LRMDA is a small leucine-rich-repeat protein that functions in pigment cells as a non-catalytic membrane-trafficking adaptor. At the limiting membrane of melanosomes, it binds active RAB32-family GTPases and the Retriever subcomplex of Commander, coupling Commander to RAB32-positive membranes during melanosome biogenesis. Human melanocytic-cell loss of LRMDA disrupts PMEL processing, stage-I-to-stage-II melanosome maturation, organelle pH, and melanin content. Biallelic human loss causes oculocutaneous albinism type 7, and zebrafish knockdown/rescue supports a conserved role in melanocyte differentiation. The reviewed human record encodes a 198-residue protein, whereas the direct RAB32/RAB38-binding, melanosome-localization, and Commander studies used a 226-residue human form; their relationship and functional equivalence remain unresolved.
Definition: A protein-containing complex composed of active RAB32, the LRMDA adaptor, and the Commander complex that associates with melanosomal membranes and participates in melanosome biogenesis.
Justification: The named RAB32-LRMDA-Commander assembly is biochemically and functionally distinct from SNX17-Commander. A specific term would distinguish the LRMDA-containing pigment-organelle assembly from the constitutive Commander core and from other Commander adaptor complexes.
Parent term: protein-containing complex
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005654 nucleoplasm | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Marked over-annotated because the source nucleoplasmic immunofluorescence is undermined by published evidence that available anti-OCA7 antibodies produced suspect nuclear and random cytoplasmic signal. Reason: The target appearing in its own IBA WITH/FROM is expected and is not evidence of circularity. The problem is instead the biological source: PMID:36334630 reports that immunofluorescence with available anti-OCA7 antibodies was suspect and gave nuclear plus random cytoplasmic signal, prompting the authors to abandon that assay for live imaging. This makes both the nucleoplasmic localization and the stronger `is_active_in` propagation biologically weak. Propagation Review Root cause: SOURCE BAD Failure modes: SOURCE EVIDENCE WEAK Sources checked: PANTHER:PTN000058168 · PANTHER phylogenetic node supporting nucleoplasm inference SOURCE WEAK OR INFERRED This is the phylogenetic node recorded by current QuickGO, but its propagated compartment claim depends on human localization evidence that a later primary study identifies as antibody-artifact-prone. UniProtKB:Q9H2I8 · human LRMDA SOURCE BAD The target self-source is structurally expected in an IBA. It is classified as bad here because PMID:36334630 reports suspect nuclear signal from available anti-OCA7 antibodies, not because it is the target itself. Supporting Evidence: PMID:36334630 Immunofluorescence experiments with available antibodies for endogenous OCA7 were suspect, resulting in nuclear and random cytoplasmic signal. |
| GO:0030318 melanocyte differentiation | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted as a core conserved role. Current QuickGO and the local GOA agree on the ordered IBA support PANTHER:PTN002922344, human LRMDA, and zebrafish lrmda (ZFIN:ZDB-GENE-050522-285). The human target and zebrafish orthologue each also carry direct IMP support from PMID:23395477. Reason: GO:0030318 is current and describes acquisition of melanocyte features. PMID:23395477 reports human loss-of-function alleles causing recessive albinism and shows that zebrafish lrmda knockdown reduces pigmentation and the apparent number of pigmented melanocytes, with rescue by wild-type but not mutant C10orf11. The experimentally supported orthologue and concordant human evidence license the phylogenetic transfer without a species, paralog, or term-scope mismatch. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN002922344 · PANTHER phylogenetic node for melanocyte differentiation SUPPORTS TRANSFER Current QuickGO records this node as the IBA inference structure shared by human and zebrafish LRMDA. UniProtKB:Q9H2I8 · human LRMDA SUPPORTS TRANSFER Human LRMDA independently carries GO:0030318 by IMP from PMID:23395477, and pathogenic human loss-of-function alleles cause OCA7. ZFIN:ZDB-GENE-050522-285 · zebrafish lrmda (UniProtKB:Q501Y0) SUPPORTS TRANSFER The zebrafish orthologue independently carries GO:0030318 by IMP from PMID:23395477; morpholino knockdown reduced pigmentation and pigmented melanocytes and was rescued by wild-type C10orf11. Supporting Evidence: PMID:23395477 Knockdown of the zebrafish ( Danio rerio ) homolog with the use of morpholinos resulted in substantially decreased pigmentation and a reduction of the apparent number of pigmented melanocytes. |
| GO:0005654 nucleoplasm | IDA GO_REF:0000052 | MARK AS OVER ANNOTATED | Summary: Marked over-annotated because later direct work reports that available anti-OCA7 antibodies produced suspect nuclear and random cytoplasmic immunofluorescence. Reason: Current QuickGO and the local GOA faithfully record the HPA IDA row, but PMID:36334630 directly cautions that immunofluorescence with available OCA7 antibodies was suspect and yielded nuclear plus random cytoplasmic signal. The authors therefore used live imaging of tagged OCA7 instead. Because the exact HPA reagent cannot be established here, the row is marked over-annotated rather than removed outright. Supporting Evidence: PMID:36334630 Immunofluorescence experiments with available antibodies for endogenous OCA7 were suspect, resulting in nuclear and random cytoplasmic signal. |
