Required for bipolar spindle formation. May act as a regulator of the p34cdc2/cyclin B kinase. Required for full activation of the plo1 kinase. However, in cut12.1 cells at restrictive temperature the H1 kinase does rise concomitant with entry into mitosis, indicating that cut12 is not required for activation of p34cdc2/cyclin B. The cut12.s11 allele may promote cdc2-independent phosphorylation of SPB proteins thereby overcoming the requirement for cdc25 in cell cycle progression. Self-associates. Interacts with plo1.