Sarcolipin (SLN) is a 31-amino-acid single-pass transmembrane peptide of the sarcoplasmic reticulum (SR) membrane and a member of the regulin family of SERCA calcium-pump regulatory subunits, with phospholamban (PLN), myoregulin, endoregulin, another-regulin and DWORF. It has a short cytosolic N-terminus, a transmembrane helix and a conserved lumenal C-terminal tail (RSYQY) that is required for function. SLN binds SERCA1a and SERCA2a in the same transmembrane groove used by PLN, lowers the pump's apparent Ca2+ affinity and so slows Ca2+ re-uptake into the SR; it acts synergistically with PLN and can bind PLN and other regulins. It is most abundant in fast-twitch skeletal muscle and, in the heart, in the atria, where its protein level falls in human atrial fibrillation. In mouse, SLN stays bound to SERCA at high Ca2+ and uncouples ATP hydrolysis from Ca2+ transport, which makes it necessary for muscle-based non-shivering thermogenesis and influences whole-body energy metabolism.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004857 enzyme inhibitor activity | ISS PMID:12032137 Sarcolipin inhibits polymerization of phospholamban to induc... | MODIFY | Summary: ISS from rabbit sarcolipin (P42532). SLN lowers the apparent Ca2+ affinity of SERCA1a and SERCA2a; human SLN behaves the same in HEK-293 co-expression. Reason: The inhibition of SERCA is the core activity of SLN and is well supported, but enzyme inhibitor activity is too general and a loose fit: in current GO the regulin target term GO:0042030 ATPase inhibitor activity sits under molecular function inhibitor activity, not under enzyme inhibitor activity. GO:0042030 names the activity inhibited and is the term carried by the paralog PLN (IDA, IBA) and used in the MRLN and ERLN reviews. Proposed replacements: ATPase inhibitor activity Supporting Evidence: PMID:12032137 NF-SLN can reduce the apparent Ca(2+) affinity of both SERCA1a (DeltaK(Ca) = -0.22 +/- 0.01 pCa units) and SERCA2a (DeltaK(Ca) = -0.37 +/- 0.04 pCa units). PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). PMID:23455424 The regulatory SLN traps the Ca(2+)-ATPase in a previously undescribed E1 state, with exposure of the Ca(2+) sites through an open cytoplasmic pathway stabilized by Mg(2+). |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | REMOVE | Summary: HuRI yeast two-hybrid interaction (UniProtKB:Q8N205). Reason: Bare protein binding from a high-throughput yeast two-hybrid screen (HuRI). The partner is an unrelated membrane protein, there is no follow-up, and a 31-residue hydrophobic transmembrane peptide is a typical promiscuous Y2H hit. The annotation is uninformative about SLN function; removal does not assert that the interaction is false. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | REMOVE | Summary: HuRI yeast two-hybrid interaction (UniProtKB:Q8N6L0). Reason: Bare protein binding from a high-throughput yeast two-hybrid screen (HuRI). The partner is an unrelated membrane protein, there is no follow-up, and a 31-residue hydrophobic transmembrane peptide is a typical promiscuous Y2H hit. The annotation is uninformative about SLN function; removal does not assert that the interaction is false. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | REMOVE | Summary: HuRI yeast two-hybrid interaction (UniProtKB:Q9BSE2). Reason: Bare protein binding from a high-throughput yeast two-hybrid screen (HuRI). The partner is an unrelated membrane protein, there is no follow-up, and a 31-residue hydrophobic transmembrane peptide is a typical promiscuous Y2H hit. The annotation is uninformative about SLN function; removal does not assert that the interaction is false. |
