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this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.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.
Identity and scope. The target is human MYO5A (myosin Va), using the user-supplied UniProt accession Q9Y4I1 and synonym MYH12. A human-protein structural study explicitly distinguishes myosin Va from the related myosins Vb (MYO5B) and Vc (MYO5C); the literature retrieved did not independently verify the accession or MYH12 synonym. Its experimentally supported motor–neck–tail architecture agrees with the supplied myosin-family and domain annotation. Findings specific to Vb or Vc are not assigned to MYO5A below. Velvarska and Niessing, published 10 December 2013. (velvarska2013structuralinsightsinto pages 1-2, velvarska2013structuralinsightsinto pages 2-3)
MYO5A encodes a cytosolic, actin-based cargo motor, not a melanin-synthesizing enzyme, membrane transporter or kinesin that runs on microtubules. Its N-terminal head binds filamentous actin and catalyzes ATP + H₂O → ADP + inorganic phosphate; ATP turnover drives force-generating changes and movement toward the actin filament’s plus, or barbed, end. ATP is its chemical substrate, whereas actin is its track and melanosomes, vesicles or organelle membranes are its transported cargos. Each of the two heavy chains has six calmodulin/light-chain-binding IQ motifs that form a lever arm, followed by a dimerizing coiled-coil stalk and a C-terminal globular tail that recognizes cargo-associated proteins. These assignments reconcile the supplied ATPase, IQ-motif and cargo-binding domain annotations with experimental class-V-myosin architecture. Zhang and colleagues, 2018; Robinson and colleagues, March 2019. (zhang2018regulationofclass pages 1-2, robinson2019theadaptorprotein pages 1-5)
This dimer is a processive motor: measurements summarized from single-molecule experiments report steps of approximately 36 nm and movement of approximately 250–450 nm/s under the studied in-vitro conditions. These are motor-assay values, not a fixed velocity for every cargo in living human tissues. The globular tail also folds back onto the head to suppress motor ATPase activity when cargo is absent; cargo-adaptor engagement and calcium-dependent conformational regulation can favor an extended, active state. An X-ray structure resolved the human MYO5A globular tail at 2.2 Å, providing direct human structural evidence for the cargo-binding region, though not an atomic structure of the entire working motor. Robinson and colleagues, 2019; Velvarska and Niessing, 2013; Zhang and colleagues, 2018. (robinson2019theadaptorprotein pages 1-5, velvarska2013structuralinsightsinto pages 2-3, zhang2018regulationofclass pages 1-2)
Melanocyte periphery: the best-established physiological cargo. On the cytoplasmic face of a melanosome, membrane-associated RAB27A binds melanophilin (MLPH), which recruits the melanocyte MYO5A splice form through a tail segment encoded by exon F. The motor moves and helps retain pigment-bearing melanosomes within the peripheral actin network and melanocyte dendrites, positioning them for transfer to neighboring keratinocytes. This is a distribution step in pigmentation, not melanin biosynthesis. Single-melanosome photobleaching experiments found that MYO5A and MLPH exchange dynamically on melanosomes while RAB27A is comparatively stable; experimentally forcing persistent cargo linkage dispersed melanosomes but impaired normal dendrite elongation. Thus, controlled attachment is biologically consequential, not merely constitutive tethering. Ménasché and colleagues, August 2003; Robinson and colleagues, March 2019. (menasche2003griscellisyndromerestricted pages 1-2, menasche2003griscellisyndromerestricted pages 5-7, robinson2019theadaptorprotein pages 1-5)
Recycling and secretory membranes: broader, splice-dependent targeting. A screen of human Rab proteins identified distinct MYO5A-binding regions in its stalk, alternatively spliced segment and globular tail. In human cultured cells, RAB11 was important for recruiting an exon-F-containing MYO5A construct to membranes; RAB10 and RAB11 could recruit the exon-D form. MYO5A depletion shifted RAB11- and RAB14-positive endosomes toward the cell center. These experiments support roles in positioning recycling/post-Golgi membranes but do not establish that every observed Rab interaction represents a distinct cargo transported by endogenous MYO5A in every tissue. Lindsay and colleagues, November 2013. (lindsay2013identificationandcharacterization pages 1-2, lindsay2013identificationandcharacterization pages 6-7)
Neuronal dendritic spines: smooth-endoplasmic-reticulum delivery. MYO5A acts at actin-rich Purkinje-cell spines, where it draws smooth endoplasmic reticulum (ER) containing inositol-trisphosphate receptors (IP₃Rs) into spines. This places a calcium-releasing organelle beside synaptic signaling machinery. Motor-mutant and rescue work summarized in a neuronal-myosin review linked motor speed to ER-entry speed, reporting a maximum observed ER-tubule entry speed of approximately 0.45 μm/s. In Myo5a-mutant mice, spine ER and IP₃Rs were depleted, parallel-fiber synaptic long-term depression was lost in juveniles, and cerebellum-dependent motor learning was impaired; neuronal knockdown likewise reduced IP₃R-positive spines. This is strong mouse mechanistic evidence for a plausible contributor to human neurological disease, rather than direct demonstration of organelle movement in a patient’s neurons. Importantly, the established activity-dependent entry of AMPA-receptor recycling endosomes into hippocampal spines is assigned prominently to MYO5B, and should not be substituted for this MYO5A-specific ER result. Hammer and Wagner, October 2013; Miyata and colleagues, April 2011. (hammer2013functionsofclass pages 2-3, miyata2011arolefor pages 1-2)
