Liu Q, Larsen B, Ricicova M, Orlicky S, Tekotte H, Tang X, Craig K, Quiring A, Le Bihan T, Hansen C, Sicheri F, Tyers M

In the budding yeast Saccharomyces cerevisiae, mother cells switch mating type between a and α forms, whereas daughter cells do not. This developmental asymmetry arises because expression of the HO endonuclease, which initiates the interconversion of a and α mating-type cassettes, is extinguished by the daughter-specific Ash1 transcriptional repressor. When ...

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daughters become mothers in the subsequent cell cycle, Ash1 must be eliminated to enable a new developmental state. Here, we report that the ubiquitin ligase SCF(Cdc4) mediates the phosphorylation-dependent elimination of Ash1. Inactivation of SCF(Cdc4) stabilizes Ash1 in vivo and, consistently, Ash1 binds to and is ubiquitinated by SCF(Cdc4) in a phosphorylation-dependent manner in vitro. Mutation of a critical in vivo cyclin-dependent kinase (CDK) phosphorylation site (Thr290) on Ash1 reduces its ubiquitination and rate of degradation in vivo, and decreases the frequency of mating type switching. Ash1 associates with active Cdc28 kinase in vivo and is targeted to SCF(Cdc4) in a Cdc28-dependent fashion in vivo and in vitro. Ash1 recognition by Cdc4 appears to be mediated by at least three phosphorylation sites that form two redundant diphosphorylated degrons. The phosphorylation-dependent elimination of Ash1 by the ubiquitin system thus underpins developmental asymmetry in the model yeast system.

Date: Nov. 22, 2010

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