| GO:0030318 melanocyte differentiation | IMP PMID:23395477 Mutations in c10orf11, a melanocyte-differentiation gene, ca... | ACCEPT | Summary: Accepted as the experimentally established core biological-process role of LRMDA. The current term GO:0030318 precisely matches the human genetic and zebrafish loss-of-function/rescue evidence in PMID:23395477. Reason: Human recessive LRMDA/C10orf11 loss-of-function alleles segregate with albinism, and the study localizes the protein to human fetal melanoblasts and melanocytes. In zebrafish, orthologue knockdown substantially reduced pigmentation and apparent pigmented-melanocyte number, and wild-type but not mutant C10orf11 rescued the phenotype. This directly supports involvement in melanocyte differentiation rather than only a downstream pigmentation phenotype. Supporting Evidence: PMID:23395477 Knockdown of the zebrafish ( Danio rerio ) homolog with the use of morpholinos resulted in substantially decreased pigmentation and a reduction of the apparent number of pigmented melanocytes. |
| GO:0030674 protein-macromolecule adaptor activity | IPI PMID:41038817 Identification of a RAB32-LRMDA-Commander membrane trafficki... | NEW | Summary: LRMDA acts as a non-catalytic adaptor linking active RAB32 to the VPS35L-containing Retriever subcomplex and the wider Commander assembly. Reason: The study demonstrates simultaneous association with active RAB32 and Commander, maps separable interaction regions, and functionally distinguishes the RAB32-LRMDA-Commander pathway in melanocytes. GO:0030674 captures the bridging activity more informatively than generic protein binding. All experiments in PMID:41038817 used the longer human A0A087WWI0 proteoform, so the gene-level annotation is retained with an explicit boundary against claiming that the reviewed 198-residue Q9H2I8 proteoform was directly tested. Supporting Evidence: PMID:41038817 Here, we identify leucine rich melanocyte differentiation associated (LRMDA) as a Commander binding protein. We reveal that LRMDA and SNX17 share a common mechanism of Commander association, and that LRMDA simultaneously associates with Commander and active RAB32, establishing distinct RAB32-LRMDA-Commander and SNX17-Commander assemblies. |
| GO:0031267 small GTPase binding | IPI PMID:36334630 OCA7 is a melanosome membrane protein that defines pigmentat... | NEW | Summary: LRMDA binds the melanosomal small GTPases RAB32 and RAB38. Reason: Recruitment through the canonical effector-binding surface of RAB32 and RAB38 directly supports small GTPase binding. The term does not imply that LRMDA has GTPase, exchange-factor, or GTPase-activating activity. The binding and recruitment constructs used the 226-residue form amplified from MNT1 cells, so the gene-level annotation does not assert that reviewed Q9H2I8 was directly tested. Supporting Evidence: PMID:36334630 We demonstrate that OCA7 is recruited through interaction with a canonical effector-binding surface of melanosome proteins Rab32 and Rab38. |
| GO:0032438 melanosome organization | IMP PMID:36334630 OCA7 is a melanosome membrane protein that defines pigmentat... | NEW | Summary: LRMDA is required for normal melanosome maturation and organization. Reason: Knockout in human MNT1 cells altered PMEL processing, melanosome fibrillation and striation, organelle pH, and the transition from stage I to stage II. These perturbation phenotypes directly support melanosome organization rather than a generic pigmentation term alone. Supporting Evidence: PMID:36334630 Using newly generated OCA7-KO MNT1 cells, we show OCA7 regulates overall melanin levels in a melanocyte autonomous manner by controlling melanosome maturation. PMID:41038817 Functional analysis in melanocytes reveals distinct roles for RAB32-LRMDA-Commander and SNX17-Commander in melanosome biogenesis. |
| GO:0033162 melanosome membrane | IDA PMID:36334630 OCA7 is a melanosome membrane protein that defines pigmentat... | NEW | Summary: LRMDA localizes to the melanosome limiting membrane. Reason: Direct pigment-cell localization places LRMDA at the melanosome limiting membrane. Because the reviewed protein lacks a transmembrane segment and is recruited by active RAB32/RAB38, the annotation describes membrane association rather than integral-membrane topology. The imaging construct was the 226-residue form amplified from MNT1 cells, not the reviewed 198-residue Q9H2I8 sequence. Supporting Evidence: PMID:36334630 Here, we discover that OCA7 localizes to the limiting membrane of melanosomes, the specialized pigment cell organelles where melanin is synthesized. |
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Download this section (compressed HTML)Q: Which LRMDA proteoform is expressed in normal human melanocytes, and do the 198- and 226-residue proteins differ in stability, localization, or partner binding?