| GO:0005515 protein binding | IPI PMID:28890335 The ER-Localized Transmembrane Protein EPG-3/VMP1 Regulates ... | MODIFY | Summary: IPI with SERCA2 (ATP2A2). The paper shows that human SLN and PLN form an inhibitory complex with SERCA that VMP1 competes against. Reason: The interaction is the physical basis of SLN function. The partner is the SERCA2 calcium pump, an ATPase, so the informative binding term is ATPase binding (GO:0051117), which SLN also carries by ISS. The inhibitory activity itself is captured by GO:0042030 elsewhere. Proposed replacements: ATPase binding Supporting Evidence: PMID:28890335 VMP1 interacts with SERCA and prevents formation of the SERCA/PLN/SLN inhibitory complex. PMID:23455424 The regulatory SLN traps the Ca(2+)-ATPase in a previously undescribed E1 state, with exposure of the Ca(2+) sites through an open cytoplasmic pathway stabilized by Mg(2+). |
| GO:0005515 protein binding | IPI PMID:28890335 The ER-Localized Transmembrane Protein EPG-3/VMP1 Regulates ... | REMOVE | Summary: IPI with VMP1. VMP1 binds SERCA and competes with PLN/SLN for it. Reason: Bare protein binding. VMP1 is not an enzyme or transporter regulated by SLN, and no more informative MF follows from this interaction; VMP1 is the protein that acts here (it displaces SLN from SERCA). Removal does not dispute the reported co-immunoprecipitation. Supporting Evidence: PMID:28890335 VMP1 interacts with SERCA and prevents formation of the SERCA/PLN/SLN inhibitory complex. |
| GO:0005515 protein binding | IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari... | REMOVE | Summary: Yeast two-hybrid interaction with TMEM79 from the HuRI variant-perturbation screen. Reason: Bare protein binding from a high-throughput yeast two-hybrid screen (HuRI). The partner is an unrelated membrane protein, there is no follow-up, and a 31-residue hydrophobic transmembrane peptide is a typical promiscuous Y2H hit. The annotation is uninformative about SLN function; removal does not assert that the interaction is false. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping to ER membrane. Reason: SLN is a single-pass ER/SR membrane peptide; when expressed in non-muscle cells it localises with SERCA to the ER membrane. The SR membrane is the more specific location in muscle. Supporting Evidence: PMID:9575189 Immunohistochemistry was used to demonstrate colocalization of NF-SLN and SERCA1 in the endoplasmic reticulum membrane and to demonstrate the cytosolic orientation of the N terminus of SLN. |
| GO:0005789 endoplasmic reticulum membrane | ISS GO_REF:0000024 | ACCEPT | Summary: ISS from rabbit SLN to ER membrane. Reason: Consistent with the HEK-293 co-localisation data for SLN and SERCA1 in the ER membrane. Supporting Evidence: PMID:9575189 Immunohistochemistry was used to demonstrate colocalization of NF-SLN and SERCA1 in the endoplasmic reticulum membrane and to demonstrate the cytosolic orientation of the N terminus of SLN. |
| GO:0006816 calcium ion transport | NAS PMID:9367679 Characterization of the gene encoding human sarcolipin (SLN)... | MODIFY | Summary: NAS from the human SLN gene paper, which describes SLN as a SERCA1-associated proteolipid. Reason: SLN does not transport calcium; it regulates the pump that does. As a regulator it should carry the regulation term for SR Ca2+ import rather than the transport process itself. Proposed replacements: negative regulation of calcium ion import into sarcoplasmic reticulum Supporting Evidence: PMID:9367679 Sarcolipin (SLN) is a low-molecular-weight protein that copurifies with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+ ATPase (SERCA1). PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). |
| GO:0016020 membrane | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the sarcolipin family signature. Reason: Correct though uninformative; SLN is an integral membrane peptide. The more specific SR membrane term is also annotated. Supporting Evidence: PMID:9367679 Homology of SLN with phospholamban (PLN) suggests that the first 7 hydrophilic amino acids are cytoplasmic, the next 19 hydrophobic amino acids form a single transmembrane helix, and the last 5 hydrophilic amino acids are lumenal. |
| GO:0016020 membrane | NAS PMID:9367679 Characterization of the gene encoding human sarcolipin (SLN)... | ACCEPT | Summary: NAS for membrane from the human SLN gene paper. Reason: Correct; topology prediction and later NMR show a single transmembrane helix. Supporting Evidence: PMID:9367679 Homology of SLN with phospholamban (PLN) suggests that the first 7 hydrophilic amino acids are cytoplasmic, the next 19 hydrophobic amino acids form a single transmembrane helix, and the last 5 hydrophilic amino acids are lumenal. PMID:11781085 SLN orients perpendicularly to the plane of the membrane bilayers. |
