Hood-DeGrenier Jennifer K, Boulton Cortney N, Lyo Victoria
Department of Biological Sciences, Wellesley College, 106 Central St., Wellesley, MA 02481, USA.
Curr Genet. 2007 Jan;51(1):1-18. doi: 10.1007/s00294-006-0102-1. Epub 2006 Oct 11.
Subcellular localization is an important determinant of substrate and functional specificity for cyclin-cyclin dependent kinase (CDK) complexes. This work addresses the cytoplasmic function of the budding yeast mitotic cyclin Clb2, which is mostly nuclear but is also present in the bulk cytoplasm and at the mother-bud neck. Clb2 contains two leucine-rich nuclear export signals (NESs)--one of which we newly describe here--that maintain its presence in the cytoplasm. Yeast strains bearing mutations in one or both of these NESs have elongated buds, indicative of a G2/M cell cycle delay. A small number of these cells exhibit a filamentous-like morphology under conditions that do not normally induce filamentous growth. These phenotypes are enhanced by deletion of the other three mitotic cyclins (CLB1,3,4) and are dependent on expression of Swe1, the yeast Cdk1 inhibitory kinase. Deltaclb1,3,4 Deltabud3 cells, which fail to localize Clb2 to the bud neck, also exhibit a Swe1-dependent elongated bud phenotype. Our results support a model in which cytoplasmic Clb2-Cdk1 is required for timely inactivation of Swe1 at the G2/M transition and bud neck targeting of Clb2 contributes to the efficiency of this process. Cytoplasmic Clb2 may also be important for repression of filamentous growth.
亚细胞定位是细胞周期蛋白 - 细胞周期蛋白依赖性激酶（CDK）复合物底物和功能特异性的重要决定因素。这项工作探讨了芽殖酵母有丝分裂细胞周期蛋白Clb2的细胞质功能，Clb2主要存在于细胞核中，但也存在于大量细胞质和母 - 芽颈处。Clb2含有两个富含亮氨酸的核输出信号（NESs）——其中一个是我们在此新描述的——这些信号维持其在细胞质中的存在。携带这些NESs中一个或两个发生突变的酵母菌株具有伸长的芽，这表明存在G2/M细胞周期延迟。在通常不会诱导丝状生长的条件下，这些细胞中有一小部分表现出丝状样形态。这些表型通过缺失其他三种有丝分裂细胞周期蛋白（CLB1、3、4）而增强，并且依赖于酵母Cdk1抑制激酶Swe1的表达。缺失clb1、3、4和bud3的细胞无法将Clb2定位到芽颈，也表现出依赖于Swe1的伸长芽表型。我们的结果支持这样一种模型，即细胞质中的Clb2 - Cdk1是G2/M转换时Swe1及时失活所必需的，并且Clb2在芽颈处的定位有助于这一过程的效率。细胞质中的Clb2对于抑制丝状生长可能也很重要。
