The role of actin and myosin II in the cell cortex of adhered and suspended cells

Adhesion induces dramatic morphological and mechanical changes to cells, which are reflected by changes to the actin cortex. Among the many different proteins involved in this sub-membranous layer, motor proteins (e.g., nonmuscle myosin II [NMII]) and actin nucleators (e.g., Arp2/3, formins) are known to have significant influences on its dynamics and structure. The different roles of NMII, Arp2/3, and formins in the dynamics, structure, and mechanics of the actin cortex depend on the adhesion state of the cell. In this study, we unravel the interplay between the dynamics, structure, and mechanics of the actin cortex in adhered cells and in cells in suspension. We show that treatments with extrinsic cellular perturbants lead to alterations of all three properties that are correlated. However, intrinsic actin cortex variations between different cell adhesion states lead to unexpected correlations. Surprisingly, we find that NMII minifilaments have a minor influence on the actin cortex. Using new microscopy techniques, we show that NMII minifilaments are not localized within the actin cortex, as previously thought, but concentrated in a layer beneath it. Our treatments affecting Arp2/3 and formin reveal correlations between the actin cortex characteristics. Our data build towards a comprehensive understanding of the actin cortex. This understanding allows the prediction and control of cortical changes, which is essential for the study of general cellular processes, such as cell migration, metastasis, and differentiation.