Roles of ADF/cofilin in actin polymerization and beyond

Abstract

In collaboration or competition with many other actin-binding proteins, the actin-depolymerizing factor/cofilins integrate transmembrane signals to coordinate the spatial and temporal organization of actin filament assembly/disassembly (dynamics). In addition, newly discovered effects of these proteins in lipid metabolism, gene regulation, and apoptosis suggest that their roles go well beyond regulating the cytoskeleton.

Figure 1.. Concentration-dependent effects of cofilin on actin dynamics

(a) Cofilin (purple) binds preferentially to ADP-actin (orange) and, at low stoichiometry with respect to actin subunits, severs filaments, creating new barbed and pointed ends. The cofilin dissociates with an actin subunit in the ADP form, and nucleotide exchange, enhanced by Srv2/CAP1 (exchange factor for actin-bound nucleotide when complexed to cofilin) and/or profilin (green), occurs on the actin. Cofilin can recycle to sever again. The pieces of filamentous actin (F-actin) generated can nucleate filament growth or can enhance depolymerization if assembly-competent ATP-actin is limiting. (b) At higher stoichiometry, cofilin binds to ADP-actin, but since binding is cooperative, regions of the F-actin become saturated and stabilized in the ‘twisted form’. Severing occurs rapidly, but as the cofilin is sequestered on the pieces of actin, severing is not persistent. Fragments are further depolymerized in the presence of actin-interacting protein 1 (Aip1) (blue) to generate monomer or can be used to nucleate growth. In cells under stress where ADP-actin levels are elevated, the cofilin-saturated F-actin assembles into rod-shaped bundles.

Figure 2.. Possible roles of cofilin and phospho-cofilin in the establishment of the leading edge

In response to signaling through a receptor tyrosine kinase (RTK), phospholipase C gamma (PLCγ) is activated and hydrolyzes phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P₂), releasing active cofilin from its inhibitory binding, allowing severing of capped quiescent filaments, and generating free barbed ends for driving assembly. Actin-related protein 2 and 3 (Arp2/3) complex is also activated via Wiskott-Aldrich syndrome protein (WASP) to set up the branched filament network driving forward protrusion of the membrane. The RTK also recruits phosphatidylinositol 3-kinase (PIK), generating PtdIns3,4,5P₃, which serves as a docking site for the binding of dedicator of cytokinesis 2 (DOCK2). The Rac1 guanine nucleotide exchange activity of DOCK2 is exposed only upon binding of the tail of DOCK2 to phosphatidic acid (PA). PA is generated from the hydrolysis of other phospholipids, such as phosphatidylcholine (PC), by the enzyme phospholipase D1 (PLD1), which is activated by P-cofilin. Active Rac1 activates the p21-activated kinase (PAK1), which activates the cofilin phosphorylation through LIM kinase (LIMK). This feed-forward cycle maintains active Rac1 at the leading edge but becomes self-limiting when cofilin phosphatases also are recruited or become active through downstream signals from these (e.g., inositol triphosphate [IP₃] → calcium → calmodulin → calcineurin → slingshot phosphatase) and/or other pathways. DAG, diacylglycerol; GEF, guanine nucleotide exchange factor; NCK, adaptor molecule with src homology domains 2 and 3; PIP₂, phosphatidyinositol diphosphate; PIP₃, phosphatidyinositol triphosphate.