Abstract
The plasma membrane and the underlying cytoskeletal cortex constitute active platforms for many cellular processes. Recent work has shown that acto-myosin dynamics modify the local membrane organization, but the molecular details are not well understood due to difficulties with experimentally accessing the relevant time and length scales. Here, we use interferometric scattering (iSCAT) microscopy to investigate a minimal acto-myosin network linked to a supported lipid bilayer membrane. Using the magnitude of the interferometric contrast, which is proportional to molecular mass, we detect, image and distinguish actin and myosin filaments. As a result, we can follow single, membrane attached actin filaments diffusing within the acto-myosin network, revealing differing types of motion depending on filament length. We go on to quantify myosin II filament dwell times and processivity as a function of ATP concentration, providing evidence for the predicted ensemble behavior of myosin head domains. Simultaneous observation of long-term network flow and organization enables us to link changes in myosin II filament dynamics with decreasing ATP concentrations to a switch in the acto-myosin network from a remodeling, fluid state to contractile behavior, and to observe the formation of vortices so far only predicted by theory.