| GO:0016529 sarcoplasmic reticulum | IDA PMID:9575189 Sarcolipin regulates the activity of SERCA1, the fast-twitch... | ACCEPT | Summary: IDA: SLN is present in SR vesicles from rabbit fast-twitch muscle and co-purifies with SERCA1. Reason: Correct location; the SR membrane term (GO:0033017) is the more precise annotation for the same evidence. Supporting Evidence: PMID:9575189 Ca2+ uptake rates in sarcoplasmic reticulum vesicles, isolated from rabbit fast-twitch muscle (tibialis anterior) subjected to chronic low frequency stimulation, were reduced by approximately 40% in 3- and 4-day stimulated muscle PMID:9367679 Sarcolipin (SLN) is a low-molecular-weight protein that copurifies with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+ ATPase (SERCA1). |
| GO:0030234 enzyme regulator activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the sarcolipin family signature to enzyme regulator activity. Reason: Correct in direction but too general and on a different branch: the regulin activity is GO:0042030 ATPase inhibitor activity, which is not a descendant of enzyme regulator activity in current GO. Proposed replacements: ATPase inhibitor activity Supporting Evidence: PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). PMID:23455424 The regulatory SLN traps the Ca(2+)-ATPase in a previously undescribed E1 state, with exposure of the Ca(2+) sites through an open cytoplasmic pathway stabilized by Mg(2+). |
| GO:0033017 sarcoplasmic reticulum membrane | EXP PMID:11781085 Structure and orientation of sarcolipin in lipid environment... | ACCEPT | Summary: EXP from the NMR structure/topology study of synthetic SLN in lipid bilayers. Reason: The cached abstract shows transmembrane insertion in model bilayers rather than localisation in muscle SR, but the location is correct for SLN and the curator had the full text, so the annotation is accepted. Supporting Evidence: PMID:11781085 Sarcolipin (SLN) is a 31 amino acid integral membrane protein that regulates Ca-ATPase activity in skeletal muscle. PMID:11781085 SLN orients perpendicularly to the plane of the membrane bilayers. |
| GO:0033017 sarcoplasmic reticulum membrane | EXP PMID:9575189 Sarcolipin regulates the activity of SERCA1, the fast-twitch... | ACCEPT | Summary: EXP from co-expression and SR-vesicle data. Reason: SLN is a resident SR membrane regulatory subunit of SERCA1; topology (cytoplasmic N-terminus) was shown in this paper. Supporting Evidence: PMID:9575189 Immunohistochemistry was used to demonstrate colocalization of NF-SLN and SERCA1 in the endoplasmic reticulum membrane and to demonstrate the cytosolic orientation of the N terminus of SLN. PMID:9575189 Ca2+ uptake rates in sarcoplasmic reticulum vesicles, isolated from rabbit fast-twitch muscle (tibialis anterior) subjected to chronic low frequency stimulation, were reduced by approximately 40% in 3- and 4-day stimulated muscle |
| GO:0033017 sarcoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping to SR membrane. Reason: Correct core location. Supporting Evidence: PMID:9367679 Sarcolipin (SLN) is a low-molecular-weight protein that copurifies with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+ ATPase (SERCA1). |
| GO:0033017 sarcoplasmic reticulum membrane | ISS GO_REF:0000024 | ACCEPT | Summary: ISS from mouse SLN. Reason: Correct core location; mouse and human SLN share the conserved TM and lumenal tail. Supporting Evidence: PMID:9367679 Sarcolipin (SLN) is a low-molecular-weight protein that copurifies with the fast-twitch skeletal muscle sarcoplasmic reticulum Ca2+ ATPase (SERCA1). |
