomal phospholipid salvage pathway that prevents complete catabolism of membrane phospholipids (he2022spns1isa pages 1-2, he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12). In the lysosome, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are hydrolyzed by lysosomal phospholipase A2 (primarily PLA2G15) to generate LPC and LPE, which can be further deacylated to produce glycerophosphodiesters such as glycerophosphocholine (GPC) and glycerophosphoethanolamine (GPE) (he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12). SPNS1 exports LPC and LPE before this terminal deacylation, salvaging these lysophospholipid intermediates for direct re-acylation (he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12).

Once exported from lysosomes, LPC is re-acylated by lysophosphatidylcholine acyltransferases (LPCATs) in the endoplasmic reticulum (ER) to regenerate PC through the Lands' cycle (he2022spns1isa pages 3-5, scharenberg2022alysosomallipid pages 9-12). Isotope tracing experiments showed that SPNS1-deficient cells produce 50% less deuterated PC species (d9-PC 32:1, d9-PC 34:1, d9-PC 34:2) from lysosome-derived d9-LPC compared to control cells, demonstrating quantitative contribution of the SPNS1-mediated salvage pathway to cellular PC pools (he2022spns1isa pages 3-5). This salvage mechanism conserves energy by avoiding futile deacylation-reacylation cycles and becomes essential under nutrient limitation (scharenberg2022alysosomallipid pages 9-12).

Metabolic Regulation and Nutrient Sensing

Choline Homeostasis:
SPNS1 function becomes critically important under conditions of choline limitation (scharenberg2022alysosomallipid pages 1-5, scharenberg2022alysosomallipid pages 5-7, scharenberg2022alysosomallipid pages 9-12). An endolysosome-focused CRISPR-Cas9 screen in pancreatic cancer cells cultured without free choline identified SPNS1 as the top-scoring essential gene (scharenberg2022alysosomallipid pages 5-7). SPNS1-knockout cells exhibit dramatic growth defects and cell death when deprived of exogenous choline, but grow normally in choline-replete medium (scharenberg2022alysosomallipid pages 5-7). This conditional essentiality reflects the dependence on lysosomal phospholipid salvage to supply choline-containing scaffolds for PC synthesis when de novo pathways are limited (scharenberg2022alysosomallipid pages 1-5, scharenberg2022alysosomallipid pages 9-12).

Triglyceride Synthesis:
Recent evidence reveals that SPNS1-mediated lysophospholipid transport quantitatively contributes to triglyceride (TAG) synthesis (he2025spns1variantscause pages 7-8, he2025spns1variantscause pages 1-2). Stable isotope tracing with d82-POPC demonstrated that fatty acids released from lysosomal LPC (following transport and further hydrolysis) are channeled into TAG synthesis, particularly under mTOR inhibition (he2025spns1variantscause pages 7-8). SPNS1-deficient cells showed more than 50% reduction in TAG containing deuterated fatty acids derived from lysosomal phospholipid catabolism (he2025spns1variantscause pages 7-8). This pathway represents an adaptive mechanism to utilize lysosome-derived fatty acids for energy storage when mTOR activity is low (he2025spns1variantscause pages 7-8).

Cholesterol Homeostasis:
SPNS1 deficiency impairs lysosomal cholesterol egress, leading to cholesterol accumulation in lysosomes (he2025spns1variantscause pages 7-8, he2025spns1variantscause pages 1-2). In SPNS1-knockout cells, filipin staining revealed punctate intracellular cholesterol accumulation, while wild-type cells showed primarily plasma membrane staining (he2025spns1variantscause pages 7-8). This phenotype was exacerbated by mTOR inhibition (he2025spns1variantscause pages 7-8). The mechanism involves inhibition of NPC1-mediated cholesterol export by accumulated lyso-ether-phospholipids (lysoplasmalogens) in SPNS1-deficient lysosomes (he2025spns1variantscause pages 7-8). Consequently, SPNS1-deficient cells exhibit reduced biogenesis of cytosolic lipid droplets containing cholesteryl esters, connecting lysosomal lysophospholipid transport to cellular lipid storage and cholesterol homeostasis (he2025spns1variantscause pages 7-8, he2025spns1variantscause pages 1-2).

Signaling Pathway Involvement

PI3K/AKT Signaling:
Loss of SPNS1 in mouse liver results in altered PI3K/AKT signaling, with decreased phosphorylation of Akt and reduced expression of ATF4, a downstream transcription factor important for metabolic stress adaptation (ha2024lackofspns1 pages 9-11, ha2024lackofspns1 pages 1-2). This dysregulation likely contributes to liver injury observed in SPNS1-deficient mice (ha2024lackofspns1 pages 9-11). The accumulation of sphingosine in lysosomes is known to inhibit Akt phosphorylation and disrupt insulin signaling (ha2024lackofspns1 pages 9-11).

Notch1 Signaling and Cardiac Development:
In zebrafish heart development, SPNS1 plays a critical role in cardiac valve morphogenesis through regulation of Notch1 signaling (chavez2024spns1dependentendocardiallysosomal pages 1-4). SPNS1-deficient (nrs mutant) zebrafish larvae display abnormal endocardial organization, impaired valve formation, and retrograde blood flow (chavez2024spns1dependentendocardiallysosomal pages 1-4). Single-nucleus transcriptome analysis revealed endocardial-specific alterations in lysosome-related genes and Notch1 signaling components (chavez2024spns1dependentendocardiallysosomal pages 1-4). Endocardial-specific overexpression of either SPNS1 or Notch1 rescued valve formation defects, establishing a functional link between lysosomal SPNS1 activity and developmental Notch signaling (chavez2024spns1dependentendocardiallysosomal pages 1-4).

Autophagy and Lysosome Biogenesis:
SPNS1 participates in autophagy regulation and autophagic lysosome reformation (ALR) (he2022spns1isa pages 1-2, scharenberg2022alysosomallipid pages 9-12). SPNS1-deficient cells exhibit enlarged lysosomes, increased basal LC3-II levels, and defective autophagosome-lysosome fusion, indicating impaired autophagic flux (he2022spns1isa pages 1-2). The protein has been implicated in synaptic pruning and neurodevelopment through autophagy-dependent processes (scharenberg2022alysosomallipid pages 9-12).

Experimental Evidence

Functional Assays

Multiple experimental approaches have validated SPNS1 function:

Cell-surface transport assays: Overexpression of SPNS1 at the plasma membrane enabled measurement of radiolabeled LPC uptake in controlled pH buffers, demonstrating saturable, pH-dependent transport with KM values in the micromolar range (he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 9-12).
Lysosomal lipid tracing: Metabolic labeling with [14C]-choline followed by lysosome isolation showed 5-fold accumulation of [14C]-LPC in lysosomes from SPNS1-KO cells (he2022spns1isa pages 1-2).
Stable isotope flux analysis: Delivery of d9-PC to lysosomes via apoE-nanodiscs combined with mass spectrometry quantification of lysosome-derived LPC and re-acylated PC species provided direct evidence for SPNS1-mediated salvage pathway (he2022spns1isa pages 3-5).
CRISPR-Cas9 screens: Genome-wide and targeted endolysosomal screens under choline limitation identified SPNS1 as essential for cell survival, with all ten sgRNAs showing strong depletion (scharenberg2022alysosomallipid pages 5-7).
Lipidomics: Comprehensive lipidomic profiling of SPNS1-deficient cells, tissues, and lysosome-enriched fractions consistently demonstrated accumulation of LPC, LPE, lysoplasmalogens, and sphingosine (he2022spns1isa pages 1-2, ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4).

