The minichromosome maintenance (MCM) complex is thought to function as the replicative helicase in archaea, separating the two strands of chromosomal DNA during replication. The catalytic activity resides within the C-terminal region of the MCM protein, while the N-terminal portion plays an important role in DNA binding and protein multimerization. An alignment of MCM homologues from several archaeal species revealed a number of conserved amino acids. Here several of the conserved residues located on the surface of the helicase have been mutated and their roles in MCM functions determined. It was found that some mutations result in increased affinity for ssDNA while the affinity for dsDNA is decreased. Other mutants exhibit the opposite effect. Thus, the data suggest that these conserved surface residues may participate in MCM-DNA interactions. 1. Introduction The minichromosome maintenance (MCM) helicase is thought to function as the replicative helicase in eukarya and archaea. Most archaeal species contain a single MCM homologue with biochemical properties that are similar to the eukaryotic enzyme. Both archaeal and eukaryal MCM helicases exhibit ATP-dependent 3′-5′ helicase activity, can bind and translocate along single-stranded (ss) and double-stranded (ds) DNA, unwind DNA-RNA hybrids while translocating on the DNA strand, and can displace proteins from DNA (reviewed in [1–3]). The archaeal MCM protein can be divided into three parts; the N-terminal region, the AAA+ catalytic core, and a C-terminal region that may form a helix-turn-helix (HTH) domain [4–7]. The three-dimensional structure of the N-terminal portion of the Methanothermobacter thermautotrophicus and Sulfolobus solfataricus MCM proteins revealed a three domain architecture [4, 5]. Biochemical studies showed that domain A participates in regulating helicase activity, domain B participates in DNA binding, and domain C is involved in hexamer formation, DNA binding and communication between the N-terminal part and the catalytic domains (reviewed in: [1, 3]). Although the N-terminal portion of MCM is less conserved than the AAA+ region, an alignment of the N-terminal region from a number of archaeal species revealed the presence of several highly conserved residues, particularly in domain C (Figure 1(a)). Several of these residues have previously been reported to play an essential role in communicating between the N-terminal DNA binding and C-terminal catalytic activity [8, 9]. In this study, several conserved residues in domain C of the M. thermautotrophicus MCM protein were individually
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