Yuan Kai, O'Farrell Patrick H
Department of Biochemistry, University of California, San Francisco, San Francisco, CA 94158-2517, USA.
Department of Biochemistry, University of California, San Francisco, San Francisco, CA 94158-2517, USA.
Curr Biol. 2015 Mar 16;25(6):811-816. doi: 10.1016/j.cub.2015.01.053. Epub 2015 Mar 5.
The timing mechanism for mitotic progression is still poorly understood. The spindle assembly checkpoint (SAC), whose reversal upon chromosome alignment is thought to time anaphase [1-3], is functional during the rapid mitotic cycles of the Drosophila embryo; but its genetic inactivation had no consequence on the timing of the early mitoses. Mitotic cyclins-Cyclin A, Cyclin B, and Cyclin B3-influence mitotic progression and are degraded in a stereotyped sequence [4-11]. RNAi knockdown of Cyclins A and B resulted in a Cyclin B3-only mitosis in which anaphase initiated prior to chromosome alignment. Furthermore, in such a Cyclin B3-only mitosis, colchicine-induced SAC activation failed to block Cyclin B3 destruction, chromosome decondensation, or nuclear membrane re-assembly. Injection of Cyclin B proteins restored the ability of SAC to prevent Cyclin B3 destruction. Thus, SAC function depends on particular cyclin types. Changing Cyclin B3 levels showed that it accelerated progress to anaphase, even in the absence of SAC function. The impact of Cyclin B3 on anaphase initiation appeared to decline with developmental progress. Our results show that different cyclin types affect anaphase timing differently in the early embryonic divisions. The early-destroyed cyclins-Cyclins A and B-restrain anaphase-promoting complex/cyclosome (APC/C) function, whereas the late-destroyed cyclin, Cyclin B3, stimulates function. We propose that the destruction schedule of cyclin types guides mitotic exit by affecting both Cdk1 and APC/C, whose activities change as each cyclin type is lost.
有丝分裂进程的时间调控机制仍未被充分理解。纺锤体组装检查点（SAC），其在染色体排列后逆转被认为是后期定时的关键[1 - 3]，在果蝇胚胎快速的有丝分裂周期中发挥作用；但其基因失活对早期有丝分裂的时间没有影响。有丝分裂周期蛋白——周期蛋白A、周期蛋白B和周期蛋白B3——影响有丝分裂进程，并按固定顺序降解[4 - 11]。通过RNA干扰敲低周期蛋白A和B会导致仅存在周期蛋白B3的有丝分裂，其中后期在染色体排列之前就开始了。此外，在这种仅存在周期蛋白B3的有丝分裂中，秋水仙碱诱导的SAC激活未能阻止周期蛋白B3的降解、染色体解聚或核膜重新组装。注射周期蛋白B蛋白恢复了SAC阻止周期蛋白B3降解的能力。因此，SAC功能取决于特定的周期蛋白类型。改变周期蛋白B3的水平表明，即使在没有SAC功能的情况下，它也会加速进入后期的进程。周期蛋白B3对后期起始的影响似乎随着发育进程而减弱。我们的结果表明，在早期胚胎分裂中，不同类型的周期蛋白对后期时间的影响不同。早期被降解的周期蛋白——周期蛋白A和B——抑制后期促进复合物/细胞周期体（APC/C）的功能，而后期被降解的周期蛋白，周期蛋白B3，则刺激其功能。我们提出，周期蛋白类型的降解时间表通过影响Cdk1和APC/C来指导有丝分裂退出，随着每种周期蛋白类型的消失，它们的活性会发生变化。
