SPC42P - A PHOSPHORYLATED COMPONENT OF THE SACCHAROMYCES-CEREVISIAE SPINDLE POLE BODY (SPB) WITH AN ESSENTIAL FUNCTION DURING SPB DUPLICATION

The 42-kD component of the S. cerevisiae spindle pole body (SPB) local izes to the electron-dense central plaque of the SPB (Rout, M.P., and J.V. Kilmartin. 1991, Cold Spring Harbor Symp. Quant. Biol. 56:687-691 ). We have cloned the corresponding gene SPC42 (spindle pole component ) and show that it is essential. Seven temperature-sensitive (ts) muta nts in SPC42 were prepared by error-prone PCR. We found that a change to a proline residue in a potential coiled-coil region of Spc42p was r esponsible for the ts phenotype in at least three alleles, suggesting that formation of the coiled-coil is essential for normal function. Th e mutant cells showed a phenotype of predominantly single or bilobed S PBs often with an accumulation of unstructured electron-dense material associated with the bridge structure adjacent to the SPB. This phenot ype suggests a defect in SPB duplication. This was confirmed by examin ing synchronized mutant cells that lose viability when SPB duplication is attempted. Spc42p is a phosphoprotein which shows some cell cycle- regulated phosphorylation. Overexpression of Spc42p causes the formati on of a disc- or dome-shaped polymer composed of phosphorylated Spc42p , which is attached to the central plaque and associated with the oute r nuclear membrane. Taken together, these data suggest that Spc42p for ms a polymeric layer at the pe riphery of the SPB central plaque which has an essential function during SPB duplication and may facilitate a ttachment of the SPB to the nuclear membrane.