Suggested experts: Human melanocyte biologists, Proteoform and transcript experts
Q: Which melanosome cargo is sorted by RAB32-LRMDA-Commander to control PMEL processing and organelle pH?
Suggested experts: Melanosome trafficking experts, Commander complex experts
Q: Does LRMDA act directly in melanocyte lineage differentiation, or does defective organelle maturation secondarily reduce the apparent number of pigmented cells?
Suggested experts: Pigment-cell developmental biologists, Human stem-cell model experts
Q: What is the endogenous subunit stoichiometry and membrane assembly cycle of the RAB32-LRMDA-Commander complex?
Suggested experts: Structural cell biologists, Small-GTPase trafficking experts
Experiment: Endogenously express each proteoform in matched LRMDA-null human melanocytes and compare RAB32/Retriever binding, melanosome localization, organelle maturation, pH, and pigmentation rescue.
Hypothesis: The reviewed 198-residue LRMDA protein can recapitulate the RAB32-Commander adaptor activity demonstrated with the 226-residue form.
Type: proteoform-resolved endogenous rescue
Experiment: Couple acute LRMDA degradation to melanosome immunoisolation, quantitative membrane proteomics, cargo-trafficking assays, and candidate-cargo rescue or epistasis.
Hypothesis: RAB32-LRMDA-Commander sorts a discrete melanosome membrane cargo whose loss causes the PMEL-processing and pH phenotypes.
Type: acute perturbation and organelle proteomics
Experiment: Use inducible LRMDA knockout at successive stages of human melanocyte differentiation and quantify lineage markers, survival, organelle ultrastructure, melanin, and rescue by RAB32- or Commander-binding-selective mutants.
Hypothesis: LRMDA's developmental melanocyte phenotype is separable from its role in mature melanosome organization.
Type: stage-resolved human differentiation genetics
Experiment: Combine endogenous multicolor tagging, fluorescence-correlation measurements, native complex mass photometry, and in-situ cryo-electron tomography to determine recruitment order, stoichiometry, and residence time.
Hypothesis: Active RAB32 nucleates a defined LRMDA-Commander assembly on melanosome membranes.
Type: endogenous complex assembly and structural analysis
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The relative expression and functional equivalence of the reviewed 198-residue Q9H2I8 protein and the 226-residue A0A087WWI0 proteoform are unknown.
OPEN BIOLOGY RESIDUAL_SUBGAP
Significance: The 2022 RAB32/RAB38-binding and localization constructs and all 2025 RAB32-Commander interaction and mutagenesis experiments used a 226-residue form. The available papers and reviewed UniProt record do not establish the sequence relationship between that form and Q9H2I8, so construct-specific interface numbering and endogenous proteoform usage cannot be assumed for Q9H2I8.
What would resolve it: Quantify both proteoforms by long-read transcriptomics and targeted proteomics in primary human melanocytes, then compare their melanosome recruitment, RAB32 and Commander binding, and rescue of LRMDA-null cells at endogenous expression levels.
Provenance (the field's own admissions):
Gap: The cargo and trafficking steps through which RAB32-LRMDA-Commander control PMEL processing, melanosome fibrillation, and lumen pH remain unresolved.
OPEN BIOLOGY BP_DARK
Significance: Knockout phenotypes establish organelle-level consequences, but they do not yet identify the membrane cargo whose sorting directly changes PMEL maturation or proton homeostasis.
What would resolve it: Combine acute endogenous LRMDA degradation with organelle-resolved proteomics, trafficking kinetics, and CRISPR rescue to identify Commander-dependent melanosome cargo and distinguish primary transport defects from downstream maturation changes.
Provenance (the field's own admissions):
Gap: How the melanosome-biogenesis mechanism relates to LRMDA's developmental role in melanocyte differentiation is unknown.
OPEN BIOLOGY BP_DARK
Significance: Zebrafish knockdown supports melanocyte differentiation, whereas the mechanistic human-cell studies examine melanosome maturation in differentiated melanoma-derived cells; these may represent linked but distinct developmental and organelle roles.
What would resolve it: Perturb LRMDA at defined stages of human melanocyte differentiation and measure lineage specification, survival, organelle biogenesis, pigmentation, and rescue by interaction-selective alleles.
Provenance (the field's own admissions):
Gap: The endogenous composition, stoichiometry, and lifetime of the RAB32-LRMDA-Commander assembly on human melanosomes are unknown.
OPEN BIOLOGY CC_DARK
Significance: Biochemical and proteomic evidence establishes a distinct assembly, but does not define whether one LRMDA bridges a complete Commander supercomplex to one active RAB32 molecule or how assembly is regulated on native organelles.
What would resolve it: Use endogenous tagging, quantitative native complex isolation, single-particle or in-situ structural analysis, and live-cell residence-time measurements in primary melanocytes.
Provenance (the field's own admissions):
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