title: Polymorphism and double hexamer structure in the archaeal minichromosome maintenance (MCM) helicase from Methanobacterium thermoautotrophicum.
abstract: Methanobacterium thermoautotrophicum minichromosome maintenance complex (mtMCM), a cellular replicative helicase, is a useful model for the more complex eukaryotic MCMs. Biochemical and crystallographic evidence indicates that mtMCM assembles as a double hexamer (dHex), but previous electron microscopy studies reported only the presence of single heptamers or single hexamers (Pape, T., Meka, H., Chen, S., Vicentini, G., Van Heel, M., and Onesti, S. (2003) EMBO Rep. 4, 1079-1083; Yu, X., VanLoock, M. S., Poplawski, A., Kelman, Z., Xiang, T., Tye, B. K., and Egelman, E. H. (2002) EMBO Rep. 3, 792-797). Here we present the first three-dimensional electron microscopy reconstruction of the full-length mtMCM dHex in which two hexamers contact each other via the structurally well defined N-terminal domains. The dHex has obvious side openings that resemble the side channels of LTag (large T antigen). 6-fold and 7-fold rings were observed in the same mtMCM preparation, but we determined that assembly as a double ring favors 6-fold structures. Additionally, open rings were also detected, which suggests a direct mtMCM loading mechanism onto DNA.
authors.affiliation: Biocomputing Unit, Centro Nacional de Biotecnología, Consejo Superior de Investigaciones Científicas, Darwin 3, 28049 Madrid, Spain.
authors.forename: Yacob
authors.initials: Y
authors.lastname: Gómez-Llorente
authors.forename: Ryan J
authors.initials: RJ
authors.lastname: Fletcher
authors.forename: Xiaojiang S
authors.initials: XS
authors.lastname: Chen
authors.forename: José M
authors.initials: JM
authors.lastname: Carazo
authors.forename: Carmen
authors.initials: C
authors.lastname: San Martín
mesh_terms: D019847:Chromosomes, Archaeal / Q000737:chemistry / Q000378:metabolism*; D004265:DNA Helicases / Q000737:chemistry* / Q000235:genetics*; D004269:DNA, Bacterial / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics; D017014:Methanobacterium / Q000201:enzymology*; D008854:Microscopy, Electron; D008958:Models, Molecular; D011110:Polymorphism, Genetic; D017510:Protein Folding; D011994:Recombinant Proteins / Q000737:chemistry
chemical_list: D004269:DNA, Bacterial; D011994:Recombinant Proteins; D004265:DNA Helicases