Members of the Molecular Genetics of Development Section are using classical mouse coat-color mutations [dilute (d), ashen (ash), leaden (ln)], as well as reverse genetics, to molecularly dissect the biochemical pathway(s) important for pigment granule transport. The d, ash, and ln mutations cause a lightening of coat color due to defects in pigment granule transport, while d also causes a fatal neurological disease due to endoplasmic reticulum (ER) transport defects in cerebellar Purkinje cells. Previously, we showed that these three mutations are suppressed by another mutation [dilute suppressor (dsu)] and provided evidence suggesting that all three mutations function in a common biochemical pathway. We also showed that dsu can suppress the eye color but not the diluted coat color associated with two other mutations [ruby-eye (ru) and ruby-eye-2 (ru2)]. In addition, we showed that d encodes unconventional myosin VA (MyoVA), a major cellular vesicle transport motor, while others showed that human MYOVA mutations produce Griscelli disease, a rare autosomal recessive disorder characterized by pigment dilution, variable cellular immunodeficiency, neurological disorders, and acute phases of uncontrolled lymphocyte and macrophage activation. In more recent studies, we used RT-PCR-based sequencing to identify the mutations responsible for 17 viable dilute alleles and yeast two-hybrid assays to identify proteins that interact with MyoVA. These studies identified important functional domains of the protein and provided support for the notion that the different MyoVA isoforms produced by alternative splicing encode important cell-type-specific functions. These studies also showed that MyoVA can bind a major cellular microtubule vesicle transport motor, ubiquitous kines in heavy chain (KhcU), and suggested for the first time that vesicle transport can be coordinated in the cell via the direct interaction of the different motor molecules. Future studies are aimed at using positional cloning to identify the gene products encoded by the other coat-color mutations and then to determine how they function in the cell.
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