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 G 1 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.The initiation of DNA replication in eukaryotic cells is a carefully regulated process requiring the orchestrated assembly of many proteins at origin sites, including the origin recognition complex and minichromosome maintenance (MCM) complex. The MCM complex consists of six subunits, MCM2 through MCM7 (MCM2-7), which form a hexamer. Studies in Saccharomyces cerevisiae, where MCM proteins were first identified (25), showed that each of the MCM subunits performs an essential function in the initiation and elongation of DNA replication (20,21). Genetic and biochemical studies conducted in yeast, Xenopus, Drosophila, and mammals point to probable roles of the MCM proteins in melting origin DNA and in functioning as the replicative helicase at replication forks (8,27).Biochemical analyses of the MCM complex have shown that MCM4, -6, and -7 are the most stably associated subunits, referred to as the helicase core (MCM4/6/7), with MCM2 and a dimer of MCM3 and MCM5 being more loosely associated with the core (13,23,30,36). MCM4, MCM6, and MCM7 on their own can form hexamers with weak but measurable DNA helicase activity. The addition of MCM2 to the MCM4/6/7 core complex disrupts the hexamer and inhibits DNA helicase activity (12, 23). The complete MCM2-7 complex has no detectable helicase activity in vitro (12, 23), but helicase activity has been reported for a larger complex containing MCM2-7, cdc45, and GINS (29). As expected, MCM complexes exhibit ATPase activity (6,23,35). ATPase activity has not been observed in individual MCM subunits but occurs when certain pairs of MCM proteins interact (6).Each of the six MCM subunits shares a region of homology referre...

The Fission Yeast Minichromosome Maintenance (MCM)-binding Protein (MCM-BP), Mcb1, Regulates MCM Function during Prereplicative Complex Formation in DNA Replication

“…Following the first identification in humans, several studies aimed to characterize and elucidate the functions of MCM-BP. Co-immunoprecipitation analysis showed that human MCM-BP interacts with all MCM subunits, except Mcm2; conversely, Mcm2 fails to recover MCM-BP (26). A similar pattern of interaction between MCM-BP and MCM subunits was observed in Xenopus laevis (27), suggesting that an alternative MCM complex is formed when Mcm2 is replaced by MCM-BP.…”

“…Immunoprecipitation and/or mass spectrometry studies in humans, Xenopus, and fission yeast showed that MCM-BPs (hMCM-BP for human, xMCM-BP for Xenopus, and Mcb1 for fission yeast) interact with all MCM complex subunits, except Mcm2 (26,27,29,30). Therefore, MCM-BP was thought to replace Mcm2 within the MCM complex (26); however, in A. thaliana, AtMcm2 was found to co-purify with ETG1, the A. thaliana MCM-BP ortholog, by mass spectrometry analysis (28).…”

“…MCM-binding protein (MCM-BP) was first identified as a protein that strongly associates with human MCM proteins. Tandem affinity purification with tagged human MCM core subunits recovers both MCM-BP and MCM proteins (26). MCM-BP is conserved in most eukaryotes (except budding yeast and Caenorhabditis elegans) and has only limited homology to MCM proteins.…”

Modification, Development, Application and Prospects of Tandem Affinity Purification Method

“…The application of TAP method has made considerable progress in mammalian systems (Bürckstümmer et al, 2006;Davison et al, 2009;Drakas et al, 2005;Gottlieb and Jackson, 1993;Holowaty et al, 2003;Jeronimo et al, 2007;Kamil and Coen, 2007;Knuesel et al, 2003;Lehmann et al, 2009;Milewska et al, 2009;Sakwe et al, 2007). For instance, human active SMAD3 protein complex was purified from cell lysates through TAP method, and HSP70 was identified as a novel combination partner of SMAD3 (Knuesel et al, 2003).…”

“…Therefore, an additional purification step might be reqired to resolve this trouble, which would be time consuming and lead to more sample loss. Sakwe et al identified a new form of the minichromosome maintenance (MCM) complex in human cells (Sakwe et al, 2007). In another study using the TAP process, Holowaty et al (Holowaty et al, 2003) expressed Epstein-Barr nuclear antigen-1 (EBNA1) protein in fusion with a TAP tag at the C-terminus in human 293T cells.…”