Key Concepts of Section 17.7

• Cell migration involves the extension of an actin-rich leading edge at the front of the cell, the formation of focal adhesions that move backward with respect to the cell, and their subsequent release, combined with rear contraction to push the cell forward (see Figure 17-39).

• Cell migration also involves a directed endocytic cycle, taking membrane and adhesion molecules from the rear of the cell and inserting them at the front.

• The assembly and function of actin filaments is controlled by signaling pathways through small GTP-binding proteins of the Rho family. Cdc42 regulates overall polarity and the formation of filopodia, Rac regulates actin network formation through the Arp2/3 complex, and Rho regulates actin filament formation by formins as well as contraction through regulation of myosin II (see Figure 17-43).

• Chemotaxis, the directed movement of cells toward extracellular chemical cues, involves signaling pathways that contribute to the regulation of the actin cytoskeleton and direction of cell migration.