Ye X S, Fincher R R, Tang A, McNeal K K, Gygax S E, Wexler A N, Ryan K B, James S W, Osmani S A
Henry Hood Research Program, Weis Center for Research, Pennsylvania State University College of Medicine, Danville, Pennsylvania 17822, USA.
J Biol Chem. 1997 Dec 26;272(52):33384-93. doi: 10.1074/jbc.272.52.33384.
Previously, it has been shown that Aspergillus cells lacking the function of nimQ and the anaphase-promoting complex (APC) component bimEAPC1 enter mitosis without replicating DNA. Here nimQ is shown to encode an MCM2 homologue. Although mutation of nimQMCM2 inhibits initiation of DNA replication, a few cells do enter mitosis. Cells arrested at G1/S by lack of nimQMCM2 contain p34(cdc2)/cyclin B, but p34(cdc2) remains tyrosine dephosphorylated, even after DNA damage. However, arrest of DNA replication using hydroxyurea followed by inactivation of nimQMCM2 and bimEAPC1 does not abrogate the S phase arrest checkpoint over mitosis. nimQMCM2, likely via initiation of DNA replication, is therefore required to trigger tyrosine phosphorylation of p34(cdc2) during the G1 to S transition, which may occur by inactivation of nimTcdc25. Cells lacking both nimQMCM2 and bimEAPC1 are deficient in the S phase arrest checkpoint over mitosis because they lack both tyrosine phosphorylation of p34(cdc2) and the function of bimEAPC1. Initiation of DNA replication, which requires nimQMCM2, is apparently critical to switch mitotic regulation from the APC to include tyrosine phosphorylation of p34(cdc2) at G1/S. We also show that cells arrested at G1/S due to lack of nimQMCM2 continue to replicate spindle pole bodies in the absence of DNA replication and can undergo anaphase in the absence of APC function.
此前研究表明，缺乏nimQ功能以及后期促进复合体（APC）组分bimEAPC1的曲霉菌细胞在未复制DNA的情况下进入有丝分裂。在此研究中发现nimQ编码一种MCM2同源物。虽然nimQMCM2突变会抑制DNA复制的起始，但仍有少数细胞进入有丝分裂。因缺乏nimQMCM2而停滞在G1/S期的细胞含有p34(cdc2)/细胞周期蛋白B，但即使在DNA损伤后，p34(cdc2)仍保持酪氨酸去磷酸化状态。然而，用羟基脲阻止DNA复制，随后使nimQMCM2和bimEAPC1失活，并不会消除有丝分裂过程中的S期停滞检查点。因此，nimQMCM2可能通过启动DNA复制，在G1到S期转换过程中触发p34(cdc2)的酪氨酸磷酸化，这可能是通过使nimTcdc25失活来实现的。同时缺乏nimQMCM2和bimEAPC1的细胞在有丝分裂过程中缺乏S期停滞检查点，因为它们既缺乏p34(cdc2)的酪氨酸磷酸化，也缺乏bimEAPC1的功能。需要nimQMCM2的DNA复制起始，显然对于将有丝分裂调控从APC转换为在G1/S期包括p34(cdc2)的酪氨酸磷酸化至关重要。我们还表明，因缺乏nimQMCM2而停滞在G1/S期的细胞在没有DNA复制的情况下继续复制纺锤极体，并且在没有APC功能的情况下可以进入后期。