Structural and Bioinformatic Analyses

Cryo-EM structure determination at 3.2 Å resolution provided atomic-level insights into substrate binding and transport mechanism (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 3-4). Molecular dynamics simulations over 200 nanoseconds revealed the dynamic process of LPC entry and translocation (chen2025molecularbasisof pages 4-6). AlphaFold prediction complemented experimental structures by providing models of alternative conformational states (chen2025molecularbasisof pages 3-4). Structure-guided mutagenesis validated functional importance of predicted substrate-binding and proton-sensing residues (chen2025molecularbasisof pages 4-6).

Disease Models

Mouse Models:
Global knockout of Spns1 results in embryonic lethality between E12.5 and E13.5, with severe developmental defects particularly affecting brain and eye development, reduced cortical thickness, decreased brain vascularization, and accumulation of membranous structures in neural cells (ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4). Postnatal conditional knockout using various tissue-specific Cre drivers produces organ-specific pathologies: liver-specific knockout causes liver inflammation and injury (he2022spns1isa pages 1-2); nervous system-specific knockout leads to dysmyelination, white matter dysplasia, oligodendrocyte loss, epilepsy, growth retardation, and death by 5 weeks of age (ichimura2024lossofspns1 pages 1-5).

Human Disease:
Biallelic loss-of-function variants in SPNS1 have been identified in human patients presenting with multiorgan disease, including progressive liver injury, striated muscle disease (skeletal and cardiac), developmental delay, neurological impairment, intellectual disability, and cerebellar hypoplasia (ha2024lackofspns1 pages 1-2, he2025spns1variantscause pages 1-2). Patient fibroblasts accumulate lysophospholipids (including lysoplasmalogens) and cholesterol in lysosomes, with reduced cellular plasmalogens and defective lipid droplet biogenesis (he2025spns1variantscause pages 1-2). These clinical presentations resemble lysosomal storage diseases, establishing SPNS1 as a disease gene (ha2024lackofspns1 pages 1-2, he2025spns1variantscause pages 1-2).

Zebrafish Models:
SPNS1-deficient zebrafish (nrs mutants) exhibit yolk opacity, early larval lethality, cardiac valve defects, and accumulation of LPC and LPE (he2022spns1isa pages 1-2, chavez2024spns1dependentendocardiallysosomal pages 1-4). These models have been instrumental in studying developmental roles of SPNS1.

SPNS2

SPNS1 shares 54-57% sequence identity with SPNS2 but exhibits distinct localization, substrate specificity, and transport mechanism (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 2-3). While SPNS1 localizes to lysosomes and transports LPC/LPE in a proton-dependent manner, SPNS2 localizes to the plasma membrane and functions as a facilitated-diffusion uniporter for S1P (chen2025molecularbasisof pages 2-3, he2022spns1isa pages 2-3). Structural comparison reveals similar overall MFS folds but critical differences: SPNS2 lacks the proton-sensing residue network present in SPNS1, and TM5/TM8 pack more tightly in SPNS2, precluding lateral lipid entry seen in SPNS1 (chen2025molecularbasisof pages 2-3, chen2025molecularbasisof pages 4-6). Chimeric experiments demonstrated that SPNS1's proton-sensing residues can confer pH dependence to SPNS2 (chen2025molecularbasisof pages 4-6).

Mfsd2a

SPNS1 and Mfsd2a both employ a lateral opening between TM5 and TM8 to engage LPC substrates, representing convergent evolution for handling amphiphilic lysolipids (chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6). However, Mfsd2a functions as a sodium-dependent LPC transporter at the blood-brain and blood-retina barriers, critical for omega-3 fatty acid (DHA) delivery to the brain and eye (chen2025molecularbasisof pages 3-4). SPNS1 is proton-coupled and functions in lysosomal export, highlighting how similar structural solutions can be adapted for different cellular locations and ion-coupling mechanisms (chen2025molecularbasisof pages 3-4).

Current Understanding and Recent Developments (2022-2025)

The period from 2022-2025 has witnessed remarkable progress in understanding SPNS1 biology:

Deorphanization (2022): Multiple independent groups simultaneously identified SPNS1 as a lysosomal lysophospholipid transporter, ending its classification as an orphan transporter (he2022spns1isa pages 1-2, he2022spns1isa pages 2-3, scharenberg2022alysosomallipid pages 1-5).
Structural determination (2024-2025): Cryo-EM structures of human SPNS1 revealed the molecular basis of substrate recognition and proton coupling (chen2025molecularbasisof pages 1-2, chen2025molecularbasisof pages 3-4, chen2025molecularbasisof pages 4-6).
Expanded substrate repertoire (2024): Recognition that SPNS1 transports not only LPC/LPE but also lysoplasmalogens, LPG, and potentially sphingosine, establishing it as a promiscuous lysolipid transporter (ha2024lackofspns1 pages 1-2, ha2024lackofspns1 pages 2-4).
Disease association (2024-2025): Identification of human patients with SPNS1 variants and characterization of multiorgan disease phenotypes linked SPNS1 to lysosomal storage disease spectrum (ha2024lackofspns1 pages 1-2, he2025spns1variantscause pages 1-2).
Metabolic integration (2025): Discovery that SPNS1-mediated transport contributes to triglyceride synthesis and cholesterol homeostasis, connecting lysosomal salvage to energy storage and lipid regulation under nutrient stress and mTOR inhibition (he2025spns1variantscause pages 7-8, he2025spns1variantscause pages 1-2).
Developmental roles (2024): Elucidation of SPNS1's role in cardiac valve development through Notch1 signaling and nervous system myelination (chavez2024spns1dependentendocardiallysosomal pages 1-4, ichimura2024lossofspns1 pages 1-5).

Conclusion

SPNS1 functions as a proton-dependent lysosomal transporter mediating the export of lysophospholipids, particularly LPC and LPE, from the lysosomal lumen to the cytosol. The protein operates through an alternating access mechanism with substrate binding in a lateral opening between TM5 and TM8, coupled to proton sensing via a network of residues centered on D94. SPNS1-mediated transport is essential for lysosomal phospholipid salvage, connecting lysosomal catabolism to ER-based biosynthetic pathways through the Lands' cycle. This function becomes critical under nutrient limitation, particularly choline deficiency, and integrates with broader cellular processes including triglyceride synthesis, cholesterol homeostasis, mTOR signaling, autophagy, and developmental signaling pathways. Loss of SPNS1 function causes embryonic lethality in mice, multiorgan disease in humans, and phenotypes consistent with lysosomal storage diseases, underscoring the physiological importance of lysosomal lysophospholipid transport.

