Kumada K, Su S, Yanagida M, Toda T
Department of Biophysics, Faculty of Science, Kyoto University, Japan.
J Cell Sci. 1995 Mar;108 ( Pt 3):895-905. doi: 10.1242/jcs.108.3.895.
Fission yeast nuc2+ gene encodes a protein of a tetratricopeptide repeat (TPR) family which is conserved throughout evolution. We previously showed that nuc2 is required for exit from the mitotic metaphase. In this study, we present evidence which shows that nuc2 has two additional roles in the cell cycle. We showed that the nuc2 mutant is sterile even at the permissive temperature and septation occurs in the absence of chromosome separation at the restrictive temperature. The nuc2 mutant fails to arrest at the G1 phase upon nitrogen starvation at the permissive temperature which is a prerequisite for conjugation. Upon starvation, however, the nuc2 mutant ceased division normally and induced starvation-dependent gene expression. Therefore, the nuc2 mutant is deficient only for failure to block DNA replication upon starvation. At the lower restrictive temperature, the nuc2 mutant showed a 'cut' phenotype where septation and cytokinesis takes place without the completion of mitosis. Ectopic overexpression of the nuc2+ gene caused multiple rounds of S and M phases in the complete absence of septum formation. We propose that nuc2 is a novel cell cycle regulator essential for three events; firstly for exit from mitosis, secondly for DNA replication restraint under nutrient starvation and thirdly for inhibition of septation and cytokinesis until the completion of mitosis.
裂殖酵母nuc2+基因编码一种四肽重复序列（TPR）家族的蛋白质，该家族在进化过程中高度保守。我们之前的研究表明，nuc2是有丝分裂中期退出所必需的。在本研究中，我们提供的证据表明nuc2在细胞周期中还有另外两个作用。我们发现，nuc2突变体即使在允许温度下也不育，并且在限制温度下，在没有染色体分离的情况下就会发生隔膜形成。nuc2突变体在允许温度下氮饥饿时无法在G1期停滞，而这是接合的前提条件。然而，饥饿时，nuc2突变体正常停止分裂并诱导饥饿依赖的基因表达。因此，nuc2突变体仅在饥饿时无法阻止DNA复制方面存在缺陷。在较低的限制温度下，nuc2突变体表现出“切割”表型，即隔膜形成和胞质分裂在有丝分裂未完成时就发生。nuc2+基因的异位过表达在完全没有隔膜形成的情况下导致多轮S期和M期。我们提出，nuc2是一种新型的细胞周期调节因子，对三个事件至关重要：首先是有丝分裂的退出，其次是营养饥饿时对DNA复制的抑制，第三是在有丝分裂完成之前对隔膜形成和胞质分裂的抑制。
