Class VI myosins are perhaps the most unconventional of the unconventional myosin classes. They traffic in the reverse direction (minus end-directed on the actin filament) as compared to all other characterized myosins. Myosin VI also has an unusual and controversial extension of its short lever arm to increase its step size. It rearranges its converter conformation to achieve a large strok and dimerizes by an unknown mechanism. Myosin VI is involved in a number of cellular functions, but is essential for maintenance of the stereocilia of the hair cells and thus mutations in myosin VI can result in deafness. While it can function as a processive myosin motor, transporting cargoes in endocytosis, it also can function as a strain-dependent anchor that is involved in organizing structures such as the Golgi apparatus. Myosin VI is found as a monomer in cells, but functions optimally as a dimer. We have proposed that this is indicative of a novel form of regulation that may be shared by myosins VII and X~ namely, cargo-initiated dimerization.
The aims of the study are: (1) further delineate the lever arm extension of myosin VI, the regions responsible for dimerization, and the mechanism of myosin VI gating~ (2) begin to probe structural aspects of myosins VIIa and X, which appear to be regulated in cells by cargo-initiated dimerization, as is the case for myosin VI~ (3) further characterize myosin VI mutations that result in deafness using both biochemical and optical trap assays~ and (4) use cellular assays (Caco-2 cells) and mutants to define anchoring vs. transport roles of myosin VI.
Mutations in myosin VI lead to deafness, and potentially to cardiomyopathy and deficits in the intestinal epithelium. Many of the design feature of myosin VI are unlike other myosin classes and are not fully understood, with a number of issues being controversial. This study will delineate the structural adaptation underlying myosin VI design and detail the impact of mutations that lead to deafness in humans. Furthermore, it seeks to determine if different mutations result in differential cellular deficits.
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