The polarization of the motile cell.

Polarization of the motile cell is associated with the formation of a distinct plasma membrane domain, the pseudopod, whose stabilization determines the directionality of cell movement. The rapid movement of cells over a substrate requires that an essential aspect of cell motility must be the supply of the necessary molecular machinery to the site of pseudopodial extension. Renewal of this pseudopodial domain requires the directed delivery to the site of pseudopodial protrusion of proteins which regulate actin cytoskeleton dynamics, cell-substrate adhesion, and localized degradation of the extracellular matrix. Polarized targeting mechanisms include the targeted delivery of beta-actin mRNA to the leading edge and microtubule-based vesicular traffic. The latter may include Golgi-derived vesicles of the biosynthetic pathway as well as clathrin-dependent and clathrin-independent endocytosis and recycling. Coordination of protrusive activities and supply mechanisms is critical for efficient cellular displacement and may implicate small GTPases of the Rho family. While the specific molecular mechanisms underlying pseudopodial protrusion of the motile cell are well-characterized, discussion of these diverse mechanisms in the context of cellular polarization has been limited.