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Rab27a is an essential component of melanosome receptor for myosin Va
Xufeng Wu1, Fei Wang, Kang Rao
1Laboratories of Cell Biology and Molecular Cardiology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Abstract:
Melanocytes that lack the GTPase Rab27a (ashen) are disabled in myosin Va-dependent melanosome capture because the association of the myosin with the melanosome surface depends on the presence of this resident melanosomal membrane protein. One interpretation of these observations is that Rab27a functions wholly or in part as the melanosome receptor for myosin Va (Myo5a). Herein, we show that the ability of the myosin Va tail domain to localize to the melanosome and generate a myosin Va null (dilute) phenotype in wild-type melanocytes is absolutely dependent on the presence of exon F, one of two alternatively spliced exons present in the tail of the melanocyte-spliced isoform of myosin Va but not the brain-spliced isoform. Exon D, the other melanocyte-specific tail exon, is not required. Similarly, the ability of full-length myosin Va to colocalize with melanosomes and to rescue their distribution in dilute melanocytes requires exon F but not exon D. These results predict that an interaction between myosin Va and Rab27a should be exon F dependent. Consistent with this, Rab27a present in detergent lysates of melanocytes binds to beads coated with purified, full-length melanocyte myosin Va and melanocyte myosin Va lacking exon D, but not to beads coated with melanocyte myosin Va lacking exon F or brain myosin Va. Moreover, the preparation of melanocyte lysates in the presence of GDP rather than guanosine-5'-O-(3-thio)triphosphate reduces the amount of Rab27a bound to melanocyte myosin Va-coated beads by approximately fourfold. Finally, pure Rab27a does not bind to myosin Va-coated beads, suggesting that these two proteins interact indirectly. Together, these results argue that Rab27a is an essential component of a protein complex that serves as the melanosome receptor for myosin Va, suggest that this complex contains at least one additional protein capable of bridging the indirect interaction between Rab27a and myosin Va, and imply that the recruitment of myosin Va to the melanosome surface in vivo should be regulated by factors controlling the nucleotide state of Rab27a.
Insights
Melanosome capture by myosin Va requires the GTPase Rab27a, which acts as part of a receptor complex. This interaction is mediated by exon F of myosin Va and regulated by Rab27a
Area of Science:
- Cell Biology
- Molecular Biology
- Melanogenesis Research
Background:
- Melanosome transport in melanocytes is crucial for pigmentation.
- Myosin Va (Myo5a) is implicated in melanosome capture and movement.
- The GTPase Rab27a (ashen) is essential for melanosome capture, but its precise role remains unclear.
Purpose of the Study:
- To elucidate the molecular mechanism by which Rab27a mediates myosin Va binding to melanosomes.
- To determine the specific structural requirements of myosin Va for melanosome interaction.
- To investigate the functional relationship between Rab27a and myosin Va in melanosome transport.
Main Methods:
- Analysis of alternatively spliced exons (Exon F and Exon D) in melanocyte-specific myosin Va isoforms.
- In vitro binding assays using purified myosin Va variants and Rab27a.
- Phenotypic rescue experiments in myosin Va-null (dilute) melanocytes.
Main Results:
- Melanocyte-specific myosin Va requires Exon F, but not Exon D, for melanosome localization and function.
- Rab27a binds to myosin Va in a manner dependent on Exon F.
- Rab27a binding to myosin Va is reduced in the presence of GDP, suggesting nucleotide-dependent regulation.
Conclusions:
- Rab27a acts as an essential component of a melanosome receptor complex for myosin Va.
- Myosin Va interacts indirectly with Rab27a, likely through an intermediary protein.
- The recruitment of myosin Va to melanosomes is regulated by the nucleotide-bound state of Rab27a.