Identification of a second myosin-II in Schizosaccharomyces pombe: Myp2p is conditionally required for cytokinesis.

As in many eukaryotic cells, fission yeast cytokinesis depends on the assembly of an actin ring. We cloned myp2(+), a myosin-II in Schizosaccharomyces 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 lacking myp2(+) function are multiseptated, elongated, and branched, indicative of a defect in cytokinesis. The presence of salt enhances these morphological defects. Additionally, Deltamyp2 cells are cold sensitive in high salt, failing to form colonies at 17 degrees 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 Deltamyp2 with temperature-sensitive mutant strains defective in cytokinesis. We observed synthetic lethal interactions between Deltamyp2 and three alleles of cdc11ts, as well as more modest synthetic interactions with cdc14ts and cdc16ts, implicating myp2(+) function for efficient cytokinesis under normal conditions.