Function of the amino-terminal region of human MCM4 in helicase activity.

The amino-terminal region of eukaryotic MCM4 is characteristic of the presence of a number of phosphorylation sites for CDK and DDK, suggesting that the region plays regulatory roles in the MCM2-7 helicase function. However, the roles are not fully understood. We analyzed the role of the amino-terminal region of human MCM4 by using MCM4/6/7 helicase as a model for MCM2-7 helicase. First we found that deletion of 35 amino acids at the amino-terminal end resulted in inhibition of DNA helicase activity of the MCM4/6/7 complex. Conversion of arginine at amino acid no. 10 and 11 to alanine had similar effect to the deletion mutant of Δ1-35, suggesting that these arginine play a role in the DNA helicase activity. The data suggest that expression of these mutant MCM4 in HeLa cells perturbed the progression of the S phase. Substitution of six CDK phosphorylation sites (3, 7, 19, 32, 54 and 110) in the amino-terminal region by phospho-mimetic glutamic acids affected the hexamer formation of the MCM4/6/7 complex. MCM4 phosphorylation by CDK may play a role in DNA replication licensing system, and the present results suggest that the phosphorylation interferes MCM function by lowering stability of MCM complex.

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