Bending actin filaments: twists of fate

Abstract

In many cellular contexts, intracellular actomyosin networks must generate directional forces to carry out cellular tasks such as migration and endocytosis, which play important roles during normal developmental processes. A number of different actin binding proteins have been identified that form linear or branched actin, and that regulate these filaments through activities such as bundling, crosslinking, and depolymerization to create a wide variety of functional actin assemblies. The helical nature of actin filaments allows them to better accommodate tensile stresses by untwisting, as well as to bend to great curvatures without breaking. Interestingly, this latter property, the bending of actin filaments, is emerging as an exciting new feature for determining dynamic actin configurations and functions. Indeed, recent studies using in vitro assays have found that proteins including IQGAP, Cofilin, Septins, Anillin, α-Actinin, Fascin, and Myosins—alone or in combination—can influence the bending or curvature of actin filaments. This bending increases the number and types of dynamic assemblies that can be generated, as well as the spectrum of their functions. Intriguingly, in some cases, actin bending creates directionality within a cell, resulting in a chiral cell shape. This actin-dependent cell chirality is highly conserved in vertebrates and invertebrates and is essential for cell migration and breaking L-R symmetry of tissues/organs. Here, we review how different types of actin binding protein can bend actin filaments, induce curved filament geometries, and how they impact on cellular functions.

The Septin Cytoskeleton is Required for Plasma Membrane Repair

“…In LLO-injured cells, the formation of F-actin knobs and loops associated myosin II was significantly decreased in SEPT7-deficient cells compared to cells treated with control siRNA (Figure 7A, B, SF6). This last result shows that the septins control the remodeling of cortical F-actin in knob and loop structures, in accordance with the known role of the septins in bending actin filaments [27]. Importantly, the F-actin loops still observed in SEPT7-siRNA-treated cells colocalized with low levels of septins, likely in cells that were less effectively silenced (Figure 7A, B).…”

“…The septins are known to organize and bend the actomyosin cytoskeleton during cytokinesis [27, 49, 50] and ANXA2 is required for cytokinesis [51]. Therefore, septin-dependent reorganization of the cortical F-actin network and ANXA2 during cytokinesis and during plasma membrane repair likely share common mechanisms.…”

“…Human septins include 13 members which assemble into hetero-hexamers (SEPT-2,-6,-7,-7,-6,-2) and hetero-octamers (SEPT-2,-6,-7,-9,- 9,-7,-6,-2) that further organize into filaments or rings in association with F-actin, microtubules or the plasma membrane [25, 26]. They form filaments and rings that determine the shape, curvature, and properties of the plasma membrane [25, 27]. The septins control major cellular functions including cytokinesis, cell motility, tissue morphogenesis, and host-pathogen interactions [28, 29].…”

Two Septin Complexes Mediate Actin Dynamics During Cell Wound Repair

“…Interestingly, Sep4 and Sep5 knockdown/mutant exhibit delayed actin ring remodeling implicating their roles in actin disassembly. The Sep4-Sep5-Pnut complex overlapping with the actin ring might give more flexibility to the ring by bending filaments to allow more efficient binding of disassembly factors to enhance disassembly (i.e., cofilin severs bent actin filament regions more efficiently) 77,78 .…”