References

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(ha2024lackofspns1 pages 1-2): Hoa T.T. Ha, SiYi Liu, Xuan T.A. Nguyen, Linh K. Vo, Nancy C.P. Leong, Dat T. Nguyen, Shivaranjani Balamurugan, Pei Yen Lim, YaJun Wu, Eunju Seong, Toan Q. Nguyen, Jeongah Oh, Markus R. Wenk, Amaury Cazenave-Gassiot, Zuhal Yapici, Wei-Yi Ong, Margit Burmeister, and Long N. Nguyen. Lack of spns1 results in accumulation of lysolipids and lysosomal storage disease in mouse models. JCI Insight, Mar 2024. URL: https://doi.org/10.1172/jci.insight.175462, doi:10.1172/jci.insight.175462. This article has 17 citations and is from a domain leading peer-reviewed journal.
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(chen2025molecularbasisof pages 4-6): Hongwen Chen, Hoa T. T. Ha, Nadia Elghobashi-Meinhardt, Nhung A. Le, Philip Schmiege, Long N. Nguyen, and Xiaochun Li. Molecular basis of spns1-mediated lysophospholipid transport from the lysosome. Proceedings of the National Academy of Sciences of the United States of America, Dec 2025. URL: https://doi.org/10.1073/pnas.2409596121, doi:10.1073/pnas.2409596121. This article has 7 citations and is from a highest quality peer-reviewed journal.
(ichimura2024lossofspns1 pages 1-5): Yoshinobu Ichimura, Yuki Sugiura, Yoshinori Katsuragi, Yu-Shin Sou, Takefumi Uemura, Naoki Tamura, Satoko Komatsu-Hirota, Takashi Ueno, Masato Koike, Satoshi Waguri, and Masaaki Komatsu. Loss of spns1, a lysosomal transporter, in the nervous system causes dysmyelination and white matter dysplasia. bioRxiv, May 2024. URL: https://doi.org/10.1101/2024.05.29.596535, doi:10.1101/2024.05.29.596535. This article has 2 citations.
(scharenberg2022alysosomallipid pages 5-7): Samantha G. Scharenberg, Wentao Dong, Kwamina Nyame, Roni Levin-Konigsberg, Aswini R. Krishnan, Eshaan S. Rawat, Kaitlyn Spees, Michael C. Bassik, and Monther Abu-Remaileh. A lysosomal lipid transport pathway that enables cell survival under choline limitation. bioRxiv, Nov 2022. URL: https://doi.org/10.1101/2022.11.27.517422, doi:10.1101/2022.11.27.517422. This article has 3 citations.
(he2025spns1variantscause pages 7-8): Menglan He, Mei Ding, Michaela Chocholouskova, Cheen Fei Chin, Martin Engvall, Helena Malmgren, Matias Wagner, Marlen C. Lauffer, Jacob Heisinger, May Christine V. Malicdan, Valerie Allamand, Madeleine Durbeej, Angelica Delgado Vega, Thomas Sejersen, Ann Nordgren, Federico Torta, and David L. Silver. Spns1 variants cause multiorgan disease and implicate lysophospholipid transport as critical for mtor-regulated lipid homeostasis. Journal of Clinical Investigation, Jul 2025. URL: https://doi.org/10.1172/jci193099, doi:10.1172/jci193099. This article has 5 citations and is from a highest quality peer-reviewed journal.
(he2025spns1variantscause pages 1-2): Menglan He, Mei Ding, Michaela Chocholouskova, Cheen Fei Chin, Martin Engvall, Helena Malmgren, Matias Wagner, Marlen C. Lauffer, Jacob Heisinger, May Christine V. Malicdan, Valerie Allamand, Madeleine Durbeej, Angelica Delgado Vega, Thomas Sejersen, Ann Nordgren, Federico Torta, and David L. Silver. Spns1 variants cause multiorgan disease and implicate lysophospholipid transport as critical for mtor-regulated lipid homeostasis. Journal of Clinical Investigation, Jul 2025. URL: https://doi.org/10.1172/jci193099, doi:10.1172/jci193099. This article has 5 citations and is from a highest quality peer-reviewed journal.
(chavez2024spns1dependentendocardiallysosomal pages 1-4): Myra N. Chávez, Prateek Arora, Marco Meer, Ines J. Marques, Alexander Ernst, Rodrigo A. Morales Castro, and Nadia Mercader. Spns1-dependent endocardial lysosomal function drives valve morphogenesis through notch1-signaling. Dec 2024. URL: https://doi.org/10.1016/j.isci.2024.111406, doi:10.1016/j.isci.2024.111406. This article has 3 citations and is from a peer-reviewed journal.

📚 Additional Documentation

Notes

(SPNS1-notes.md)

SPNS1 review notes

Curation scope

Falcon deep research has now completed successfully and is saved as
SPNS1-deep-research-falcon.md (15 citations; see the synthesis section below).
This review draws on that report together with cached publications,
UniProt/GOA-derived files, PN project mappings, and these manual curation notes.
(An earlier attempt had timed out before the deep_research_unified template/ID
bugs were fixed.)

Core function

SPNS1 is a lysosomal multi-pass major facilitator superfamily transporter. The
strongest current evidence supports a lysosomal lysophospholipid export/salvage
function, especially for LPC and LPE, with LPG and lysoplasmalogens also
transported in some assays. He et al. 2022 identified SPNS1 as a
proton-dependent LPC/LPE transporter and showed SPNS1-dependent efflux from
lysosomes followed by re-acylation into cytoplasmic phospholipid pools
PMID:36161949.
Scharenberg et al. 2023 independently found SPNS1 in an endolysosomal CRISPR
screen under choline limitation and concluded that SPNS1 exports LPC from the
lysosome to support phospholipid synthesis and cell survival
PMID:37075117.

Chen et al. 2025 provide structural support for this molecular function: a
cryo-EM structure of human SPNS1 in an LPC-bound lumen-facing conformation,
transport assays for cavity residues, and a proton-sensing residue network
PMID:39739806.
He et al. 2025 extend the physiology to human disease, reporting biallelic
SPNS1 loss-of-function variants with lysosomal lysophospholipid/cholesterol
accumulation and altered triglyceride/cholesterol homeostasis
PMID:40608416.

Proteostasis network context

The Proteostasis Network places SPNS1 under
Autophagy-Lysosome Pathway|Autophagic lysosome reformation|Efflux of autophagy products.
That placement is biologically plausible because autophagy-derived membrane
lipids are degraded in autolysosomes and their products must exit the lysosomal
lumen for reuse. However, the PN row is a curation hypothesis, not primary
evidence. The relevant direct evidence is that SPNS1 exports lysosomal LPC/LPE
and that loss of SPNS1 disrupts lysosomal lipid homeostasis, lysosomal function,
and nutrient-stress survival [PMID:36161949; PMID:37075117; PMID:40608416].

The PN projected term GO:0007041 lysosomal transport is defensible as a new
annotation because SPNS1 transports lysosomal degradation products across the
lysosomal membrane. It is still broad and does not capture the specific cargo or
direction. A more precise GO term such as "lysosomal lysophospholipid export" or
"lysosomal phospholipid salvage" would better represent the biology.

Annotation decisions

Accept lysosomal membrane/lysosome localization as core, with PMID:36161949
and UniProt supporting the lysosomal transporter model. The high-throughput
placental lysosomal membrane proteomics study is consistent but not the main
evidence PMID:17897319.
Accept lysophospholipid transport and phospholipid efflux as core. Treat
GO:0051978 lysophospholipid:sodium symporter activity as mechanistically
incorrect for SPNS1 because the papers describe proton-gradient dependence;
use the broad transmembrane transporter activity term pending the proposed
proton-specific ontology term.
Accept broad transmembrane transporter activity as accurate pending a
proton-specific lysophospholipid symporter term. Modify transmembrane transport, lipid transport, and generic membrane annotations to more
specific SPNS1 terms already supported by experiments.
Keep the mitochondrial inner membrane annotation as non-core. UniProt records
occasional mitochondrial localization from the early HSpin1 study, but the
dominant conserved function is lysosomal.
Mark protein binding as over-annotated. The BCL2/BCL2L1 interaction was
reported in the early HSpin1 cell-death study PMID:12815463, but GO:0005515
is not informative for SPNS1's core molecular function.

