Clotet J, Garí E, Aldea M, Ariño J
Departament de Bioquímica i Biologia Molecular, Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Barcelona, Spain.
Mol Cell Biol. 1999 Mar;19(3):2408-15. doi: 10.1128/MCB.19.3.2408.
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.
过表达丝氨酸/苏氨酸蛋白磷酸酶Ppz1的酵母细胞表现出生长缓慢的表型。这些细胞从α-因子或营养耗尽诱导的G1期停滞中恢复缓慢，在芽出现以及G1期细胞周期蛋白Cln2和Clb5的表达方面显示出相当大的延迟。因此，过量的Ppz1磷酸酶会干扰从G1期到S期的正常转变。通过过表达编码Ppz1负调节因子的HAL3/SIS2基因可以挽救生长缺陷。据报道，HAL3/SIS2的高拷贝数表达可改善细胞生长并增加sit4磷酸酶突变体中G1期细胞周期蛋白的表达。我们在此表明，HAL3/SIS2对sit4突变体的上述作用完全由Ppz1磷酸酶介导。在G1期细胞周期蛋白水平较低的菌株（如bck2Δ或cln3Δ突变体）中，PPZ1过表达引起的生长缺陷会加剧，而PPZ1的突变可挽救sit4 cln3突变体的合成致死表型。这些结果揭示了Ppz1作为酵母细胞周期调节成分的作用，强化了Hal3/Sis2作为Ppz1生物学功能负调节因子的观点，并表明Sit4和Ppz1丝氨酸/苏氨酸磷酸酶在控制G1/S转变中起相反作用。
