Proteolysis and Tyrosine Phosphorylation of p34 /Cyclin B

Previously, it has been shown thatAspergillus cells lacking the function of nimQand the anaphase-promoting complex (APC) componentbimE 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 ofnimQ 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 andbimE 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 G1to S transition, which may occur by inactivation ofnimT cdc25. Cells lacking bothnimQ MCM2 and bimE APC1are 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.