Actin turnover dynamics in cells

Actin filaments turn over rapidly in cells, exchanging subunits rapidly with a pool of unpolymerized actin monomer in cytoplasm. Rapid non-equilibrium turnover of actin filaments enables cells to remodel their shape and internal organization in response to their environments, and also generates forces that enable cells to undergo continuous directed movement. Despite over three decades of investigation, the mechanisms underlying actin filament turnover in cells are still not well understood. My dissertation seeks to understand how actin filaments turn over in cells. To elucidate the kinetic pathway of actin turnover, I imaged actin filaments both in vitro and in live cells, and also studied simple dynamical models of filament turnover. Imaging of single actin filaments in vitro revealed a pathway where filaments disassemble in bursts that involve concurrent destabilization of filament segments hundreds of subunits in length. Bursts of disassembly initiate preferentially, but not exclusively, from filament ends. Quantitative imaging of actin turnover in cells, together with dynamical models, disfavor turnover pathways driven by filament severing, and instead favor pathways involving either (1) slow filament shrinkage from ends, or (2) rapid filament destabilization following a slow catastrophic transition. The latter pathway may correspond to that observed in vitro in the regime where a burst leads to destabilization of an entire filament. Taking these studies together, I propose a new mechanism of actin turnover, where filaments exist in a long-lived stable state before disassembling rapidly through cooperative separation of the two filament strands. I also report here that pure actin filaments become more stable as they age. This phenomenon runs contrary to the classical prediction that dynamic cytoskeletal polymers become less stable with age, as a result of hydrolysis of polymer-bound nucleotide triphosphate. I propose that dynamic filament stabilization arises from structural arrangements after polymerization, and speculate that it may help cells maintain actin cytoskeletal assemblies with vastly different stabilities.