Neuromuscular-junction postsynapse: receptor recycling and local signaling. A 2024 expert review synthesizes mouse experiments in which MYO5A associates with endocytosed nicotinic acetylcholine receptor (nAChR) carriers near the postsynaptic actin cortex. MYO5A, nAChR, its binding partner rapsyn, and protein kinase A regulatory subunit RIα colocalize in subsynaptic puncta. MYO5A depletion reduced RIα enrichment and receptor stability and dispersed receptor carriers; the proposed mechanism is that MYO5A retains recycling vesicles within a local cAMP–PKA signaling microdomain, while rapsyn anchors RIα. This describes MYO5A as an organelle-positioning component of signaling, not the enzyme that produces cAMP or phosphorylates receptors. The integrated capture-and-recycling sequence remains a model derived mainly from mouse experiments. Rudolf, published 4 January 2024. (rudolf2024myosinvacapturing pages 1-2, rudolf2024myosinvacapturing pages 3-4)
| Molecular process | Intracellular site and precise motor/cargo partners | Direct support and evidence limitations |
|---|---|---|
| Melanosome capture, cortical transport and pigment distribution | Actin-rich melanocyte periphery and dendrites; melanosomal RAB27A–MLPH–MYO5A complex. RAB27A binds MLPH, and MLPH binds the exon-F-containing MYO5A tail. | Human genetics identified a homozygous 2,439-bp deletion spanning MYO5A exon F; this caused isolated hypopigmentation without neurological disease, demonstrating that exon F is required for melanocyte transport but not the principal neuronal function. Live-cell work further showed dynamic MYO5A/MLPH association with melanosomes. Most motility kinetics derive from cultured mammalian melanocytes or reconstituted motors rather than intact human skin (robinson2019theadaptorprotein pages 1-5, menasche2003griscellisyndromerestricted pages 1-2, menasche2003griscellisyndromerestricted pages 5-7). |
| RAB-positive endosome positioning | Human HeLa-cell recycling and post-Golgi membranes; MYO5A associates with RAB10, RAB11A/B and RAB14 through distinct tail regions. | A systematic human-Rab screen and cell perturbations showed that RAB11 is crucial for membrane recruitment of the F splice isoform, whereas RAB10 and RAB11 can recruit the D isoform. MYO5A depletion clustered RAB11- and RAB14-positive endosomes perinuclearly. These are human-cell results, but overexpression, ionomycin and RNAi limit inference about endogenous tissue physiology (lindsay2013identificationandcharacterization pages 1-2, lindsay2013identificationandcharacterization pages 6-7). |
| Smooth-ER entry into neuronal spines and cerebellar plasticity | Purkinje-cell dendritic spines; MYO5A pulls smooth ER bearing IP3 receptors along actin, supporting local Ca²⁺ release and long-term depression. | Motor-mutant rescue linked slower MYO5A movement to slower ER-tubule entry; a reported maximum ER-entry velocity was 0.45 μm/s. Myo5a-mutant mice showed depleted spine ER/IP3 receptors, abolished juvenile LTD and impaired motor learning; RNAi reduced IP3-receptor-positive spines. Strong causal mouse evidence, but not direct imaging in human neurons (hammer2013functionsofclass pages 2-3, miyata2011arolefor pages 1-2). |
| Postsynaptic nAChR recycling and cAMP/PKA compartmentation | Mouse neuromuscular-junction postsynapse; MYO5A captures nAChR–rapsyn–PKA RIα endocytic/recycling vesicles near the actin-rich postsynaptic membrane. | MYO5A colocalized and co-precipitated with endocytosed nAChRs; genetic or acute inhibition dispersed carriers and reduced nAChR stability, NMJ size and PKA-RIα enrichment. Rapsyn is proposed to anchor PKA RIα as an AKAP. The 2024 synthesis integrates older mouse experiments; the complete complex and transport sequence remain a mechanistic model rather than a human in-vivo demonstration (rudolf2024myosinvacapturing pages 3-4, rudolf2024myosinvacapturing pages 1-2). |
| Local actin-track assembly for melanosome dispersion | Mouse melanosome surface; RAB27A–SPIRE1–FMN1 nucleates/elongates local actin, while MLPH or SPIRE1 recruits/activates MYO5A for movement along those tracks. | The 2023 review presents this as a model supported by mouse melanocyte experiments, interaction data and simulations. Actin nucleation is performed by SPIRE1/FMN1—not by MYO5A—and generalization to other cargos remains speculative (welz2023theroleof pages 7-8, welz2023theroleof pages 8-9). |
| Structurally permitted RAB11A–MYO5A–SPIRE2 assembly | MYO5A globular tail: RAB11A binds subdomain 2 and the SPIRE2 GTBM binds the opposing subdomain 1. | Crystal structures of MYO5A–RAB11A and MYO5A–SPIRE2 complexes support a sterically compatible tripartite model. This is direct structural evidence for binding, not proof that the intact tripartite complex operates in vivo. In mouse oocytes the established transport motor is MYO5B, so that physiology must not be reassigned to MYO5A (welz2023theroleof pages 4-5, welz2023theroleof pages 3-4, welz2023theroleof pages 8-9). |
Table: Evidence-ranked summary of MYO5A cargo transport, localization and signaling functions, explicitly separating human evidence from mouse models and structural inference. MYO5B- and MYO5C-specific findings are excluded or flagged to prevent paralog misannotation.
