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Molecular and Cellular Biology, November 2000, p. 8143-8156, Vol. 20, No. 21
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Loss of a Protein Phosphatase 2A Regulatory Subunit (Cdc55p) Elicits Improper Regulation of Swe1p Degradation

Haifeng Yang, Wei Jiang, Matthew Gentry, and Richard L. Hallberg*

Department of Biology, Syracuse University, Syracuse, New York 13244

Received 20 March 2000/Returned for modification 12 May 2000/Accepted 2 August 2000

CDC55 encodes a Saccharomyces cerevisiae protein phosphatase 2A (PP2A) regulatory subunit. cdc55-null cells growing at low temperature exhibit a failure of cytokinesis and produce abnormally elongated buds, but cdc55-null cells producing the cyclin-dependent kinase Cdc28-Y19F, which is unable to be inhibited by Y19 phosphorylation, show a loss of the abnormal morphology. Furthermore, cdc55-null cells exhibit a hyperphosphorylation of Y19. For these reasons, we have examined in wild-type and cdc55-null cells the levels and activities of the kinase (Swe1p) and phosphatase (Mih1p) that normally regulate the extent of Cdc28 Y19 phosphorylation. We find that Mih1p levels are comparable in the two strains, and an estimate of the in vivo and in vitro phosphatase activity of this enzyme in the two cell types indicates no marked differences. By contrast, while Swe1p levels are similar in unsynchronized and S-phase-arrested wild-type and cdc55-null cells, Swe1 kinase is found at elevated levels in mitosis-arrested cdc55-null cells. This excess Swe1p in cdc55-null cells is the result of ectopic stabilization of this protein during G2 and M, thereby accounting for the accumulation of Swe1p in mitosis-arrested cells. We also present evidence indicating that, in cdc55-null cells, misregulated PP2A phosphatase activity is the cause of both the ectopic stabilization of Swe1p and the production of the morphologically abnormal phenotype.

* Corresponding author. Mailing address: Department of Biology, 411 Lyman Hall, Syracuse University, Syracuse, NY 13244. Phone: (315) 443-1104. Fax: (315) 443-2156. E-mail: hallberg{at}syr.edu.

Molecular and Cellular Biology, November 2000, p. 8143-8156, Vol. 20, No. 21
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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