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

doi:10.1046/j.1365-2958.2001.02601.x

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Wiley InterScience

Molecular Microbiology

Volume 42 Issue 2, Pages 345 - 353

Published Online: 7 Jul 2008

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

Helen J. McBride ¹ ** ^† Anita Sil ² ** ^‡ Vivien Measday ³ ^§ Yaxin Yu ¹ Jason Moffat ³ Mary E. Maxon ² ^¶ Ira Herskowitz ² Brenda Andrews ³ David J. Stillman ¹ *

¹ Department of Pathology, University of Utah, Salt Lake City, UT 84132, USA. ² Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA, 94143–0448, USA. ³ Department of Molecular and Medical Genetics, University of Toronto, Toronto, M5S 1A8, Canada.

*For correspondence. E-mail david.stillman@path.utah.edu; Tel. (+1) 801 581 5429; Fax (+1) 801 581 4517.

Present addresses: ^†Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.

^‡Department of Microbiology and Immunology, University of California, San Francisco, San Francisco, CA 94143–0441, USA.

^§The Centre for Molecular Medicine and Therapeutics, University of British Columbia, Vancouver, Canada, V5Z 4H4.

^¶Cytokinetics, South San Francisco, CA 94080, USA.

**The first two authors contributed equally to this work.

ABSTRACT

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 data suggest that phosphorylation of Ash1 by Pho85 governs stability of Ash1 protein.