The protein kinase Pho85 is required for asymmetric accumulation of the Ash1 protein in Saccharomyces cerevisiae

The Ash1 protein is a daughter cell-specific repressor of HO gene transcription in Saccharomyces cerevisiae. Both ASH1 mRNA and protein are localized to the incipient daughter cell at the end of mitosis; Ash1 then inhibits HO transcription in the daughter cell after cytokinesis. Mother cells, in contrast, contain little or no Ash1 and thus are able to transcribe HO. We show that deletion of PHO85, which encodes a cyclin-dependent protein kinase, causes reduced transcription of HO and that this reduction is dependent on ASH1. In pho85 mutants, Ash1 protein is no longer asymmetrically localized and is present, instead, in both mother and daughter cells. Initially, it appears to be localized properly but then persists as daughter cells mature into mother cells. In contrast, ASH1 mRNA is localized appropriately to daughter cells in pho85 mutants. We observe that Ash1 protein is phosphorylated by Pho85 in vitro and that Ash1 stability increases in a pho85 mutant. These ta suggest that phosphorylation of Ash1 by Pho85 governs stability of Ash1 protein.