Modeling contractility kit-mediated cytoskeletal network assembly

The proper function of the cytoskeleton enables mechanoresponsiveness, the ability of cells to sense and respond to mechanical cues. Myosin II, cortexillin I and IQGAP proteins preassemble in the cytoplasm of Dictyostelium cells into contractility kits (CKs). These CKs are thought to play a pivotal role in the regulation and delivery of contractile proteins to sites of mechanical stress. The IQGAPs complementarily regulate the mechanoresponsiveness of cortexillin I-myosin II elements within CKs. Whereas cells that have only IQGAP2 are highly mechanoresponsive, those that have only IQGAP1 are not. Moreover, double mutants lacking both IQGAPs are also mechanoresponsive, suggesting that IQGAP1 serves as an inhibitor of cortexillin I and myosin II, and part of the role of IQGAP2 is to alleviate IQGAP1's inhibition. To investigate the mechanism of CK self-assembly and the possible molecular means for IQGAP regulation, we developed in SpringSaLaD a coarse-grained excluded-volume molecular model in which all protein polymers are represented by nm-sized spheres connected by spring-like links. The model is parameterized using experimentally measured values acquired through fluorescence cross-correlation spectroscopy (FCCS) and fluorescence correlation spectroscopy (FCS), which describe the interaction affinities and diffusion coefficients for individual molecular components. Simulations of the model provided cluster diffusion coefficients, distributions of cluster sizes, cluster component distributions, and IQGAP ratios in the clusters. Simulations of wild-type and null-mutant conditions revealed that the temporal order of assembly of these kits is dominated by myosin II dimer formation and that IQGAP proteins mediate cluster growth. Additionally, our simulations predicted, and FCCS experiments confirmed, the existence of CKs that incorporate both IQGAP1 and IQGAP2. This molecular model serves to describe the formation of CKs and how their assembly enables and regulates mechanoresponsiveness.