Yang J, Bardes E S, Moore J D, Brennan J, Powers M A, Kornbluth S
Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710 USA.
Genes Dev. 1998 Jul 15;12(14):2131-43. doi: 10.1101/gad.12.14.2131.
Activation of the Cyclin B/Cdc2 kinase complex triggers entry into mitosis in all eukaryotic cells. Cyclin B1 localization changes dramatically during the cell cycle, precipitously transiting from the cytoplasm to the nucleus at the beginning of mitosis. Presumably, this relocalization promotes the phosphorylation of nuclear targets critical for chromatin condensation and nuclear envelope breakdown. We show here that the previously characterized cytoplasmic retention sequence of Cyclin B1, responsible for its interphase cytoplasmic localization, is actually an autonomous nuclear export sequence, capable of directing nuclear export of a heterologous protein, and able to bind specifically to the recently identified export mediator, CRM1. We propose that the observed cytoplasmic localization of Cyclin B1 during interphase reflects the equilibrium between ongoing nuclear import and rapid CRM1-mediated export. In support of this hypothesis, we found that treatment of cells with leptomycin B, which disrupted Cyclin B1-CRM1 interactions, led to a marked nuclear accumulation of Cyclin B1. In mitosis, Cyclin B1 undergoes phosphorylation at several sites, a subset of which have been proposed to play a role in Cyclin B1 accumulation in the nucleus. Both CRM1 binding and the ability to direct nuclear export were affected by mutation of these phosphorylation sites; thus, we propose that Cyclin B1 phosphorylation at the G2/M transition prevents its interaction with CRM1, thereby reducing nuclear export and facilitating nuclear accumulation.
细胞周期蛋白B/Cdc2激酶复合物的激活会触发所有真核细胞进入有丝分裂。细胞周期蛋白B1的定位在细胞周期中会发生显著变化，在有丝分裂开始时从细胞质急剧转移到细胞核。据推测，这种重新定位促进了对染色质凝聚和核膜破裂至关重要的核靶点的磷酸化。我们在此表明，先前鉴定的负责细胞周期蛋白B1间期细胞质定位的细胞质保留序列实际上是一个自主核输出序列，能够指导异源蛋白的核输出，并能与最近鉴定的输出介质CRM1特异性结合。我们提出，在间期观察到的细胞周期蛋白B1的细胞质定位反映了持续的核输入与快速的CRM1介导的输出之间的平衡。为支持这一假设，我们发现用雷帕霉素B处理细胞会破坏细胞周期蛋白B1与CRM1的相互作用，导致细胞周期蛋白B1在细胞核中显著积累。在有丝分裂过程中，细胞周期蛋白B1在多个位点发生磷酸化，其中一部分位点被认为在细胞周期蛋白B1在细胞核中的积累中起作用。这些磷酸化位点的突变会影响CRM1的结合以及指导核输出的能力；因此，我们提出在G2/M期转变时细胞周期蛋白B1的磷酸化会阻止其与CRM1的相互作用，从而减少核输出并促进核积累。
