SPINDLE POLE BODY SEPARATION IN SACCHAROMYCES-CEREVISIAE REQUIRES DEPHOSPHORYLATION OF THE TYROSINE-19 RESIDUE OF CDC28

In eukaryotes, mitosis requires the activation of cdc2 kinase via asso ciation with cyclin B and dephosphor ylation of the threonine 14 and t yrosine 15 residues, It is known that in the budding yeast Saccharomyc es cerevisiae, a homologous kinase, Cdc28, mediates the progression th rough M phase, but it is not clear what specific mitotic function its activation by the dephosphorylation of an equivalent tyrosine (Tyr-19) serves, We report here that cells expressing cdc28-E19 (in which Tyr- 19 is replaced by glutamic acid) perform Start-related functions, comp lete DNA synthesis, and exhibit high levels of Clb2-associated kinase activity but are unable to form bipolar spindles, The failure of these cells to form mitotic spindles is due to their inability to segregate duplicated spindle pole bodies (SPBs), a phenotype strikingly similar to that exhibited by a previously reported mutant defective in both k inesin-like motor proteins Cin8 and Kip1, We also find that the overex pression of SWE1, the budding-yeast homolog of weel, also leads to a f ailure to segregate SPBs, These results imply that dephosphorylation o f Tyr-19 is required for the segregation of SPBs, The requirement of T yr-19 dephosphorylation for spindle assembly is also observed under co nditions in which spindle formation is independent of mitosis, suggest ing that the involvement of Cdc28/Clb kinase in SPB separation is dire ct, On the basis of these results, we propose that one of the roles of Tyr-19 dephosphorylation is to promote SPB separation.