Fission yeast myosin-II isoforms assemble into contractile rings at distinct times during mitosis.

Myosin-II is required for cytokinesis in Schizosaccharomyces pombe [1-3], but unlike other unicellular organisms, S. pombe has two structurally distinct myosin-IIs, Myo2p and Myp2p, which are required under different conditions [4]. Disruption of myo2(+) is lethal, whereas disruption of myp2(+) leads to defects in cytokinesis when nutrients are limiting and to cold-sensitivity in 1 M KCl. In dividing cells, both myosin-IIs localize to a ring in the center of the cell, which is thought to contract, separating the cytoplasms of the daughter cells. Using deconvolution microscopy, we obtained three-dimensional reconstructions of fission yeast cells expressing green fluorescent protein-labeled (GFP)-myosin-II, providing for the first time detailed images of GFP-myosin-II rings. By time-lapse microscopy, we observed ring assembly and contraction in three dimensions using GFP-tubulin as a cell cycle marker. We determined that the Myo2p ring forms in metaphase/anaphase A whereas the Myp2p ring forms much later, at the end of anaphase B. Myo2p initiates ring formation while Myp2p acts later to increase the efficiency of cytokinesis.