Identification and Characterization of a Novel Component of the Human Minichromosome Maintenance Complex

Abstract

Minichromosome maintenance (MCM) complex replicative helicase complexes play essential roles in DNA replication in all eukaryotes. Using a tandem affinity purification-tagging approach in human cells, we discovered a form of the MCM complex that contains a previously unstudied protein, MCM binding protein (MCM-BP). MCM-BP is conserved in multicellular eukaryotes and shares limited homology with MCM proteins. MCM-BP formed a complex with MCM3 to MCM7, which excluded MCM2; and, conversely, hexameric complexes of MCM2 to MCM7 lacked MCM-BP, indicating that MCM-BP can replace MCM2 in the MCM complex. MCM-BP-containing complexes exhibited increased stability under experimental conditions relative to those containing MCM2. MCM-BP also formed a complex with the MCM4/6/7 core helicase in vitro, but, unlike MCM2, did not inhibit this helicase activity. A proportion of MCM-BP bound to cellular chromatin in a cell cycle-dependent manner typical of MCM proteins, and, like other MCM subunits, preferentially associated with a cellular origin in G₁ but not in S phase. In addition, down-regulation of MCM-BP decreased the association of MCM4 with chromatin, and the chromatin association of MCM-BP was at least partially dependent on MCM4 and cdc6. The results indicate that multicellular eukaryotes contain two types of hexameric MCM complexes with unique properties and functions.

We gratefully acknowledge Jack Greenblatt and Mahel Zeghouf for pMZI and pMZS3F vectors. We also thank Rolf Knippers and Masahide Ishibashi for cDNA clones of human MCM6 and MCM7, respectively, and Grant Brown for critical reading of the manuscript.

This work was supported by a grant from the Canadian Institutes of Health Research (CIHR) to L.F. and by a CIHR postdoctoral fellowship to A.M.S. L.F. is a Canada Research Chair in Molecular Virology.