UC San Diego

Myosin VI is a molecular motor that harnesses the conformational changes caused by ATP hydrolysis to generate motion along the actin filament (F-actin). As with all myosins, the motor domain of myosin VI contains an actin binding site and nucleotide binding site. The lever arm of myosin VI is followed by a coiled coil motif that mediates dimerization, and a cargo binding tail domain. Myosin VI is the only myosin that moves to the minus-end of F-actin. It has been implicated in multiple functions in disparate model systems. The ability of myosin VI to move processively across the actin filament provides a role for myosin VI in cargo transport, including uncoated endocytic vesicle (UCV) transport. Alternatively, kinetic studies have shown that myosin VI can be made to spend more of its actomyosin cycle tightly bound to actin when under increased load conditions, allowing myosin VI to also serve as a tension sensor that tethers membrane and protein elements to the actin cytoskeleton. The regulation of myosin VI at the level of the motor for it to act as a motor or tether is an intriguing possibility. In addition, myosin VI is mainly found in a cytoplasmic pool suggesting that its docking to cargoes is regulated, separately from its interaction with actin. In Chapter One, I show that myosin VI functions to transport uncoated endocytic vesicles (UCV) through the actin meshwork in our model retinal pigmented epithelial cell system, and in doing so demonstrate that the cortical actin meshwork acts as a barrier to uncoated endocytic vesicle trafficking to the pericentriolar region. In Chapter Two, I provide evidence that in vivo, myosin VI is regulated to act either as a transporter or a tension sensing anchor to actin following alteration at threonine 406. Finally in Chapter Three, I delineate a mechanism for myosin VI recruitment to the UCV cargo through its adapter GIPC/synectin. I posit that recruitment occurs via activation of the GIPC/synectin PDZ domain by PDZ motif- containing receptors being trafficked through the UCV