In this section we explore a most conspicuous and well-studied function of the actin cytoskeleton: its ability to serve as the engine for cell crawling. By driving the expansion of the plasma membrane in the direction of cell advancement, actin polymerization initiates the crawling cycle (Fig. 9-8). The networks formed by polymerization evolve to structures that provide mechanical support for cell extensions; that link the cell to its substrate; and that support the myosin-based contractions needed for cell translation. The network must also disassemble to recycle its constituents for further rounds of assembly. Thus the actin network at the leading edge of motile cells provides both the structure and the forces needed for crawling (see Fig. 9-9). Here we review the current understanding of the geometry and dynamics of these networks, and address the important question of how polymerization might lead to pushing forces.

Fig. 9-8: The four steps in cell migration. The classic schematic of crawling breaks the process down into a four-step cycle. The cycle begins with the protrusion of the leading edge driven by actin polymerization. The extended cell forms new attachments in advance of its body and then contracts against this attachment to break tail adhesions and translate forward. From Mitchison and Cramer, 1996.

Fig. 9-9: Actin dynamics in crawling cells. (A) For steady crawling, actin polymerization and depolymerization must complete a balanced cycle. However, the demand for assembling monomers at the leading edge causes a spatial segregation of these processes...