Title: Force Transmission in a Reconstituted Actomyosin Cortex
Abstract: The cellular cortex is a dense, quasi 2D network of actin filaments coupled to the inner leaflet of the cell membrane and underlies numerous morphological and physical behaviors of cells, including division, migration and adhesion. The mechanical properties of the actin cortex determine how forces generated from myosin II motors are transmitted to facilitate contraction and tension build up at cellular (10-30 um) length scales. We assembled a reconstituted actomyosin cortex which serves as a model system to understand the roles of actin filament length, actin cross-linking proteins and actin-membrane adhesion to elucidate the mechanisms of cortex force transmission. We form a quasi-2D network of actin filaments of variable length near the surface of a supported lipid bilayer and control the actin-actin or actin-lipid interactions by the addition of cross-linking proteins. Upon the addition of myosin II motors, we observe the movement of actin and myosin by timelapse confocal microscopy. In the absence of adhesion to the membrane and actin cross-linking proteins, contractility at 10-100 um length scales is only observed for sufficiently long (10 um) actin filaments. To facilitate contraction of short filaments (∼ 1 um), the addition of an actin cross-linker protein is required. Increasing adhesion to the lipid bilayer reduces the rate of contraction while facilitating tension build up. Our results demonstrate the roles of actin network connectivity and membrane adhesion in modulating the nature of force transmission in a biomimetic model of the actin cortex.