Mechanism of Generating Pulling Force via Actin Polymerization

Actin polymerization is the primary mechanism for overcoming the large turgor pressure that opposes endocytosis in yeast. Actin-based pulling forces are less well understood than pushing forces. We stochastically simulate a system of 144 semiflexible actin filaments in a square network, treating all subunits explicitly. Each filament interacts with the membrane via a potential having both attractive and repulsive components. The protein Sla2, which binds actin filaments to the membrane, is assumed to slow the growth of the filaments near the array center by having a strongly attractive potential. The (de)polymerization rates depend on the filament-membrane gap. We include both actin network elasticity and filament-tip bending. We find that that the outer filaments push on the membrane, while the inner filaments pull on it. We calculate the force distribution for various model parameters, including the potential depths, the free filament on- and off-rates, the numbers of fast- and slow-growing filaments, and the network rigidity. Under the most favorable conditions, the total pulling force is the sum of the stall forces of all the pushing filaments.