Mechanisms for rapid assembly and disassembly of the actin cytoskeleton in Saccharomyces cerevisiae
The actin cytoskeleton is conserved across eukaryotic species and is required for the generation of force for a wide variety of cellular processes. Rapid actin filament assembly and turnover produces the force needed for these processes. Because of the ubiquitous nature of actin dependent processes, a central problem in cell biology is understanding how different actin-associated proteins regulate these processes. I have studied proteins involved in actin dynamics using the budding yeast S. cerevisiae because it is highly amenable to both genetic and biochemical approaches, and its actin structures are readily distinguished. This thesis describes the characterization of the genetic, cellular, and biochemical functions of three actin-binding proteins: coronin, ARPC1/Arc40, and Srv2/CAP. Coronin was previously described as an actin cross-linking protein with a weak actin-assembly promoting activity. I show that coronin directly binds to the actin-nucleating Arp2/3 complex, recruits the complex to the sides of pre-existing actin filaments, and in the absence of actin filaments inhibits Arp2/3 complex nucleation activity. ARPC1/Arc40 is one of seven conserved subunits of the Arp2/3 complex. I define four essential and distinct surfaces on ARPC1/Arc40 through mutagenesis. Mutations of residues that establish contacts between ARPC1/Arc40 and two other subunits of the Arp2/3 complex specifically alter the actin nucleation activity of the Arp2/3 complex. Further, I show that ARPC1/Arc40 directly binds both actin monomers and WASp, an Arp2/3 complex nucleation-promoting factor. Srv2/CAP has previously shown to be an actin monomer binding protein, and deletion of the SRV2 gene causes disorganization of the actin cytoskeleton and cell morphology defects. I show that native Srv2/CAP exists in a high molecular weight complex of approximately 600 kDa, and that this complex promotes dissociation of cofilin from ADP-actin monomers to increase the rate of actin filament disassembly. This role for Srv2/CAP in actin filament turnover is supported by both genetic and cellular evidence. Finally, I demonstrate that cofilin, together with Aip1, a co-factor of cofilin, form a barbed end cap on severed actin filaments to promote net actin depolymerization.