These observations suggest that myosin-II along with actin crosslinkers establish local cortical tension and elasticity, allowing for contractility independent of a circumferential cytoskeletal array. Furthermore, myosin-II and actin crosslinkers may influence each other as they modulate the dynamics and mechanics of cell-shape change.

During cytokinesis, global and equatorial pathways deform the cell cortex in a stereotypical manner, which leads to daughter cell separation. Equatorial forces are largely generated by myosin-II and the actin crosslinker, cortexillin-I. In contrast, global mechanics are determined by the cortical cytoskeleton, including the actin crosslinker, dynacortin. We used direct morphometric characterization and laser-tracking microrheology to quantify cortical mechanical properties of wild-type and cortexillin-I and dynacortin mutant Dictyostelium cells. Both cortexillin-I and dynacortin influence cytokinesis and interphase cortical viscoelasticity as predicted from genetics and biochemical data using purified dynacortin proteins. Our studies suggest that the regulation of cytokinesis ultimately requires modulation of proteins that control the cortical mechanical properties that establish the force-balance that specifies the shapes of cytokinesis. The combination of genetic, biochemical, and biophysical observations suggests that the cell's cortical mechanical properties control how the cortex is remodeled during cytokinesis.

Cell cortices rearrange dynamically to complete cytokinesis, crawl in response to chemoattractant, build tissues, and make neuronal connections. Highly enriched in the cell cortex, actin, myosin II, and actin crosslinkers facilitate cortical movements. Because cortical behavior is the consequence of nanoscale biochemical events, it is essential to probe the cortex at this level. Here, we use highresolution laser-based particle tracking to examine how myosin II mechanochemistry and dynacortin-mediated actin crosslinking control cortex dynamics in Dictyostelium. Consistent with its low duty ratio, myosin II does not directly drive active bead motility. Instead, myosin II and dynacortin antagonistically regulate other active processes in the living cortex.cell mechanics ͉ dynacortin ͉ laser-based particle tracking ͉ nanoscale M ediated by an actin filament network, the myosin II motor protein has a central role in many cellular shape changes (1-3) and uses energy from ATP hydrolysis to mechanically move and slide actin filaments (4). However, unlike the highly ordered structure of muscle, nonmuscle myosin II usually works on a less-ordered array of short cortical actin filaments. During its mechanochemical cycle, myosin II tightly binds actin and swings its lever arm, moving the filament Ϸ8 nm (for Dictyostelium myosin II) (5) (Fig. 1A). The maximum velocity with which the motor moves an actin filament is limited by the strongly bound-state time ( s ϭ 1͞k ADP-release ; s ϭ 2.4 ms for Dictyostelium myosin II) and the step size of the motor. Importantly, the motor is not simply a dynamic crosslinker; rather it changes the actin network by generating force to slide the actin filaments.Various myosin II isoforms differ dramatically in their duty ratios and thick-filament assembly states. Muscle myosin IIs are tuned so that each thick filament has tens of heads associating with the actin filaments, allowing the sarcomere to contract and maintain tension (6, 7). In contrast, mammalian nonmuscle myosin IIs assemble into small minithick filaments and have a range of duty ratios so that as few as one or two to tens of motor heads per thick filament engage actin at a time, depending on the isoform (nmhcII-A, -B, or -C) (8-13). With so few heads associated, low duty-ratio motors may not sustain tension as readily or slide actin filaments significant distances as expected for muscle myosin II.Expressing many of the same cytoskeletal proteins, Dictyostelium cells behave similarly to higher eukaryotic cells, such as human neutrophils, but offer exquisite genetic control over the proteins. Dictyostelium are also relatively mechanically simple because they do not appear to have stress fibers or intermediate filaments. Similar to mammalian nmhcII-A, Dictyostelium myosin II has a low duty ratio and thick-filament assembly state that allows as few as one head per thick filament to engage actin at a time (5,14,15). We demonstrate here that Dictyostelium myosin II mechanochemistry promotes nonmyosin II-driven cortical rearrangements. R...