Identification of a Second Myosin-II inSchizosaccharomyces pombe:

Abstract

As in many eukaryotic cells, fission yeast cytokinesis depends on the assembly of an actin ring. We clonedmyp2⁺, a myosin-II inSchizosaccharomyces pombe, conditionally required for cytokinesis. myp2⁺, the second myosin-II identified in S. pombe, does not completely overlap in function with myo2⁺. The catalytic domain of Myp2p is highly homologous to known myosin-IIs, and phylogenetic analysis places Myp2p in the myosin-II family. The Myp2p sequence contains well-conserved ATP- and actin-binding motifs, as well as two IQ motifs. However, the tail sequence is unusual, since it is predicted to form two long coiled-coils separated by a stretch of sequence containing 19 prolines. Disruption of myp2⁺is not lethal but under nutrient limiting conditions cells lackingmyp2⁺ function are multiseptated, elongated, and branched, indicative of a defect in cytokinesis. The presence of salt enhances these morphological defects. Additionally,Δmyp2 cells are cold sensitive in high salt, failing to form colonies at 17°C. Thus, myp2⁺ is required under conditions of stress, possibly linking extracellular growth conditions to efficient cytokinesis and cell growth. GFP-Myp2p localizes to a ring in the middle of late mitotic cells, consistent with a role in cytokinesis. Additionally, we constructed double mutants of Δmyp2 with temperature-sensitive mutant strains defective in cytokinesis. We observed synthetic lethal interactions between Δmyp2 and three alleles ofcdc11ts, as well as more modest synthetic interactions with cdc14ts and cdc16ts, implicatingmyp2⁺ function for efficient cytokinesis under normal conditions.