SUMMARYMicrotubules act as "railways" for motor-driven intracellular transport, interact with accessory proteins to assemble into larger structures such as the mitotic spindle, and provide an organizational framework to the rest of the cell. Key to these functions is the fact that microtubules are "dynamic." As with actin, the polymer dynamics are driven by nucleotide hydrolysis and influenced by a host of specialized regulatory proteins, including microtubule-associated proteins. However, microtubule turnover involves a surprising behavior-termed dynamic instability-in which individual polymers switch stochastically between growth and depolymerization. Dynamic instability allows microtubules to explore intracellular space and remodel in response to intracellular and extracellular cues. Here, we review how such instability is central to the assembly of many microtubule-based structures and to the robust functioning of the microtubule cytoskeleton.

Srv2/cyclase-associated protein is expressed in virtually all plant, animal, and fungal organisms and has a conserved role in promoting actin depolymerizing factor/cofilin-mediated actin turnover. This is achieved by the abilities of Srv2 to recycle cofilin from ADP-actin monomers and to promote nucleotide exchange (ATP for ADP) on actin monomers. Despite this important and universal role in facilitating actin turnover, the mechanism underlying Srv2 function has remained elusive. Previous studies have demonstrated a critical functional role for the G-actin-binding C-terminal half of Srv2. Here we describe an equally important role in vivo for the N-terminal half of Srv2 in driving actin turnover. We pinpoint this activity to a conserved patch of surface residues on the N-terminal dimeric helical folded domain of Srv2, and we show that this functional site interacts with cofilin-actin complexes. Furthermore, we show that this site is essential for Srv2 acceleration of cofilin-mediated actin turnover in vitro. A cognate Srv2-binding site is identified on a conserved surface of cofilin, suggesting that this function likely extends to other organisms. In addition, our analyses reveal that higher order oligomerization of Srv2 depends on its N-terminal predicted coiled coil domain and that oligomerization optimizes Srv2 function in vitro and in vivo. Based on these data, we present a revised model for the mechanism by which Srv2 promotes actin turnover, in which coordinated activities of its N-and C-terminal halves catalyze sequential steps in recycling cofilin and actin monomers.Remodeling of cell shape during cell motility, cell division, and cell morphogenesis requires not only the rapid assembly of new actin filaments but also the coordinated disassembly of older filaments. Dynamic turnover provides cells with the plasticity necessary to remodel actin networks rapidly in response to cues, and replenishes the pool of assembly competent ATP-bound actin monomers available for new growth. Although major advancements have been made in determining the mechanisms that promote actin assembly (1, 2), comparatively little is known about the mechanisms governing actin disassembly and turnover. The rate-limiting step in filament disassembly is dissociation of subunits from filament ends (3). actin depolymerizing factor/cofilin (referred to herein as cofilin) accelerates this step by severing and depolymerizing older (ADP-rich) actin filaments (4 -7). Cofilin remains bound to dissociated ADP-actin monomers and strongly inhibits nucleotide exchange (ATP for ADP) on monomeric actin (8). This leads to an accumulation of cofilin-bound ADP-actin monomers and depletes available cofilin and ATP-actin monomers. For this reason, cells require a mechanism for rapidly displacing cofilin from ADP-G-actin to maintain rapid actin turnover.Recent studies suggest that this function is performed by Srv2/cyclase-associated protein (CAP), 2 which here we refer to as Srv2 (9, 10). Srv2 is expressed ubiquitously in all animal, plant, and fungal or...

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Bil1 binds to the regulatory sequence of Bud6, unmasking its ability to stimulate the activity of the formin Bni1 and promote actin nucleation.

We show that polymers displaying dynamic instability (DI) have at least two experimentally distinguishable critical concentrations (CCs), typical DI occurs between these two CCs, and the separation between the CCs depends on the NTP hydrolysis rate. We demonstrate how these CCs relate to various existing experimental and theoretical definitions of CC.