| GO:0043242 negative regulation of protein-containing complex disassembly | ISS PMID:12032137 Sarcolipin inhibits polymerization of phospholamban to induc... | KEEP AS NON CORE | Summary: ISS from rabbit SLN: NF-SLN made the PLN-SERCA complex resistant to Ca2+-induced dissociation in co-immunoprecipitation. Reason: A single co-immunoprecipitation observation with overexpressed tagged rabbit SLN in HEK-293 microsomes. It is part of the proposed super-inhibition mechanism rather than a separate role, and has not been tested for human SLN in muscle. Supporting Evidence: PMID:12032137 NF-SLN induced resistance to Ca(2+) dissociation of the PLN.SERCA complex. PMID:26743715 This binding of SLN could disrupt PLB pentamer formation and increase the interaction of PLB with SERCA, thereby inhibiting SERCA to a much greater extent. |
| GO:0051117 ATPase binding | ISS PMID:12032137 Sarcolipin inhibits polymerization of phospholamban to induc... | ACCEPT | Summary: ISS from rabbit SLN; SLN binds SERCA1a and SERCA2a. Reason: Direct binding to SERCA is well established, including a crystal structure of the SERCA1a-SLN complex; human SLN co-immunoprecipitates with SERCA2. Supporting Evidence: PMID:23455424 The regulatory SLN traps the Ca(2+)-ATPase in a previously undescribed E1 state, with exposure of the Ca(2+) sites through an open cytoplasmic pathway stabilized by Mg(2+). PMID:28890335 VMP1 interacts with SERCA and prevents formation of the SERCA/PLN/SLN inhibitory complex. |
| GO:0051924 regulation of calcium ion transport | ISS GO_REF:0000024 | ACCEPT | Summary: ISS from rabbit SLN. Reason: Correct but general; the more specific negative regulation of calcium ion import into SR is also annotated. Supporting Evidence: PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). |
| GO:0070296 sarcoplasmic reticulum calcium ion transport | ISS GO_REF:0000024 | MODIFY | Summary: ISS from mouse SLN for SR calcium ion transport. Reason: SLN is a regulator of the SERCA pump, not a participant that moves Ca2+. The regulation term describes its role. Proposed replacements: negative regulation of calcium ion import into sarcoplasmic reticulum Supporting Evidence: PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). PMID:12032137 NF-SLN can reduce the apparent Ca(2+) affinity of both SERCA1a (DeltaK(Ca) = -0.22 +/- 0.01 pCa units) and SERCA2a (DeltaK(Ca) = -0.37 +/- 0.04 pCa units). |
| GO:0090281 negative regulation of calcium ion import | ISS PMID:12032137 Sarcolipin inhibits polymerization of phospholamban to induc... | ACCEPT | Summary: ISS from rabbit SLN. Reason: Correct; a less specific parent of negative regulation of calcium ion import into SR. Supporting Evidence: PMID:12032137 NF-SLN can reduce the apparent Ca(2+) affinity of both SERCA1a (DeltaK(Ca) = -0.22 +/- 0.01 pCa units) and SERCA2a (DeltaK(Ca) = -0.37 +/- 0.04 pCa units). |
| GO:0120162 positive regulation of cold-induced thermogenesis | ISS PMID:22961106 Sarcolipin is a newly identified regulator of muscle-based t... | KEEP AS NON CORE | Summary: ISS from mouse: Sln-null mice fail to maintain body temperature in the cold, and Sln re-expression restores muscle thermogenesis. Reason: Strong mouse genetic evidence, with a mechanism (SERCA uncoupling and futile Ca2+ cycling) that follows from the core SERCA-regulatory activity. It is a physiological consequence rather than the molecular role, and human data are lacking, so it is kept as non-core. Supporting Evidence: PMID:22961106 When challenged to acute cold (4 °C), Sln(-/-) mice were not able to maintain their core body temperature (37 °C) and developed hypothermia. PMID:22961106 Here we present data to suggest that Sln can continue to interact with Serca in the presence of Ca(2+), which can promote uncoupling of the Serca pump and cause futile cycling. PMID:26743715 Using genetically engineered mouse models for SLN, we recently showed that SLN is an important regulator of muscle-based thermogenesis and metabolism (14, 15). |
| GO:1901077 regulation of relaxation of muscle | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS from mouse SLN for regulation of muscle relaxation. Reason: Follows from SERCA inhibition (SR Ca2+ re-uptake terminates contraction); a physiological consequence of the core activity. Supporting Evidence: PMID:21640081 Ablation of SLN selectively increases the atrial SERCA pump activity [10], SR Ca2+ load and Ca2+ transients PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). |
