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Molecular and Cellular Biology, March 1999, p. 2408-2415, Vol. 19, No. 3
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

The Yeast Ser/Thr Phosphatases Sit4 and Ppz1 Play Opposite Roles in Regulation of the Cell Cycle

Josep Clotet,1 Eloi Garí,2 Martí Aldea,2 and Joaquín Ariño1,*

Departament de Bioquímica i Biologia Molecular, Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Barcelona,1 and Departament de Ciencies Mèdiques Bàsiques, Facultat de Medicina, Universitat de Lleida, 25198 Lleida,2 Spain

Received 23 July 1998/Returned for modification 5 October 1998/Accepted 8 December 1998

Yeast cells overexpressing the Ser/Thr protein phosphatase Ppz1 display a slow-growth phenotype. These cells recover slowly from alpha -factor or nutrient depletion-induced G1 arrest, showing a considerable delay in bud emergence as well as in the expression of the G1 cyclins Cln2 and Clb5. Therefore, an excess of the Ppz1 phosphatase interferes with the normal transition from G1 to S phase. The growth defect is rescued by overexpression of the HAL3/SIS2 gene, encoding a negative regulator of Ppz1. High-copy-number expression of HAL3/SIS2 has been reported to improve cell growth and to increase expression of G1 cyclins in sit4 phosphatase mutants. We show here that the described effects of HAL3/SIS2 on sit4 mutants are fully mediated by the Ppz1 phosphatase. The growth defect caused by overexpression of PPZ1 is intensified in strains with low G1 cyclin levels (such as bck2Delta or cln3Delta mutants), whereas mutation of PPZ1 rescues the synthetic lethal phenotype of sit4 cln3 mutants. These results reveal a role for Ppz1 as a regulatory component of the yeast cell cycle, reinforce the notion that Hal3/Sis2 serves as a negative modulator of the biological functions of Ppz1, and indicate that the Sit4 and Ppz1 Ser/Thr phosphatases play opposite roles in control of the G1/S transition.

* Corresponding author. Mailing address: Dept. Bioquímica i Biologia Molecular, Fac. Veterinària, Universidad Autònoma de Barcelona, Bellaterra 08193, Barcelona, Spain. Phone: 34-93-5812182. Fax: 34-93-5812006. E-mail: J.Arino{at}CC.UAB.ES.

Molecular and Cellular Biology, March 1999, p. 2408-2415, Vol. 19, No. 3
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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