Falcon deep research synthesis (2026-06-21)

The Falcon deep research report (file:human/SPNS1/SPNS1-deep-research-falcon.md)
corroborates the lysosomal lysophospholipid-salvage model above and adds three
strands of newer evidence not previously captured in these notes. Citations
below use the DOIs given in the Falcon report; PMIDs already verified for the
core papers are reused from the sections above.

Structural / mechanistic detail (Chen et al. 2025, PMID:39739806;
doi:10.1073/pnas.2409596121). The 3.2 Å cryo-EM structure captures SPNS1 in an
outward/lumen-facing state with LPC bound in a lateral opening between TM5 and
TM8 (substrate residues Y203, G200, L201, I204 on TM5 and S322, G326, L327, C330
on TM8; G200L/G326L/Y203A abolish transport). Proton coupling is mediated by a
hydrophilic network rather than a single residue: D94 is central, with E87, S97,
R300 and S442 forming a salt-bridge/H-bond network (D94A abolishes, D94N partly
rescues; R300A nearly abolishes). Chimeric transfer of E87/S97 into the
pH-independent paralog SPNS2 confers pH dependence, and the reciprocal SPNS1
E87Q/S97A makes SPNS1 active at neutral pH — strong evidence that this network is
the proton sensor. The TM5/TM8 lateral-entry geometry parallels the plasma-
membrane Na+-dependent LPC transporter Mfsd2a (convergent handling of
amphipathic lysolipids), reinforcing that SPNS1 is a lysosomal proton-coupled
exporter, not a Na+-symporter (supports keeping GO:0051978
lysophospholipid:sodium symporter activity as mechanistically incorrect).

Mouse-genetic confirmation of substrate range and physiology (Ha et al. 2024,
JCI Insight, doi:10.1172/jci.insight.175462). Spns1 knockout causes lysosomal
accumulation of LPC, LPE, lysoplasmalogens, LPG, LPI and sphingosine (the
sphingosine buildup rivals NPC1-deficient models), with localization to LAMP1+/
Rab7+ late endosomes-lysosomes (not Rab5+ early endosomes). Global KO is
embryonic-lethal (~E12.5–E13.5) with brain/eye defects; liver-specific KO causes
inflammation/injury accompanied by reduced Akt phosphorylation and lower ATF4.
This places SPNS1 loss upstream of PI3K/AKT–ATF4 dysregulation and broadens the
transported-cargo set beyond LPC/LPE, while noting sphingosine export remains
indirectly (not yet directly) demonstrated.

New developmental roles. (i) In zebrafish, endocardial lysosomal SPNS1
activity drives cardiac valve morphogenesis via Notch1 signaling; endocardial
re-expression of SPNS1 or Notch1 rescues valve defects (Chávez et al. 2024,
iScience, doi:10.1016/j.isci.2024.111406). (ii) Nervous-system–specific Spns1
loss causes dysmyelination and white-matter dysplasia with oligodendrocyte
loss and epilepsy (Ichimura et al. 2024, doi:10.1101/2024.05.29.596535,
preprint). These are tissue-specific developmental consequences of the lysosomal
lipid-salvage defect and are best treated as non-core (downstream of the core
transporter function) rather than as primary molecular-function annotations.

Upstream/coupled lipid handling. The report notes lysosomal PLA2G15 generates
the LPC/LPE that SPNS1 exports, exported LPC is re-acylated by ER LPCATs (Lands'
cycle), and accumulated lyso-ether-phospholipids inhibit NPC1-mediated cholesterol
egress — the mechanistic link between SPNS1 loss and the lysosomal cholesterol
accumulation reported by He et al. 2025 (PMID:40608416).

Net effect on the review: no change to the core molecular-function call
(proton-coupled lysosomal lysophospholipid exporter / phospholipid salvage). The
new material strengthens (a) rejection of the Na+-symporter MF, (b) the case for a
proton-coupled lysophospholipid-export NTR, and (c) the framing of the
neurodevelopmental, cardiac, and hepatic phenotypes as non-core downstream
consequences.

Pn Notes

(SPNS1-pn-notes.md)

SPNS1 PN Consistency Notes

Generated: 2026-06-18
Project: PROTEOSTASIS
Scope: PN consistency rereview against local AIGR review and available deep-research artifacts
UniProt: Q9H2V7
AIGR review status: COMPLETE
Review batch: proteostasis-pr-1217 (PR 1217)
Batch change status: added

Source Files Checked

AIGR review YAML: present - genes/human/SPNS1/SPNS1-ai-review.yaml
AIGR review HTML: present - genes/human/SPNS1/SPNS1-ai-review.html
Gene notes: present - genes/human/SPNS1/SPNS1-notes.md
GOA TSV: present - genes/human/SPNS1/SPNS1-goa.tsv
UniProt record: present - genes/human/SPNS1/SPNS1-uniprot.txt
PN dossier: projects/PROTEOSTASIS/reports/phase1_dossiers/SPNS1.md
PN consistency section: projects/PROTEOSTASIS/reports/phase1_dossiers/_sections/SPNS1.md
PN projection report: projects/PROTEOSTASIS/reports/pn_projection/pn_projected_gene_go_summary.tsv
PN mapping audit: projects/PROTEOSTASIS/reports/pn_mapping_audit/current_mapping_scrutiny.tsv

Deep Research Files

No *-deep-research*.md file found in this gene directory.

AIGR Review Snapshot

Description: SPNS1 is a multi-pass lysosomal membrane protein in the major facilitator superfamily and Spinster family. Its core function is proton-gradient-dependent export of lysophospholipids, especially lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE), from the lysosomal lumen to the cytosol. The exported lysophospholipids are reused in phospholipid salvage pathways and can support phosphatidylcholine synthesis, neutral-lipid storage, cholesterol homeostasis, and survival under nutrient limitation. SPNS1 deficiency causes lysosomal accumulation of LPC/LPE and related lysolipids, perturbs lysosomal function and lipid homeostasis, and is now linked to human multiorgan disease caused by biallelic loss-of-function variants.
Existing/core annotation action counts: ACCEPT: 11; KEEP_AS_NON_CORE: 1; MARK_AS_OVER_ANNOTATED: 1; MODIFY: 4; NEW: 1

PN Consistency Summary

Consistency: Consistent. Notes, review YAML and PN agree SPNS1 is a lysosomal proton-driven lysophospholipid (LPC/LPE) efflux transporter, and that "efflux of autophagy products" is biologically plausible. The review explicitly accepts the PN GO:0007041 projection as a NEW annotation and cites the PN mapping file. No contradictions.
PN story / NEW pressure: PN asserts a lysosomal-export role not previously in GOA. GO:0007041 lysosomal transport is verified real (OLS: "movement into/out of/within a lysosome") and is genuinely absent from SPNS1 GOA (confirmed in goa.tsv) — the review correctly adds it as NEW. The review goes further with two proposed_new_terms (lysophospholipid:proton symporter activity; lysosomal lysophospholipid export), the right altitude for the precise cargo/direction the broad GO:0007041 misses. Conclude: ADD GO:0007041 (done as NEW) + proposed_new_terms; defensible.
Evidence alignment: Partial overlap. PN cites the Spinster ALR/mTOR PNAS paper and a trafficking review (titles only). Review's core evidence is the direct human/mouse transporter literature (PMID:36161949, 37075117, 39739806, 40608416) — stronger, mechanism-level; the PN ALR/mTOR paper is not in review supported_by but is consistent.
Verdict: Fully consistent; PN projection correctly actioned as NEW. No edits needed.

