Building a three-dimensional model for the contractile actomyosin ring of fission yeast

Abstract

Cytokinesis is a crucial step in cell division. In fission yeast, cytokinesis occurs by the formation and constriction of a contractile ring concurrently with septum regression. Although the composition of proteins in the contractile ring is known, these proteins are too densely packed to be resolved by conventional confocal fluorescence microscopy. This leaves us with many unanswered questions about protein organization within the ring and the constriction mechanism. We used the data from cross-linking mass spectrometry of isolated rings to map the organization of proteins present in the ring. We generated homology models for the fission yeast cytokinesis proteins and mapped the crosslinks on these structures using Chimera and Xlink analyzer. The node proteins Cdc12, Cdc15, Mid1, Myo2, and Rng2, along with actin, are present in abundance in the ring. Myo2p is a major motor involved in force generation in the ring. We propose that Myo2p has a staggered heptagonal arrangement in the ring. Such an arrangement facilitates deeper penetration within the F-actin network of the ring. Such a complex of fission yeast Myo2p, or any non-muscle myosin, has not been captured experimentally before.