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Volume 272, Number 52, Issue of December 26, 1997 pp. 33384-33393

Proteolysis and Tyrosine Phosphorylation of p34^cdc2/Cyclin B
THE ROLE OF MCM2 AND INITIATION OF DNA REPLICATION TO ALLOW TYROSINE PHOSPHORYLATION OF p34^cdc2

(Received for publication, September 23, 1997, and in revised form, October 21, 1997)

Xiang S. Ye , Russell R. Fincher , Alice Tang , Kimberly K. McNeal ^§ , Scott E. Gygax ^§ , Adam N. Wexler ^§ , Kevin B. Ryan ^§ , Steven W. James ^§ and Stephen A. Osmani

From the  Henry Hood Research Program, Weis Center for Research, Pennsylvania State University College of Medicine, Danville, Pennsylvania 17822 and the ^§ Department of Biology, Gettysburg College, Gettysburg, Pennsylvania 17325

Previously, it has been shown that Aspergillus cells lacking the function of nimQ and the anaphase-promoting complex (APC) component bimE^APC1 enter mitosis without replicating DNA. Here nimQ is shown to encode an MCM2 homologue. Although mutation of nimQ^MCM2 inhibits initiation of DNA replication, a few cells do enter mitosis. Cells arrested at G₁/S by lack of nimQ^MCM2 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 nimQ^MCM2 and bimE^APC1 does not abrogate the S phase arrest checkpoint over mitosis. nimQ^MCM2, 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 nimT^cdc25. Cells lacking both nimQ^MCM2 and bimE^APC1 are deficient in the S phase arrest checkpoint over mitosis because they lack both tyrosine phosphorylation of p34^cdc2 and the function of bimE^APC1. Initiation of DNA replication, which requires nimQ^MCM2, 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 nimQ^MCM2 continue to replicate spindle pole bodies in the absence of DNA replication and can undergo anaphase in the absence of APC function.

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