Model of the Role of Actin Crosslinker Proteins during Contractile Ring Assembly in Fission Yeast

Abstract

Actin cross linkers play a crucial role in various processes during cell cycle. Contractile ring formation in fission yeast is one of the processes where the role of two actin crosslinkers fimbrin and alpha actinin is important. In fission yeast contractile ring assembles from a broad band of cortical nodes. Fission yeast nodes are macromolecular complexes containing several myosin-II heads and a few formin dimers. The condensation of the broad band of nodes into a ring has been described by a search, capture, pull and release (SCPR) model. In this model, formin nucleates filaments along random directions parallel to the cell membrane. When a growing filament comes in close proximity to a myosin of a neighboring node, myosin grabs the filament and exerts force that moves nodes toward one other. Using analytical calculations and numerical simulations, we have shown that the SCPR model predicts that nodes coalesce into rings or clumps (i.e. disconnected aggregates), depending on the values of parameters. In experimental images of fission yeast, myosin nodes appear to get stabilized into linear elements towards the late stages of ring assembly, presumably due to crosslinkers. This process may assist ring formation by preventing the formation of clumps. To study this possible role of crosslinkers, we incorporated a short-range aligning force to the SCPR model, representing a short range aligning mechanism. We used simulations and scaling arguments to quantify the resulting dynamics and morphology of myosin aggregates. We describe regions in parameter space in which local alignment prevents clump formation. We compare the results to experiments on fission yeast cells in the process of contractile ring formation.