Actin Based Pulling Forces in Endocytosis

Abstract

Endocytosis in yeast requires actin polymerization to overcome the large turgor pressure opposing invagination. While the generation of pushing forces by actin polymerization is understood, it is not clear how actin polymerization generates the required pulling forces. Previous work using a finite element approach has suggested that spatial variation of the polymerization rate could lead to pulling forces1. We extend this work by performing calculations using discrete actin filaments with all subunits explicitly treated. We simulate a growing array of up to 200 actin filaments in a hexagonal network. Each filament interacts with the membrane via an interaction potential that has both attractive and repulsive components. The inner filaments of the array are bound more strongly to the membrane and thus grow more slowly. Elasticity of the actin network is modeled by linear springs connecting the filaments to each other. We calculate the spatial distribution of the filament-generated forces. The time-averaged force of the outer filaments pushes on the membrane, while the time-averaged force of the inner filaments pulls on the membrane. We calculate the total force of the pulling filaments as a function of several model parameters, including the potential depths, the free filament on-rates, the numbers of fast- and slow-growing filaments, and the network rigidity.[1] A. E. Carlsson and P. V. Bayly, Biophys. J. 106:1596-1606(2014).