Full Consistency Review

UniProt: Q9H2V7 (Spinster homolog 1) · batch: proteostasis-pr-1217 · review status: COMPLETE
PN placement: Autophagy-Lysosome Pathway → Autophagic lysosome reformation → Efflux of autophagy products (1 row, ALP) ; PN-node mapping: leaf group=mapped/ok_for_propagation_to_go→GO:0007041 lysosomal transport; class (ALR)=context_only/too_broad→GO:0007040; branch=no_mapping. Projects GO:0007041 (new_to_goa).
Consistency: Consistent. Notes, review YAML and PN agree SPNS1 is a lysosomal proton-driven lysophospholipid (LPC/LPE) efflux transporter, and that "efflux of autophagy products" is biologically plausible. The review explicitly accepts the PN GO:0007041 projection as a NEW annotation and cites the PN mapping file. No contradictions.
PN story / NEW pressure: PN asserts a lysosomal-export role not previously in GOA. GO:0007041 lysosomal transport is verified real (OLS: "movement into/out of/within a lysosome") and is genuinely absent from SPNS1 GOA (confirmed in goa.tsv) — the review correctly adds it as NEW. The review goes further with two proposed_new_terms (lysophospholipid:proton symporter activity; lysosomal lysophospholipid export), the right altitude for the precise cargo/direction the broad GO:0007041 misses. Conclude: ADD GO:0007041 (done as NEW) + proposed_new_terms; defensible.
Mapping strategy: Gene does not change the node; GO:0007041 is appropriately broad (an over-broad "lysosomal transport" is acceptable here as the only fitting current term, and the review flags it as broad). Status/scope correct.
Evidence alignment: Partial overlap. PN cites the Spinster ALR/mTOR PNAS paper and a trafficking review (titles only). Review's core evidence is the direct human/mouse transporter literature (PMID:36161949, 37075117, 39739806, 40608416) — stronger, mechanism-level; the PN ALR/mTOR paper is not in review supported_by but is consistent.
Verdict: Fully consistent; PN projection correctly actioned as NEW. No edits needed.

PN Dossier Context

review_batch: proteostasis-pr-1217
review_yaml: genes/human/SPNS1/SPNS1-ai-review.yaml
PN workbook rows: 1

PN row 1: Autophagy-Lysosome Pathway | Autophagic lysosome reformation | Efflux of autophagy products

UniProt: Q9H2V7
In branches: ALP
Notes: Lysosome efflux permease that is essential for mTOR reactivation and ALR after prolonged starvation.
PN references (titles):
- Membrane Trafficking in Autophagy - ScienceDirect
- Spinster is required for autophagic lysosome reformation and mTOR reactivation following starvation | PNAS
PN-node mapping records (path + ancestors):
- [group] Autophagy-Lysosome Pathway|Autophagic lysosome reformation|Efflux of autophagy products
  status=mapped scope=ok_for_propagation_to_go GO=[GO:0007041 lysosomal transport]
  rationale: This PN leaf denotes export of degradation products from the autolysosome during late-stage autophagic lysosome reformation. The closest current GO process term is lysosomal transport, because the key shared semantics are transport across the lysosomal/autolysosomal membrane rather than a specific cargo chemistry.
- [class] Autophagy-Lysosome Pathway|Autophagic lysosome reformation
  status=context_only scope=too_broad_to_propagate GO=[GO:0007040 lysosome organization]
  rationale: Autophagic lysosome reformation is the lysosome-regeneration phase that follows autolysosome formation and cargo degradation. As a class, it is better aligned to lysosome organization than to generic autophagy, but the PN members are mechanistically mixed across membrane remodeling, tubulation, product efflux, and unknown late-stage roles, so class-level propagation would still over-annotate.
- [branch] Autophagy-Lysosome Pathway
  status=no_mapping scope= GO=[]
  rationale: Reviewed as the top-level PN branch. It is a project taxonomy umbrella rather than a direct GO assertion; all propagation must come from manually curated child nodes.

Projected GO annotations (1)

GO:0007041 lysosomal transport | scope=ok_for_propagation_to_go | goa_status=new_to_goa | from=Autophagy-Lysosome Pathway|Autophagic lysosome reformation|Efflux of autophagy products

Note

This file is generated from the current PROTEOSTASIS phase-1 dossier and local gene-review artifacts. Edit the source review, PN mapping, or dossier rather than this generated note when correcting the underlying curation.

