Eukaryotic MCM proteins: beyond replication initiation

Abstract

The minichromosome maintenance (or MCM) protein family is composed of six related proteins that are conserved in all eukaryotes. They were first identified by genetic screens in yeast and subsequently analyzed in other experimental systems using molecular and biochemical methods. Early data led to the identification of MCMs as central players in the initiation of DNA replication. More recent studies have shown that MCM proteins also function in replication elongation, probably as a DNA helicase. This is consistent with structural analysis showing that the proteins interact together in a heterohexameric ring. However, MCMs are strikingly abundant and far exceed the stoichiometry of replication origins; they are widely distributed on unreplicated chromatin. Analysis of mcm mutant phenotypes and interactions with other factors have now implicated the MCM proteins in other chromosome transactions including damage response, transcription, and chromatin structure. These experiments indicate that the MCMs are central players in many aspects of genome stability.

FIG. 1.

Phylogenetic tree of eukaryotic MCMs, assembled using ClustalX (ftp://ftp-igbmc.u-strasbg.fr/pub/ClustalX/) and Phylip 3.6 (http://evolution.genetics.washington.edu/phylip.html) for Macintosh. Colors correspond to the seven MCM subfamilies. Dashed line, loose relationship. Accession numbers are as follows. S. pombe: SpMcm2, CAB58403; SpMcm3, P30666; SpMcm4, P29458, SpMcm5, CAA93299 and CAB61472; SpMcm6, CAB75412; SpMcm7, O75001. S. cerevisiae: ScMcm2, NP_009530; ScMcm3, NP_010882; ScMcm4, S56050; ScMcm5, A39631; ScMcm6, NP_011314; ScMcm7, S34027. Human: HsMcm2, P49736; HsMcm3, P25205; HsMcm4, NP_005905; HsMcm5, AAH03656; HsMcm6, NP_005906; HsMcm7, P33993; HsMcm8, NP_115874. Xenopus: Xmcm2, JC5085; Xmcm3, I51685; Xmcm4, T47223; Xmcm5, PC4225; Xmcm6Z, AAC41267; Xmcm6, T47222; Xmcm7, T47221. Arabidopsis: AtMcm2, NP_175112.1; AtMcm3, NP_199440.1; AtMcm4, NP_179236.2; AtMcm5, NP_178812.1; AtMcm6, NP_680393.1; AtMcm7, NP_192115.1; AtMcm8?, NP_187577.1; unknown Mcm, NP_179021.1. Drosophila: DmMcm2, AAF54207; DmMcm3, NP_511048.2; DmMcm4, S59872; DmMcm5, NP_524308.2; DmMcm6, NP_511065.1; DmMcm7, NP_523984.1.

FIG. 2.

Structural features of the Mcm protein family, derived from the alignment used in Fig.1. Abbreviations: Z, zinc finger; A, Walker A ATPase motif; B, Walker B ATPase motif; R, arginine finger motif.

FIG. 3.

Pairwise interactions in vitro suggest this organization of the MCM heterohexamer. The P-loop (Walker A motif) and SRF motif (arginine finger) are proposed to act together as ATPase domains. In isolation, this complex does not have helicase activity in vitro (see the text). Reprinted from reference with permission from the publisher.

FIG. 4.

Model for replication initiation driven by MCM proteins. Panels A to F show stages of replication that are described in the text.

FIG. 5.

Associations between MCMs and other factors are grouped by function and the MCM subunit with which they interact. Interactions with Mcm7 occur through its C terminus, while interactions with Mcm2 occur through its N terminus. Abbreviations: E6, papillomavirus E6 protein; FHL2, heart-specific LIM-domain protein; Hbo1, MYST family HAT; Rad17, checkpoint protein; RNAPol-CTD; carboxy-terminal domain of RNA pol II. For references and discussion, see the text.