The Minichromosome Maintenance Replicative Helicase

The eukaryotic replicative helicase, the minichromosome maintenance (MCM) complex, is composed of six distinct, but related, subunits MCM(2–7). The relationship between the sequences of the subunits indicates that they are derived from a common ancestor and indeed, present-day archaea possess a homohexameric MCM. Recent progress in the biochemical and structural studies of both eukaryal and archaeal MCM complexes are beginning to shed light on the mechanisms of action of this key component of the replisome.The minichromosome maintenance (MCM) complex subunits are members of the AAA⁺ superfamily of ATPases and thus use energy derived from cycles of ATP binding and hydrolysis to move or reorganize bound substrates. In the case of the MCM complex, the energy is harnessed to effect DNA unwinding. The AAA⁺ proteins can be classified into seven distinct clades, based on the topography of their active sites. MCMs are members of clade 7, being characterized by the presence of an additional α-helix when compared with the classical AAA⁺ fold (Iyer et al. 2004; Erzberger and Berger 2006). In addition to the AAA⁺ domain, MCMs also have an amino-terminal domain (NTD) that plays a role in higher-order structure assembly. Finally, following the AAA⁺ domain is a degenerate winged helix (wH) structure (Fig. 1). Although the archaeal MCMs possess this simple NTD–AAA⁺–wH domain architecture, many of the eukaryal MCM(2–7) subunits are embellished with amino- or carboxy-terminal extensions that play roles in the regulation or recruitment of MCM(2–7). The eukaryal MCM complex is an important target for regulatory posttranslational modifications; the nature and consequences of these modifications are dealt with in Bell and Kaguni (2013), Tanaka and Araki (2013), and Siddiqui et al. (2013). Much of what we know regarding the inner workings of the MCM helicase has been learned from structural and mechanistic studies of the simple archaeal model (Sakakibara et al. 2009). Additionally, the crystal structures of distantly related superfamily three helicases, SV40 LTAg, and the E1 helicase of bovine papilloma virus, have provided important structural frameworks for understanding the mode of action of hexameric helicases (Gai et al. 2004; Enemark and Joshua-Tor 2006, 2008). In the following, we shall describe structural and mechanistic insights derived from studies of the simpler archaeal MCMs before extending our discussion to the eukaryotic assembly.Open in a separate windowFigure 1.(A) Linear representation of a monomer of the archaeal MCM. (Gray) The central AAA⁺ domain; (white) the flanking amino-terminal domains and winged helix (wH). The position of key secondary structural elements—(Zn) zinc-binding; (ACL) allosteric communication loop; (NBH) amino-terminal β-hairpin; (EXT-HP) external β-hairpin; (H2I) helix 2 insert; and (PS1BH) pre-sensor 1 β-hairpin—are indicated above the figure and shown by colored blocks, the colors corresponding to those used in panels B and C. Key residues involved in the ATPase active site are indicated below the figure. (Orange lines) A, B, and S1, shown as orange lines are the Walker A lysine (K346), Walker B glutamate (E404), and Sensor 1 asparagine (N448), respectively, and constitute “cis”-acting residues. (Black lines) “Trans”-acting residues T1 (R331), T2 (Q423), arginine finger (R473), and Sensor 2 (R560). Numbering is from SsoMCM.(B) Structure of a monomer of SsoMCM (lacking detail of the wH domain). Secondary structure elements are labeled and colored in cartoon format, using the color scheme in panel A. (Purple spheres) The atoms of the zinc-coordinating residues; (orange spheres) the cis-acting residues; (black spheres) the trans-residues.(C) Model of a symmetric hexamer of SsoMCM. (Left) View down the central cavity of SsoMCM, looking from the carboxy-terminal face. (Right) The same hexamer rotated 90° to show a side view. The two tiers corresponding to the amino-terminal and AAA⁺ domains are indicated. The color scheme is as in panels A and B. Panels B and C were generated from PDB entry 3F9V using PyMOL (http://www.pymol.org).