Assembly and turnover of short actin filaments by the formin INF2 and profilin [Cell Biology]

June 29th, 2015 by Gurel, P. S., A, M., Guo, B., Shu, R., Mierke, D. F., Higgs, H. N.

Inverted Formin 2 (INF2) is a formin protein with unique biochemical effects on actin. In addition to the common formin ability to accelerate actin nucleation and elongation, INF2 can also sever filaments and accelerate their depolymerization. While we understand key attributes of INF2-mediated severing, we do not understand the mechanism by which INF2 accelerates depolymerization subsequent to severing. Here, we show that INF2 can create short filaments (< 60 nm) that continuously turnover actin subunits through a combination of barbed end elongation, severing, and WH2 motif-mediated depolymerization. This pseudo-steady state condition occurs whether starting from actin filaments or monomers. The rate-limiting step of the cycle is nucleotide exchange of ADP for ATP on actin monomers after release from the INF2/actin complex. Profilin addition has two effects: 1) to accelerate filament turnover six-fold by accelerating nucleotide exchange; and 2) to shift the equilibrium toward polymerization, resulting in longer filaments. In sum, our findings show that the combination of multiple interactions between INF2 and actin can work in concert to increase ATP turnover rate. Depending on the ratio of INF2:actin, this increased flux can result in rapid filament depolymerization or maintenance of short filaments. We also show that high concentrations of cytochalasin D accelerate ATP turnover by actin, but through a different mechanism from that of INF2.