Hexameric ring structure of the full‐length archaeal MCM protein complex

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Abstract
In eukaryotes, a family of six homologous minichromosome maintenance (MCM) proteins has a key function in ensuring that DNA replication occurs only once before cell division. Whereas all eukaryotes have six paralogues, in some Archaea a single protein forms a homomeric assembly. The complex is likely to function as a helicase during DNA replication. We have used electron microscopy to obtain a three‐dimensional reconstruction of the full‐length MCM from Methanobacterium thermoautotrophicum. Six monomers are arranged around a sixfold axis, generating a ring‐shaped molecule with a large central cavity and lateral holes. The channel running through the molecule can easily accommodate double‐stranded DNA. The crystal structure of the amino‐terminal fragment of MCM and a model for the AAA+ hexamer have been docked into the map, whereas additional electron density suggests that the carboxy‐terminal domain is located at the interface between the two domains. The structure suggests that the MCM complex is likely to act in a different manner to traditional hexameric helicases and is likely to bear more similarity to the SV40 large T antigen or to double‐stranded DNA translocases.