| GO:1901881 positive regulation of protein depolymerization | ISS PMID:12032137 Sarcolipin inhibits polymerization of phospholamban to induc... | MARK AS OVER ANNOTATED | Summary: ISS from rabbit SLN: NF-SLN binds PLN and reduces PLN pentamers in HEK-293 cells. Reason: The observation was that SLN reduces PLN pentamer formation in co-expression; it was not shown that SLN actively depolymerises PLN, and the physiological significance in human muscle (where SLN and PLN co-occur mainly in atria) is untested. Regulins also form mixed oligomers, which complicates the interpretation. The term overstates a single overexpression observation. Supporting Evidence: PMID:12032137 NF-SLN binds directly to PLN and that NF-SLN inhibits the formation of PLN pentamers. PMID:36523160 In particular, sarcolipin was able to join oligomers when it was the minority species but did not readily accommodate other micropeptides in the reciprocal experiment when it was expressed in fivefold excess. |
| GO:1901894 regulation of ATPase-coupled calcium transmembrane transporter activity | IDA PMID:11781085 Structure and orientation of sarcolipin in lipid environment... | ACCEPT | Summary: IDA citing the NMR structure paper on SLN. Reason: Correct process for SLN, though the negative child (GO:1901895) is more precise. The cached abstract reports structure, not activity assays, but the curator had the full text and the function is well established. Supporting Evidence: PMID:11781085 Sarcolipin (SLN) is a 31 amino acid integral membrane protein that regulates Ca-ATPase activity in skeletal muscle. PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). |
| GO:1901895 negative regulation of ATPase-coupled calcium transmembrane transporter activity | IDA PMID:28890335 The ER-Localized Transmembrane Protein EPG-3/VMP1 Regulates ... | ACCEPT | Summary: IDA: human SLN forms an inhibitory complex with SERCA2. Reason: Core process: SLN lowers SERCA Ca2+ affinity and Ca2+ transport. Supporting Evidence: PMID:28890335 VMP1 interacts with SERCA and prevents formation of the SERCA/PLN/SLN inhibitory complex. PMID:12032137 NF-SLN can reduce the apparent Ca(2+) affinity of both SERCA1a (DeltaK(Ca) = -0.22 +/- 0.01 pCa units) and SERCA2a (DeltaK(Ca) = -0.37 +/- 0.04 pCa units). PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). |
| GO:1902081 negative regulation of calcium ion import into sarcoplasmic reticulum | ISS PMID:12032137 Sarcolipin inhibits polymerization of phospholamban to induc... | ACCEPT | Summary: ISS from rabbit SLN. Reason: Core process: SLN slows SERCA-mediated Ca2+ uptake into the SR. Supporting Evidence: PMID:12032137 NF-SLN can reduce the apparent Ca(2+) affinity of both SERCA1a (DeltaK(Ca) = -0.22 +/- 0.01 pCa units) and SERCA2a (DeltaK(Ca) = -0.37 +/- 0.04 pCa units). PMID:9575189 Coexpression of native rabbit SLN or NF-SLN with SERCA1 decreased the apparent affinity of SERCA1 for Ca2+ but stimulated maximal Ca2+ uptake rates (Vmax). |
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Download this section (compressed HTML)Q: Should SERCA inhibition by regulins (PLN, SLN, MRLN, ERLN, ALN) be annotated as GO:0042030 ATPase inhibitor activity or GO:0141110 transporter inhibitor activity? SLN in particular inhibits Ca2+ transport more than ATP hydrolysis (uncoupling), which arguably fits the transporter term better; PLN currently carries both.
Q: Does human SLN uncouple SERCA and contribute to non-shivering thermogenesis in human skeletal muscle, as shown in mouse?
Experiment: Co-reconstitute synthetic human SLN with purified human SERCA1a in proteoliposomes and measure ATPase activity and Ca2+ uptake in parallel to obtain the coupling ratio; include the C-terminal RSYQY deletion as a control.
Hypothesis: Human SLN uncouples SERCA1a ATP hydrolysis from Ca2+ transport.
Type: in vitro reconstitution enzyme kinetics
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