A 2023 Journal of Cell Science review integrates a more elaborate melanosome-transport mechanism: melanosome-associated RAB27A recruits SPIRE1, which cooperates with the formin FMN1 to assemble actin tracks; MLPH links RAB27A to MYO5A, which moves on those tracks. The review’s melanocyte-specific model is illustrated in its cropped Figure 3C. SPIRE/FMN proteins nucleate and elongate actin; MYO5A supplies motor force. Structural studies also support binding of RAB11A and SPIRE2 to different surfaces of a MYO5A globular tail, making a three-part assembly structurally possible. That structural compatibility does not prove that this particular complex operates as a unit in living human cells: the prominently described RAB11A-vesicle transport in mouse oocytes instead involves MYO5B. These distinctions are essential to isoform-specific annotation. Welz and Kerkhoff, March 2023. (welz2023theroleof pages 7-8, welz2023theroleof media fdb9a5ca, welz2023theroleof pages 3-4, welz2023theroleof pages 8-9)
The broader 2024 research context places class-V myosins, Rab proteins, SPIRE nucleators and formins within an evolutionarily conserved organelle-transport system, but conservation of the module is not itself proof that any specified human MYO5A cargo uses the same molecular assembly. Kollmar and colleagues, July 2024; Welz and Kerkhoff, 2023. (karreis2025diversityofmammalianc pages 31-33, welz2023theroleof pages 8-9)
The most compelling human functional validation is Griscelli syndrome type 1 (GS1): damaging MYO5A variants associate pigment dilution and characteristic silvery hair with primary neurological impairment. Genotype matters. In one patient, a homozygous 2,439-base-pair genomic deletion spanning MYO5A exon F produced hypopigmentation without neurological manifestations. This natural human experiment supports a precise functional division: exon-F-containing MYO5A is required for MLPH-dependent melanosome movement, while an exon-F-negative isoform can support essential neuronal functions. Ménasché and colleagues, August 2003. (menasche2003griscellisyndromerestricted pages 1-2, menasche2003griscellisyndromerestricted pages 5-7)
This distinction has a direct diagnostic application: pigment dilution with neurological findings points toward MYO5A-related GS1, whereas the major immune-deficient/hemophagocytic form, GS2, is caused by RAB27A, and pigmentation-only GS3 is classically caused by MLPH. An exon-F-specific MYO5A lesion can also yield pigmentation-only disease; phenotype alone therefore does not establish the causal gene, and molecular testing must distinguish these pathways. A 2024 clinical review reiterates their differing clinical implications. No prevalence or treatment-effect percentage from an RAB27A/GS2 cohort should be reported as an MYO5A/GS1 statistic. Ménasché and colleagues, 2003; Mazzetto and colleagues, June 2024. (menasche2003griscellisyndromerestricted pages 1-2, menasche2003griscellisyndromerestricted pages 5-7, mazzetto2024skinhypopigmentationin pages 13-13)
Overall assessment. The defensible primary annotation is ATP-powered, plus-end-directed movement and positioning of selected intracellular cargo along F-actin, with the RAB27A–MLPH–MYO5A melanosome pathway supported by human genetics and direct cell biology. ER delivery into neuronal spines and postsynaptic receptor-vesicle retention explain additional tissue-specific functions, but their detailed causal mechanisms depend substantially on mouse experiments. The 2023–2024 literature refines how local actin assembly and cAMP microdomains may cooperate with this motor; it does not justify reassigning MYO5B- or MYO5C-specific activities to MYO5A. (robinson2019theadaptorprotein pages 1-5, menasche2003griscellisyndromerestricted pages 5-7, miyata2011arolefor pages 1-2, rudolf2024myosinvacapturing pages 1-2, welz2023theroleof pages 8-9)
References
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