Phosphorylations of Sds23/Psp1/Moc1 by stress-activated kinase and cAMP-dependent kinase are essential for regulating cell viability in prolonged stationary phase.

Under nutritional deprivation caused by prolonged culture, actively growing cells prepare to enter stationary phase. We showed here that Sds23/Psp1/Moc1 was phosphorylated by both cAMP-dependent kinase and stress-activated MAP kinase Sty1 upon entry into stationary phase. Overexpression of the phosphorylation-defective mutant Sds23/Psp1/Moc1 induced cell death in prolonged culture and blocked re-entry into the cell division cycle. These phosphorylations are likely to be required for cell survival during stationary phase and for binding of Ufd2, a Schizosaccharomyces pombe homologue of multi-ubiquitin chain assembly factor E4. Deletion of the Ufd2 gene and overexpression of Sds23/Psp1/Moc1 increased cell viability in prolonged stationary phase. These results suggested that Ufd2 induces cell death in prolonged nutrient deprivation, that Sds23/Psp1/Moc1 may be a target protein of the ubiquitin-fusion degradation pathway for regulation of cell viability under this stress condition, and that Sty1 and activity in stationary phase is essential for interaction between Sds23/Psp1/Moc1 and Ufd2.

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