UC Berkeley

The high turgor pressure across the plasma membrane of yeasts creates a requirement for substantial forces, produced by polymerization of an actin filament network, for clathrin-mediated endocytosis (CME). Endocytic internalization is impeded in the absence of fimbrin, an actin filament crosslinking protein called Sac6 in budding yeast. In the first chapter of this dissertation, I use live-cell microscopy to gain new insights into the role of actin filament crosslinking proteins in force generation. Quantitative measurement of the numbers of fimbrin and transgelin molecules at sites of CME reveals an interplay between recruitment of these two actin crosslinking proteins. Manipulation of turgor pressure shows that sites with more fimbrin are more effective at internalization under high load. Super-resolved microscopy of actin patches in fixed yeast reveals that sites with more fimbrin also have a higher density of actin filaments, implicating crosslinking proteins in assisting in force generation by the actin network during CME under increased load.In Chapter 2, I use simulations of an experimentally constrained, agent-based mathematical model of CME to recapitulate the result that endocytic networks with more double-bound fimbrin internalize the plasma membrane against elevated turgor pressure more effectively. Networks with large numbers of crosslinks also have more growing actin filament barbed ends at the plasma membrane, where the addition of new actin monomers contributes to force generation and vesicle internalization. These results provide a richer understanding of the crucial role played by actin filament crosslinking proteins during actin network force generation, highlighting the contribution of these proteins to the self-organization and force generation of the actin filament network. The myosin-Is, Myo3 and Myo5 in budding yeast, are also implicated in force generation and assist in assembly of the actin network during CME. The myosin-Is consist of a motor domain, a membrane binding tail homology 1 (TH1) domain, and an Src homology 3 (SH3) domain that works in concert with a central acidic (CA) region to activate the Arp2/3 complex and promote branched actin assembly. In the third chapter of this dissertation, I examine the ability of Myo5 domain mutant proteins to complement each other in diploid budding yeast. Through a growth assay and live cell microscopy, I reveal that the force generating motor domain and NPF activity from the SH3 domain of Myo5 each must be coupled to the membrane binding TH1 domain for successful progression of CME. However, motor and NPF activity are modular and separable, providing interesting insight into the function of the essential myosin-I in actin network assembly and force generation during budding yeast CME.