Cell cycle control of spindle pole body duplication and splitting by Sfi1 and Cdc31 in fission yeast

Spindle pole biogenesis and segregation are tightly coordinated to produce a bipolar mitotic spindle. In yeasts, the spindle pole body (SPB) half-bridge composed of Sfi1 and Cdc31 duplicates to promote biogenesis of a second SPB. Sfi1 accumulates at the half-bridge in two phases in S. pombe, from anaphase to early septation, and throughout G2 phase. We found that Sfi1/Cdc31 function in SPB duplication is accomplished before septation ends and G2 accumulation starts. Thus, Sfi1 early accumulation at mitotic exit may correspond to half-bridge duplication. We further show that Cdc31 phosphorylation on serine 15 in a CDK1 consensus is required for the dissociation of a significant pool of Sfi1 from the bridge and timely segregation of SPBs at mitotic onset. This suggests that Cdc31 N-terminus modulates the stability of Sfi1/Cdc31 arrays in fission yeast, and impacts on the timing of spindle bipolarity establishment.