Membrane bending by actin polymerization

Abstract

Actin polymerization provides driving force to aid several types of processes that involve pulling the plasma membrane into the cell, including phagocytosis, cellular entry of large viruses, and endocytosis. In endocytosis, actin polymerization is especially important under conditions of high membrane tension or high turgor pressure. Recent modeling efforts have shown how actin polymerization can give rise to a distribution of forces around the endocytic site, and explored how these forces affect the shape dynamics; experiments have revealed the structure of the endocytic machinery in increasing detail, and demonstrated key feedback interactions between actin assembly and membrane curvature. Here we provide a perspective on these findings and suggest avenues for future research.

Figure 1

Membrane bending by inhomogeneous actin polymerization. Red denotes extent of polymerization near the membrane (green). A focus of polymerization leads to localized pushing force balanced by a ring of pulling force; a local slowing of polymerization leads to localized pulling force balanced by a ring of pushing force.

Figure 2

Assumed distributions of forces used to calculate effect of actin polymerization on vesicle budding. A) Balancing, distributed vertical forces. B) Single pulling force at middle of invagination. C) Lateral forces.

Figure 3

Simulation results. A) Zero turgor pressure. Small actin force drives transition from “U”-shaped to “Ω”-shaped bud at an intermediate membrane tension. Taken from Ref. (21). B) Force balance during endocytosis in cells with high turgor pressure. The turgor pressure Π pushes up on the membrane, while actin-polymerization forces and forces from the cell wall pull down. Modified from Ref. (18) by inverting. C) Stochastically modeled actin network structure for a Sla2-null cell. There is no membrane bending, consistent with Ref. (25), but actin forms a comet tail. Taken from Ref. (24).

Figure 4

Lateral force generation by Type-I myosin. A) 150 myosins in lipid bilayer on bead. Lower panel is covariance trace of beads used to hold actin filament. Inset is expanded view of first 15 seconds. B) Leftward motion of myosins creates pulling forces on membrane. Modified from Ref. (37).