THE FISSION YEAST SPB COMPONENT CUT12 LINKS BIPOLAR SPINDLE FORMATIONTO MITOTIC CONTROL

During fission yeast mitosis, the duplicated spindle pole bodies (SPBs ) nucleate microtubule arrays that interdigitate to form the mitotic s pindle. cut12.1 mutants form a monopolar mitotic spindle, chromosome s egregation fails, and the mutant undergoes a lethal cytokinesis. The c ut12(+) gene encodes a novel 62-kD protein with two predicted coiled c oil regions, and one consensus phosphorylation site for p34(cdc2) and two for MAP kinase. Cut12 is localized to the SPB throughout the cell cycle, predominantly around the inner face of the interphase SPB, adja cent to the nucleus. cut12(+) is allelic to stf1(+); stf1.1 is a gain- of-function mutation bypassing the requirement for the Cdc25 tyrosine phosphatase, which normally dephosphorylates and activates the p34(cdc 2)/cyclin B kinase to promote the onset of mitosis. Expressing a cut12 (+) cDNA carrying the stf1.1 mutation also suppressed cdc25.22. The sp indle defect in cut12.1 is exacerbated by the cdc25.22 mutation, and s tf1.1 cells formed defective spindles in a cdc25.22 background at high temperatures. We propose that Cut12 may be a regulator or substrate o f the p34(cdc2) mitotic kinase.