Reassembly of contractile actin cortex

The contractile cortex is a 50-nm–2-μm-thick layer of cytoskeleton under the plasma membrane that is rich in actin fi laments, myosin II, and actin-binding proteins (Bray and White, 1988). Assembly dynamics and contractility of this layer are thought to generate cortical tension, drive cytokinesis, and play a central role in cell locomotion and tissue morphogenesis (Bray and White, 1988; Alberts et al., 2004). Despite its importance, the structural organization, dynamics, membrane connections, and assembly pathway of contractile cortex are not well understood. The classic unit of contractile actomyosin organization is the sarcomere, a structure that is well characterized in muscle cells and present in stress fi bers of nonmuscle cells (Cramer et al., 1997; Alberts et al., 2004). However, typical contractile cortex does not contain well-organized sarcomeres by light or electron microscopy, and how its actin and myosin fi laments are structurally organized is unclear. Understanding the structure and dynamics of the cortex is important because it determines how cells respond to mechanical force or generate force for shape change and movement. Cells are composite materials, with each constituent conferring different mechanical properties. The membrane bilayer enables the maintenance of a specifi c microenvironment, but cannot expand or retain a stable shape when subjected to environmental forces (Hamill and Martinac, 2001). The plasma membrane of red blood cells is stiffened by a submembranous cytoskeleton consisting of a meshwork of spectrin tetramers tethered both to plasma membrane proteins and to short actin fi laments by linking proteins, notably ankyrin and protein 4.1 (Bennett and Baines, 2001). Motile cells contain all of these proteins, but the extent and function of a submembranous cytoskeleton is unclear. They also have a much thicker and stiffer cortex under the plasma membrane, consisting of a shell of cross-linked actin fi laments oriented tangential to the cell surface, which enables cells to better resist mechanical deformation (Bray and White, 1988). This shell can produce force either through myosin II–driven contraction or actin polymerization. Myosin-driven contraction generates cortical tension that can be converted into different types of motility by appropriate symmetry breaking (Bray and White, 1988). Biochemically, the proteinaceous composition of the cortex is dominated by actin, actin-bundling proteins, and myosin II. How cortex is regulated and attached to the plasma membrane is unclear. The small GTPase RhoA is probably the most important regulator of contractile cortex assembly (EtienneManneville and Hall, 2002). Its activation leads to both actin polymerization and myosin II recruitment through several pathways in cytokinesis and chemotaxis (Lee et al., 2004; Bement et al., 2005; Kamijo et al., 2006), but its role in the regulation of generic contractile actin cortex is less well understood. Reassembly of contractile actin cortex in cell blebs