Regulation of the Anaphase-promoting Complex/Cyclosome by bimA(APC3) and Proteolysis of NIMA

Surprisingly, although highly temperature-sensitive, the bimA1(APC3) anaphase-promoting complex/cyclosome (APC/C) mutation does not cause arrest of mitotic exit. Instead, rapid inactivation of bimA1(APC3) is shown to promote repeating oscillations of chromosome condensation and decondensation, activation and inactivation of NIMA and p34(cdc2) kinases, and accumulation and degradation of NIMA, which all coordinately cycle multiple times without causing nuclear division. These bimA1(APC3)-induced cell cycle oscillations require active NIMA, because a nimA5 + bimA1(APC3) double mutant arrests in a mitotic state with very high p34(cdc2) H1 kinase activity. NIMA protein instability during S phase and G2 was also found to be controlled by the APC/C. The bimA1(APC3) mutation therefore first inactivates the APC/C but then allows its activation in a cyclic manner; these cycles depend on NIMA. We hypothesize that bimA(APC3) could be part of a cell cycle clock mechanism that is reset after inactivation of bimA1(APC3). The bimA1(APC3) mutation may also make the APC/C resistant to activation by mitotic substrates of the APC/C, such as cyclin B, Polo, and NIMA, causing mitotic delay. Once these regulators accumulate, they activate the APC/C, and cells exit from mitosis, which then allows this cycle to repeat. The data indicate that bimA(APC3) regulates the APC/C in a NIMA-dependent manner.

令人意外的是，尽管该bimA1(APC3)后期促进复合物/周期体（anaphase-promoting complex/cyclosome, APC/C）突变对温度高度敏感，但它并不会引发有丝分裂退出受阻。与之相反，bimA1(APC3)的快速失活可诱导染色体凝缩与解凝、NIMA激酶及p34(cdc2)激酶的激活与失活，以及NIMA蛋白的积累与降解出现周期性重复振荡；上述过程可协同循环多次，却不会引发核分裂。此类由bimA1(APC3)诱导的细胞周期振荡依赖于有活性的NIMA激酶：当nimA5与bimA1(APC3)构成双突变体时，细胞会停滞在有丝分裂状态，此时p34(cdc2)介导的组蛋白H1激酶活性显著升高。研究还发现，S期与G2期内NIMA蛋白的不稳定调控同样受APC/C介导。因此，bimA1(APC3)突变首先会使APC/C失活，随后又以循环方式允许其重新激活；这类循环过程完全依赖于NIMA激酶。我们推测，bimA(APC3)可能是细胞周期时钟机制的组成部分，该机制可在bimA1(APC3)失活后完成重置。此外，bimA1(APC3)突变可能会使APC/C对其自身的有丝分裂底物（如细胞周期蛋白B、Polo激酶及NIMA）的激活产生抗性，进而引发有丝分裂延迟。一旦这些调控因子积累至足够浓度，它们便会激活APC/C，使细胞退出有丝分裂，从而允许该循环再次启动。上述实验数据表明，bimA(APC3)以NIMA依赖的方式调控APC/C的功能。