Modeling chemoattractant-elicited relocalization of myosin filaments in Dictyostelium

Dictyostelium myosin is able to assemble into filaments that, when visualized under normal conditions, appear to be uniformly distributed throughout the cytoplasm. After stimulation by the chemoattractant cAMP, these filaments quickly diminish in the cellular medulla and accumulate in the cortex. A general hypothesis to explain the mechanism of this relocalization proposes that one or more of the chemical coefficients governing filament polymerization is precisely regulated by some sort of intracellular second messenger. To investigate this hypothesis we formulated a simple theoretical model of myosin polymerization and then used this model to analyze the resting state of the cell and various scenarios for initializing a transition to the activated state. In general, we found that the relocalization of filaments could be realized if a second messenger increased the elongation and (or) the nucleation coefficients for filament assembly in cortical ectoplasm and (or) if the messenger decreased these parameters in the cellular medulla. By comparing these limiting cases with experimental observations, we concluded that models in which redistribution of myosin is achieved by decreasing filament stability in the medulla are the most likely candidates.