Date of Completion

10-24-2013

Embargo Period

10-24-2013

Keywords

Myosin Vb, Single molecule imaging, Processivity, Filopodia, Barbed-end binding, sptPALM, Capping protein, VASP

Major Advisor

Dr. Ji Yu

Associate Advisor

Dr. Vladimir Rodionov

Associate Advisor

Dr. John H. Carson

Associate Advisor

Dr. Srdjan D. Antic

Field of Study

Biomedical Science

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

The myosin superfamily is a large and diverse family of proteins that are capable of binding to actin filaments and generating mechanical forces by hydrolyzing ATP. Myosins perform a diverse set of functions in cell, including endocytosis, exocytosis, movement of pigment granules or mRNA, and cell motility. More specifically, Class-V myosins are a group of barbed-end directed motors that have been implicated in organelle transport. For example, in vitro motility assays suggest that myosin Va is a processive motor with a step size of 36 nm and moves at the speed of 0.6 µm/sec, which is consistent with a functional role in actin-based transport process. Although the properties of myosin V in vitro are relatively well-studied, much less is understood about the motility and protein-protein interaction of myosin V in vivo.

Previous studies of molecular motor proteins in vivo have analyzed intracellular cargoes, which are decorated with multiple types of motor proteins, making it difficult to deduce the exact dynamics and the function of a specific motor protein. In this work, we carried out experiments to analyze the localization and the dynamics of myosin Vb in live mammalian cells at both the ensemble level and the level of individual myosin Vb molecules. We found that: 1) motor molecules localize and accumulate at regions enriched in F-actin barbed-ends; 2) individual motor molecules have a high stalling rate when bound to F-actin and do not undergo continuous long-distance movement in cells and 3) over-expression of myosin Vb motor domains promotes filopodia growth. Based on these observations, we propose a novel cellular function of myosin Vb proteins: myosin Vb facilitates linear F-actin growth by moving towards, and eventually binding to, the barbed-ends of individual actin filaments, which in turn prevents capping of the F-actin. Therefore, our study provides new insights into the mechanism of myosin’s function in filopodia formation.

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