📄 View Raw YAML

id: Q9H2V7
gene_symbol: SPNS1
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  SPNS1 is a multi-pass lysosomal membrane protein in the major facilitator
  superfamily and Spinster family. Its core function is proton-gradient-dependent
  export of lysophospholipids, especially lysophosphatidylcholine (LPC) and
  lysophosphatidylethanolamine (LPE), from the lysosomal lumen to the cytosol.
  The exported lysophospholipids are reused in phospholipid salvage pathways and
  can support phosphatidylcholine synthesis, neutral-lipid storage, cholesterol
  homeostasis, and survival under nutrient limitation. SPNS1 deficiency causes
  lysosomal accumulation of LPC/LPE and related lysolipids, perturbs lysosomal
  function and lipid homeostasis, and is now linked to human multiorgan disease
  caused by biallelic loss-of-function variants.
alternative_products:
  - name: '1'
    id: Q9H2V7-1
  - name: '2'
    id: Q9H2V7-2
    sequence_note: VSP_028196
  - name: 3 (CRA_d)
    id: Q9H2V7-3
    sequence_note: VSP_028195, VSP_028196
  - name: '4'
    id: Q9H2V7-4
    sequence_note: VSP_028194
  - name: '5'
    id: Q9H2V7-5
    sequence_note: VSP_036389
existing_annotations:
  - term:
      id: GO:0022857
      label: transmembrane transporter activity
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    qualifier: enables
    review:
      summary: >-
        SPNS1 is a transmembrane transporter, and the broad term is accurate for
        the currently supported lysosomal lysophospholipid export activity.
      action: ACCEPT
      reason: >-
        Retain the broad transporter term rather than replacing it with
        GO:0051978, whose sodium-coupled label does not match the
        proton-gradient-dependent SPNS1 evidence. A proton-specific
        lysophospholipid symporter term is requested below.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            the major facilitator superfamily protein Spns1 that is ubiquitously
            expressed in all tissues as a proton-dependent lysophosphatidylcholine
            (LPC) and lysophosphatidylethanolamine (LPE) transporter
        - reference_id: PMID:39739806
          supporting_text: >-
            Spns1 mediates the rate-limiting efflux of lysophospholipids from
            the lysosome to the cytosol
  - term:
      id: GO:0051977
      label: lysophospholipid transport
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    qualifier: involved_in
    review:
      summary: >-
        This is a core SPNS1 biological process. SPNS1 exports lysosomal LPC and
        LPE to the cytosol, and loss of SPNS1 causes lysosomal accumulation of
        these lysophospholipids.
      action: ACCEPT
      reason: >-
        The process accurately captures the conserved cargo-level biology, even
        though a more precise lysosomal export term would be preferable.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            Spns1 deficiency in cells, zebrafish embryos, and mouse liver
            resulted in lysosomal accumulation of LPC and LPE species
        - reference_id: PMID:37075117
          supporting_text: >-
            leads to intralysosomal accumulation of lysophosphatidylcholine (LPC) and
            lysophosphatidylethanolamine (LPE).
  - term:
      id: GO:0033700
      label: phospholipid efflux
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    qualifier: involved_in
    review:
      summary: >-
        SPNS1 mediates efflux of lysosomal lysophospholipids that derive from
        lysosomal phospholipid degradation. These products are exported to the
        cytosol for re-acylation and broader lipid salvage.
      action: ACCEPT
      reason: >-
        Although the transported substrates are lysophospholipids rather than
        intact phospholipids, the term is a reasonable process-level description
        of lysosomal phospholipid catabolite efflux and salvage.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            Flux analysis using stable isotope-labeled phospholipid apolipoprotein
            E nanodiscs targeted to lysosomes showed that LPC was transported out
            of lysosomes in an Spns1-dependent manner
        - reference_id: PMID:40608416
          supporting_text: >-
            SPNS1 is a lysosomal transporter that mediates the salvage of
            lysoglycerophospholipids
  - term:
      id: GO:0016020
      label: membrane
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    qualifier: is_active_in
    review:
      summary: >-
        SPNS1 is a multi-pass membrane transporter, but generic membrane is too
        broad. The experimentally and physiologically relevant active location is
        the lysosomal membrane.
      action: MODIFY
      reason: >-
        Replace the generic membrane term with the specific lysosomal membrane
        localization.
      proposed_replacement_terms:
        - id: GO:0005765
          label: lysosomal membrane
      supported_by:
        - reference_id: file:human/SPNS1/SPNS1-uniprot.txt
          supporting_text: 'SUBCELLULAR LOCATION: Lysosome membrane'
        - reference_id: PMID:39739806
          supporting_text: >-
            While Spns1 primarily localizes to the lysosomal membrane to export
            its substrates from lysosomal lumen under physiological conditions
  - term:
      id: GO:0005743
      label: mitochondrial inner membrane
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    qualifier: located_in
    review:
      summary: >-
        Early HSpin1 work and UniProt note occasional mitochondrial localization,
        but later biochemical and structural work establishes lysosomal membrane
        transport as the dominant SPNS1 function.
      action: KEEP_AS_NON_CORE
      reason: >-
        Keep as a possible non-core/overexpression-associated localization rather
        than treating it as a defining location for SPNS1 activity.
      supported_by:
        - reference_id: file:human/SPNS1/SPNS1-uniprot.txt
          supporting_text: >-
            Mitochondrion inner membrane {ECO:0000269|PubMed:12815463};
            Multi-pass membrane protein {ECO:0000269|PubMed:12815463}.
            Note=Ocassionally localizes to mitochondria.
  - term:
      id: GO:0005765
      label: lysosomal membrane
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    qualifier: located_in
    review:
      summary: >-
        Lysosomal membrane is the core active location for SPNS1. Transport
        assays, lysosome lipidomics, human disease work, and structural analyses
        all interpret SPNS1 as a lysosomal membrane lysophospholipid transporter.
      action: ACCEPT
      reason: Core localization for the lysosomal efflux function.
      supported_by:
        - reference_id: file:human/SPNS1/SPNS1-uniprot.txt
          supporting_text: 'SUBCELLULAR LOCATION: Lysosome membrane'
        - reference_id: PMID:40608416
          supporting_text: >-
            SPNS1, a ubiquitously expressed lysosomal transmembrane protein
            belonging to the major facilitator superfamily
  - term:
      id: GO:0006869
      label: lipid transport
    evidence_type: IEA
    original_reference_id: GO_REF:0000117
    qualifier: involved_in
    review:
      summary: >-
        SPNS1 is involved in lipid transport, but the reviewed literature defines
        the more specific process as lysosomal lysophospholipid transport and
        phospholipid-catabolite efflux.
      action: MODIFY
      reason: >-
        The broad lipid transport term should be replaced by more informative
        lysophospholipid transport and phospholipid efflux terms.
      proposed_replacement_terms:
        - id: GO:0051977
          label: lysophospholipid transport
        - id: GO:0033700
          label: phospholipid efflux
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            Our findings identify a phospholipid salvage pathway from lysosomes
            to the cytosol that is dependent on Spns1
  - term:
      id: GO:0022857
      label: transmembrane transporter activity
    evidence_type: IEA
    original_reference_id: GO_REF:0000002
    qualifier: enables
    review:
      summary: >-
        The InterPro-derived transporter annotation is broad but accurate for
        SPNS1 lysosomal lysophospholipid export activity.
      action: ACCEPT
      reason: >-
        Retain the broad transporter term rather than replacing it with
        GO:0051978, whose sodium-coupled label does not match the
        proton-gradient-dependent SPNS1 evidence. A proton-specific
        lysophospholipid symporter term is requested below.
      supported_by:
        - reference_id: PMID:39739806
          supporting_text: >-
            Our results reveal molecular insights into lysosomal LPC transport
            and the proton-sensing mechanism by Spns1.
  - term:
      id: GO:0055085
      label: transmembrane transport
    evidence_type: IEA
    original_reference_id: GO_REF:0000002
    qualifier: involved_in
    review:
      summary: >-
        SPNS1 mediates transmembrane movement, but the specific process is
        lysosomal lysophospholipid export/salvage.
      action: MODIFY
      reason: >-
        Replace the broad transmembrane transport term with the specific
        lysophospholipid transport process.
      proposed_replacement_terms:
        - id: GO:0051977
          label: lysophospholipid transport
      supported_by:
        - reference_id: PMID:37075117
          supporting_text: >-
            SPNS1 functions to export LPC species from the lysosomal lumen to the
            cytosol for reacylation to PC
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:12815463
    qualifier: enables
    review:
      summary: >-
        The early HSpin1 study reported interaction with BCL2 and BCL2L1, but
        GO:0005515 is uninformative and does not describe SPNS1's core molecular
        function.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Avoid protein binding as a retained functional annotation. The interaction
        may be valid as interaction data, but it should not define SPNS1 function.
      supported_by:
        - reference_id: PMID:12815463
          supporting_text: >-
            HSpin1 bound to Bcl-2 and apoptosis regulator Bcl-X (Bcl-xL)
  - term:
      id: GO:0005764
      label: lysosome
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    qualifier: is_active_in
    review:
      summary: >-
        SPNS1 acts in lysosomes as a lysophospholipid exporter. This orthology
        transfer is consistent with direct human and model-organism evidence.
      action: ACCEPT
      reason: Core active cellular location.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            The lysosome is central to the degradation of proteins,
            carbohydrates, and lipids and their salvage back to the cytosol for
            reutilization.
        - reference_id: PMID:40608416
          supporting_text: >-
            SPNS1 is a lysosomal transporter that mediates the salvage of
            lysoglycerophospholipids
  - term:
      id: GO:0051977
      label: lysophospholipid transport
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    qualifier: involved_in
    review:
      summary: >-
        Orthology-based lysophospholipid transport is directly supported by human
        SPNS1 transport assays, lysosome flux experiments, and disease variant
        studies.
      action: ACCEPT
      reason: Core biological process.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            Taken together, these data indicate that endogenous Spns1 mediates
            the transport of LPCs and LPEs out of lysosomes
        - reference_id: PMID:40608416
          supporting_text: >-
            lysophospholipids transported by SPNS1 into the cytosol quantitatively
            contributed to triglyceride synthesis
  - term:
      id: GO:0005764
      label: lysosome
    evidence_type: IDA
    original_reference_id: PMID:36161949
    qualifier: is_active_in
    review:
      summary: >-
        The 2022 PNAS study directly investigated SPNS1 function in lysosomes and
        isolated lysosome-enriched fractions showing substrate accumulation in
        SPNS1-deficient cells.
      action: ACCEPT
      reason: Core active location.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            Lysosomes isolated from [14C]-choline-labeled Spns1 KO cells showed
            accumulation of [14C]-LPC
  - term:
      id: GO:0051977
      label: lysophospholipid transport
    evidence_type: IDA
    original_reference_id: PMID:36161949
    qualifier: involved_in
    review:
