Structure and function of archaeal MCM helicase

The mini-chromosome maintenance protein (MCM) complex is an essential helicase for DNA replication in Archaea and Eukaryotes. While the eukaryotic complex consists of six homologous proteins (MCM2-7), the archaeon Sulfolobus solfataricus has only one MCM protein (ssoMCM), six subunits of which form a homohexamer. Here, I first discuss my 4.35A crystal structure of the near full-length ssoMCM. The structure shows an elongated fold, with five sub-domains that are organized into two large N- and C-terminal domains. A near full-length ssoMCM hexamer generated based on the 6-fold symmetry of the N-terminal Methanothermobacter thermautotrophicus (mtMCM) hexamer shows inter-subunit distances suitable for bonding contacts, including the interface around the ATP pocket. Four unusual beta-hairpins of each subunit are located inside the central channel or around the side channels in the hexamer. Additionally, the hexamer fits well into the double-hexamer EM map of mtMCM. Mutational analysis of residues at the inter-subunits interface and around the side channels demonstrates their critical roles for hexamerization and helicase function. I also present a series of 25 structure based mutations, 9 of 10 of which have been characterized by DNA binding, ATPase and helicase assays. Finally, modeling of ATP binding and hydrolysis based on the above evidence is presented.