Abstract
Srv2/cyclase-associated protein (CAP) is a ubiquitously expressed actin monomer binding protein required for proper organization and rapid remodeling of cellular actin networks. CAP catalyzes the dissociation of cofilin-bound ADP-actin complexes, elevating cofilin levels available for filament disassembly. In addition, CAP and profilin promote exchange of nucleotide (ATP for ADP) on G-actin, then CAP releases profilin-bound ATP-G-actin to replenish the actin monomer pool. These functions are highly conserved, as expression of animal and plant CAPs complement cellular defects of yeast cap mutants. Unlike most actin monomer binding proteins, CAP oligomerizes, likely into hexamers. Within the high molecular weight complex formed, the C-terminal half of each CAP molecule binds one actin monomer. In addition, the N-terminus of CAP binds to cofilin-G-actin complexes and the middle region binds to Abpl and profilin. Abpl tethers CAP to filamentous actin networks. Cofilin and profilin function together with CAP to accelerate actin turnover, through a series of actin monomer handoffs guided by the changing nucleotide state of actin. Thus, the emerging view of CAP function is that it serves as a large molecular hub where multiple actin binding proteins interact to recycle actin monomers and cofilin. This macromolecular complex plays a key role in remodeling the actin cytoskeleton during events such as endocytosis, cell polarity, and cell motility.