      summary: >-
        This is one of the central conclusions of the 2022 PNAS study. SPNS1
        transports LPC and LPE out of lysosomes and enables their reuse in
        phospholipid pools.
      action: ACCEPT
      reason: Core biological process supported by direct assays and flux data.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            data from our flux analysis support the conclusion that LPCs are
            transported out of lysosomes by Spns1 and re-acylated into PC pools
  - term:
      id: GO:0051978
      label: lysophospholipid:sodium symporter activity
    evidence_type: IDA
    original_reference_id: PMID:36161949
    qualifier: enables
    review:
      summary: >-
        This term captures lysophospholipid transport, but its sodium-symporter
        label is mechanistically incorrect for SPNS1 because SPNS1 is
        proton-gradient dependent.
      action: MODIFY
      reason: >-
        Replace with the broad transmembrane transporter activity term pending a
        proton-specific lysophospholipid symporter term, which is requested
        below.
      proposed_replacement_terms:
        - id: GO:0022857
          label: transmembrane transporter activity
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            The uptake of [14C]-LPC-oleate by Spns1 was concentration dependent
            and saturable, with a pH optimum between pH 5.0 and 6.0
        - reference_id: PMID:39739806
          supporting_text: >-
            we identify a five-residue network that is crucial for proton-sensing
            by Spns1
  - term:
      id: GO:0005764
      label: lysosome
    evidence_type: IDA
    original_reference_id: PMID:25365221
    qualifier: located_in
    review:
      summary: >-
        The seeded original reference is an autophagic lysosome reformation paper
        focused on spastizin and spatacsin rather than direct SPNS1 evidence.
        Nevertheless, lysosomal localization of SPNS1 is strongly supported by
        later direct SPNS1 studies and UniProt.
      action: ACCEPT
      reason: >-
        Accept the term because it is correct, while noting that PMID:25365221 is
        not the best primary evidence for SPNS1 itself.
      supported_by:
        - reference_id: file:human/SPNS1/SPNS1-uniprot.txt
          supporting_text: 'SUBCELLULAR LOCATION: Lysosome membrane'
        - reference_id: PMID:39739806
          supporting_text: >-
            Spns1 is located on the lysosomal membrane and mediates
            lysophospholipid efflux depending on the proton gradient
  - term:
      id: GO:0005765
      label: lysosomal membrane
    evidence_type: HDA
    original_reference_id: PMID:17897319
    qualifier: located_in
    review:
      summary: >-
        High-throughput lysosomal membrane proteomics is consistent with the
        established SPNS1 lysosomal membrane localization, but the direct
        transporter papers are stronger support.
      action: ACCEPT
      reason: Accurate cellular component annotation consistent with core function.
      supported_by:
        - reference_id: PMID:17897319
          supporting_text: >-
            We searched for novel proteins in lysosomal membranes, tentatively
            participating in molecular transport across the membrane
        - reference_id: file:human/SPNS1/SPNS1-uniprot.txt
          supporting_text: 'SUBCELLULAR LOCATION: Lysosome membrane'
  - term:
      id: GO:0007041
      label: lysosomal transport
    evidence_type: IDA
    original_reference_id: PMID:36161949
    qualifier: involved_in
    review:
      summary: >-
        Proposed new annotation from the Proteostasis Network context. SPNS1
        exports lysosomal degradation products across the lysosomal membrane,
        placing its specific lysophospholipid salvage activity within the broader
        process of lysosomal transport.
      action: NEW
      reason: >-
        GOA already captures lysophospholipid transport and phospholipid efflux,
        but not the broader lysosomal-transport context highlighted by the PN
        entry. This should be added conservatively as a broad process annotation;
        a more precise lysosomal lysophospholipid export term would be better.
      supported_by:
        - reference_id: PMID:36161949
          supporting_text: >-
            Our findings identify a phospholipid salvage pathway from lysosomes
            to the cytosol that is dependent on Spns1
        - reference_id: PMID:37075117
          supporting_text: >-
            SPNS1 functions to export LPC species from the lysosomal lumen to the
            cytosol for reacylation to PC
        - reference_id: file:human/SPNS1/SPNS1-notes.md
          supporting_text: >-
            The Proteostasis Network places SPNS1 under
            `Autophagy-Lysosome Pathway|Autophagic lysosome reformation|Efflux
            of autophagy products`.
core_functions:
  - description: >-
      Proton-gradient-dependent export of lysophospholipids from the lysosomal
      lumen to the cytosol. SPNS1 transports LPC and LPE, and also transports
      LPG and lysoplasmalogen species in reported assays. This lysosomal efflux
      enables phospholipid salvage through downstream re-acylation and supports
      lipid homeostasis under nutrient stress.
    molecular_function:
      id: GO:0022857
      label: transmembrane transporter activity
    directly_involved_in:
      - id: GO:0051977
        label: lysophospholipid transport
      - id: GO:0033700
        label: phospholipid efflux
      - id: GO:0007041
        label: lysosomal transport
    locations:
      - id: GO:0005765
        label: lysosomal membrane
      - id: GO:0005764
        label: lysosome
    supported_by:
      - reference_id: PMID:36161949
        supporting_text: >-
          major facilitator superfamily protein Spns1 that is ubiquitously
          expressed in all tissues as a proton-dependent lysophosphatidylcholine
          (LPC) and lysophosphatidylethanolamine (LPE) transporter
      - reference_id: PMID:37075117
        supporting_text: >-
          SPNS1 functions to export LPC species from the lysosomal lumen to the
          cytosol for reacylation to PC
      - reference_id: PMID:39739806
        supporting_text: >-
          Our results reveal molecular insights into lysosomal LPC transport and
          the proton-sensing mechanism by Spns1.
      - reference_id: PMID:40608416
        supporting_text: >-
          lysophospholipids transported by SPNS1 into the cytosol quantitatively
          contributed to triglyceride synthesis
proposed_new_terms:
  - proposed_name: lysophospholipid:proton symporter activity
    proposed_definition: >-
      Enables the coupled transfer of a lysophospholipid and a proton from one
      side of a membrane to the other, driven by the transmembrane proton
      gradient.
    justification: >-
      The current closest GO molecular-function term for SPNS1 is
      GO:0051978 lysophospholipid:sodium symporter activity, but SPNS1 is
      repeatedly described experimentally as proton dependent, not sodium
      dependent.
    proposed_parent:
      id: GO:0015295
      label: solute:proton symporter activity
    supported_by:
      - reference_id: PMID:36161949
        supporting_text: >-
          The uptake of [14C]-LPC-oleate by Spns1 was concentration dependent
          and saturable, with a pH optimum between pH 5.0 and 6.0
      - reference_id: PMID:39739806
        supporting_text: >-
          we identify a five-residue network that is crucial for proton-sensing
          by Spns1
  - proposed_name: lysosomal lysophospholipid export
    proposed_definition: >-
      The directed movement of lysophospholipids from the lysosomal lumen across
      the lysosomal membrane to the cytosol.
    justification: >-
      Existing GO terms capture lysophospholipid transport and broad lysosomal
      transport, but do not represent the specific lysosomal export/salvage
      process established for SPNS1.
    proposed_parent:
      id: GO:0051977
      label: lysophospholipid transport
    proposed_mappings:
      - predicate: skos:relatedMatch
        target_term:
          id: GO:0007041
          label: lysosomal transport
    supported_by:
      - reference_id: PMID:36161949
        supporting_text: >-
          Our findings identify a phospholipid salvage pathway from lysosomes
          to the cytosol that is dependent on Spns1
      - reference_id: PMID:37075117
        supporting_text: >-
          SPNS1 functions to export LPC species from the lysosomal lumen to the
          cytosol for reacylation to PC
suggested_questions:
  - question: >-
      Should GO:0051978 be renamed or should a proton-dependent
      lysophospholipid symporter child term be added for SPNS1-like lysosomal
      transporters?
    experts:
      - David L. Silver
      - Li X
  - question: >-
      Should SPNS1 receive a broad GO:0007041 lysosomal transport annotation now,
      or should curation wait for a more precise lysosomal lysophospholipid export
      biological-process term?
    experts:
      - Menglan He
      - Susanna G. Scharenberg
  - question: >-
      How much of SPNS1-associated disease is driven by lysophospholipid salvage,
      ether-lysophospholipid/plasmalogen salvage, cholesterol egress, or secondary
      autophagy defects in specific tissues?
    experts:
      - Menglan He
      - David L. Silver
suggested_experiments:
  - description: >-
      Reconstitute purified human SPNS1 into proteoliposomes with controlled
      proton and sodium gradients and measure transport of LPC, LPE, LPG, and
      lysoplasmalogen substrates.
    experiment_type: transport_reconstitution
    hypothesis: >-
      SPNS1 is a lysophospholipid:proton symporter rather than a sodium-coupled
      lysophospholipid transporter.
  - description: >-
      Use autophagy-induced lysosomal cargo flux assays in SPNS1 knockout and
      rescue cells to measure export of autolysosome-derived lysophospholipids
      after starvation and recovery.
    experiment_type: lipidomics/genetic_rescue
    hypothesis: >-
      SPNS1 mediates lysosomal export of autophagy-derived lysophospholipid
      catabolites during lysosome recovery and lipid salvage.
  - description: >-
      Compare wild-type SPNS1, disease variants, and transport-site mutants for
      lysosomal LPC/LPE accumulation, cholesterol egress, triglyceride synthesis,
      and autophagy markers under mTOR inhibition.
    experiment_type: disease_variant_functional_assay
    hypothesis: >-
      Disease variants cause partial loss of lysosomal lysophospholipid export
      that secondarily disrupts cholesterol egress and nutrient-stress lipid
      storage.
references:
  - id: GO_REF:0000002
    title: Gene Ontology annotation through association of InterPro records with GO terms
    findings: []
  - id: GO_REF:0000033
    title: Annotation inferences using phylogenetic trees
    findings: []
  - id: GO_REF:0000044
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
    findings: []
  - id: GO_REF:0000107
    title: Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara
    findings: []
  - id: GO_REF:0000117
    title: Electronic Gene Ontology annotations created by ARBA machine learning models
    findings: []
  - id: PMID:12815463
    title: HSpin1, a transmembrane protein interacting with Bcl-2/Bcl-xL, induces a caspase-independent autophagic cell death.
    findings: []
  - id: PMID:17897319
    title: Integral and associated lysosomal membrane proteins.
    findings: []
  - id: PMID:25365221
    title: Spastic paraplegia proteins spastizin and spatacsin mediate autophagic lysosome reformation.
    findings: []
  - id: PMID:36161949
    title: Spns1 is a lysophospholipid transporter mediating lysosomal phospholipid salvage.
    findings: []
  - id: PMID:37075117
    title: An SPNS1-dependent lysosomal lipid transport pathway that enables cell survival under choline limitation.
    findings: []
  - id: PMID:39739806
    title: Molecular basis of Spns1-mediated lysophospholipid transport from the lysosome.
    findings: []
  - id: PMID:40608416
    title: SPNS1 variants cause multiorgan disease and implicate lysophospholipid transport as critical for mTOR-regulated lipid homeostasis.
    findings: []
  - id: file:human/SPNS1/SPNS1-uniprot.txt
    title: UniProt record for human SPNS1
    findings: []
  - id: file:human/SPNS1/SPNS1-notes.md
    title: SPNS1 curation notes
    findings: []
  - id: file:projects/PROTEOSTASIS/mappings/autophagy_lysosome_pathway.yaml
    title: Proteostasis Network autophagy-lysosome pathway mappings
    findings: []

