Aip1 and Cofilin Promote Rapid Turnover of Yeast Actin Patches and Cables: A Coordinated Mechanism for Severing and Capping Filaments

Abstract

Rapid turnover of actin structures is required for dynamic remodeling of the cytoskeleton and cell morphogenesis, but the mechanisms driving actin disassembly are poorly defined. Cofilin plays a central role in promoting actin turnover by severing/depolymerizing filaments. Here, we analyze the in vivo function of a ubiquitous actin-interacting protein, Aip1, suggested to work with cofilin. We provide the first demonstration that Aip1 promotes actin turnover in living cells. Further, we reveal an unanticipated role for Aip1 and cofilin in promoting rapid turnover of yeast actin cables, dynamic structures that are decorated and stabilized by tropomyosin. Through systematic mutagenesis of Aip1 surfaces, we identify two well-separated F-actin-binding sites, one of which contributes to actin filament binding and disassembly specifically in the presence of cofilin. We also observe a close correlation between mutations disrupting capping of severed filaments in vitro and reducing rates of actin turnover in vivo. We propose a model for balanced regulation of actin cable turnover, in which Aip1 and cofilin function together to "prune" tropomyosin-decorated cables along their lengths. Consistent with this model, deletion of AIP1 rescues the temperature-sensitive growth and loss of actin cable defects of tpm1⌬ mutants.

ADF/cofilin family proteins control formation of oriented actin-filament bundles in the cell body to trigger fibroblast polarization

“…Here, we uncovered a new role for AC, the formation of oriented actin-filament bundles within the cell body during cell polarization. Our work is consistent with the known requirement for AC in the formation of oriented actin cables in the cell body for polarized growth in yeast (Okada et al, 2006). This might imply that AC functions both in cell protrusions and within the cell body in general in migrating cells, although this remains to be directly tested in a single assay in a single cell type.…”

“…It is becoming clear that the ADF/cofilin (AC) family of proteins, which catalyze actin-filament severing and depolymerization in cells (reviewed by Moon and Drubin, 1995;Sarmiere and Bamburg, 2003;Zigmond, 2004), is necessary for cell migration (reviewed by Bamburg and Wiggan, 2002). AC proteins are also known regulators of cell polarity, both in yeast (Drees et al, 2001;Okada et al, 2006) and also in migrating cell types, both to sustain already established polarity (Dawe et al, 2003) and to trigger cell polarization (Helen R. Dawe. The role of ADF/cofilin family proteins in the acquisition and maintenance of cell polarity during fibroblast migration.…”

Actin Interacting Protein1 and Actin Depolymerizing Factor Drive Rapid Actin Dynamics inPhyscomitrella patens

“…AIP1 nulls are synthetic lethal with viable cofilin mutant alleles placing them in a common genetic pathway (Iida and Yahara, 1999;Rodal et al, 1999). Furthermore, AIP1 nulls aberrantly accumulate cofilin along actin cables (Iida and Yahara, 1999;Rodal et al, 1999), and these cables are less sensitive to turnover by latrunculin A (Okada et al, 2006). Collectively, these studies highlight a functional link between AIP1 and actin, demonstrating that AIP1 has evolved variable roles and requirements in fungi, animals, and slime molds.…”

“…However, in the presence of ADF/cofilin, disassembly occurs at a higher rate than either protein alone (Aizawa et al, 1999;Okada et al, 1999;Rodal et al, 1999). While the mechanism of cooperative disassembly is still unclear, evidence supports a role for AIP1 in capping the barbed end of ADF/cofilin-severed actin filaments (Okada et al, 1999(Okada et al, , 2002(Okada et al, , 2006Balcer et al, 2003;Mohri et al, 2004), enhancing actin filament severing by ADF/cofilins (Aizawa et al, 1999;Mohri et al, 2004), or a combination of these mechanisms. AIP1's activity ultimately results in further fragmentation of ADF/cofilin-severed actin filaments (Aizawa et al, 1999;Okada et al, 1999;Okreglak and Drubin, 2010), showing that AIP1 functions in the turnover of F-actin.…”

Apoptosis and the yeast actin cytoskeleton

“…19 Actin cables also show rapid turnover, and a current model proposes that the cables stabilizing protein tropomyosin, which readily decorates cables and influences turnover by stabilizing filaments, along with the actin regulators cofilin and actin interactin protein 1 (AIP1) cooperate to appropriately regulate cable filament turnover. 20,21 During cytokinesis, bud separation is achieved through the action of an actomyosin contractile ring that is directed by the formation of a septum at the neck. The assembly of a septin ring, comprising Cdc3p, Cdc10p, Cdc11p and Cdc12p, generates a scaffold for actin ring formation.…”