Differential activity of nonmuscle myosin IIA and IIB isoforms generates a dynamic actomyosin network in a concentration-dependent manner

Cell-scale actin remodeling requires rapid actin depolymerization beyond that generated by cofilin and gelsolin. Previous reports had indicated that the activity of myosin restricted the length of actin bundles. However, it was unknown whether the ubiquitous nonmuscle myosin II isoforms (NMIIA and IIB) could generate cell-scale actin dynamics. Using linear actomyosin network simulation, we observed higher network tension and faster network rupture with NMIIA than NMIIB. Live-cell imaging of the actin network in COS7 cells also showed a similar result with numerous network severing events recorded in the presence of NMIIA while NMIIB produced fewer bundle severing events. Moreover, NMIIA was required for the formation of peripheral actin arcs and long actin fibers that were absent in cells-expressing NMIIB. We also observed the peripheral localization of cofilin in the presence of NMIIA supporting the live-cell imaging data of increased actin severing by NMIIA. Finally, using fluorescence recovery after photobleaching, optical trap-based cortical force measurements and live-cell imaging of actin network, we showed that the dynamics of the actin network increased with the increasing expression of NMIIA but not NMIIB. Thus, we established NMIIA as the predominant generator of cell-scale actin dynamics.