Su T T, O'Farrell P H
Department of Biochemistry and Biophysics, University of California San Francisco 94143-0448, USA.
J Cell Biol. 1997 Oct 6;139(1):13-21. doi: 10.1083/jcb.139.1.13.
Minichromosome maintenance (MCM) proteins are essential DNA replication factors conserved among eukaryotes. MCMs cycle between chromatin bound and dissociated states during each cell cycle. Their absence on chromatin is thought to contribute to the inability of a G2 nucleus to replicate DNA. Passage through mitosis restores the ability of MCMs to bind chromatin and the ability to replicate DNA. In Drosophila early embryonic cell cycles, which lack a G1 phase, MCMs reassociate with condensed chromosomes toward the end of mitosis. To explore the coupling between mitosis and MCM-chromatin interaction, we tested whether this reassociation requires mitotic degradation of cyclins. Arrest of mitosis by induced expression of nondegradable forms of cyclins A and/or B showed that reassociation of MCMs to chromatin requires cyclin A destruction but not cyclin B destruction. In contrast to the earlier mitoses, mitosis 16 (M16) is followed by G1, and MCMs do not reassociate with chromatin at the end of M16. dacapo mutant embryos lack an inhibitor of cyclin E, do not enter G1 quiescence after M16, and show mitotic reassociation of MCM proteins. We propose that cyclin E, inhibited by Dacapo in M16, promotes chromosome binding of MCMs. We suggest that cyclins have both positive and negative roles in controlling MCM-chromatin association.
微小染色体维持（MCM）蛋白是真核生物中保守的重要DNA复制因子。在每个细胞周期中，MCM蛋白在与染色质结合和游离的状态之间循环。人们认为染色质上缺乏MCM蛋白会导致G2期细胞核无法复制DNA。通过有丝分裂可恢复MCM蛋白与染色质结合的能力以及复制DNA的能力。在缺乏G1期的果蝇早期胚胎细胞周期中，MCM蛋白在有丝分裂末期重新与浓缩染色体结合。为了探究有丝分裂与MCM-染色质相互作用之间的联系，我们测试了这种重新结合是否需要细胞周期蛋白的有丝分裂降解。通过诱导表达不可降解形式的细胞周期蛋白A和/或B来阻滞有丝分裂，结果表明MCM蛋白与染色质的重新结合需要细胞周期蛋白A的降解，而不需要细胞周期蛋白B的降解。与早期有丝分裂不同，有丝分裂16（M16）之后是G1期，并且在M16末期MCM蛋白不会重新与染色质结合。dacapo突变体胚胎缺乏细胞周期蛋白E的抑制剂，在M16之后不会进入G1静止期，并且显示出MCM蛋白的有丝分裂重新结合。我们提出，在M16中被Dacapo抑制的细胞周期蛋白E促进了MCM蛋白与染色体的结合。我们认为细胞周期蛋白在控制MCM-染色质结合方面具有正负两方面的作用。