SPNS1

Gene Description

Proposed New Ontology Terms

lysophospholipid:proton symporter activity

lysosomal lysophospholipid export

Existing Annotations Review

Core Functions

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Comprehensive Research Report: SPNS1 (Protein Spinster Homolog 1)

Gene Identity and Protein Family

Primary Function and Substrate Specificity

Lysosomal Lysophospholipid Transporter

Sphingosine Transport

Subcellular Localization

Molecular Mechanism of Transport

Structural Basis

Proton-Coupling Mechanism

Transport Cycle

Biochemical Pathways and Physiological Roles

Lysosomal Phospholipid Salvage Pathway

Metabolic Regulation and Nutrient Sensing

Signaling Pathway Involvement

Experimental Evidence

Functional Assays

Structural and Bioinformatic Analyses

Disease Models

Comparison to Related Transporters

SPNS2

Mfsd2a

Current Understanding and Recent Developments (2022-2025)

Conclusion

📚 Additional Documentation

Notes

SPNS1 review notes

Curation scope

Core function

Proteostasis network context

Annotation decisions

Falcon deep research synthesis (2026-06-21)

Pn Notes

SPNS1 PN Consistency Notes

Source Files Checked

Deep Research Files

AIGR Review Snapshot

PN Consistency Summary

Full Consistency Review

PN Dossier Context

PN row 1: Autophagy-Lysosome Pathway | Autophagic lysosome reformation | Efflux of autophagy products

Projected GO annotations (1)

Note

📄 View Raw YAML