The Methanobacterium thermoautotrophicum MCM protein can form heptameric rings

Abstract

Mini-chromosome maintenance (MCM) proteins form a conserved family found in all eukaryotes and are essential for DNA replication. They exist as heteromultimeric complexes containing as many as six different proteins. These complexes are believed to be the replicative helicases, functioning as hexameric rings at replication forks. In most archaea a single MCM protein exists. The protein from Methanobacterium thermoautotrophicum (mtMCM) has been reported to assemble into a large complex consistent with a dodecamer. We show that mtMCM can assemble into a heptameric ring. This ring contains a C-terminal helicase domain that can be fit with crystal structures of ring helicases and an N-terminal domain of unknown function. While the structure of the ring is very similar to that of hexameric replicative helicases such as bacteriophage T7 gp4, our results show that such ring structures may not be constrained to have only six subunits.

Biochemical Characterization of the Methanothermobacter thermautotrophicus Minichromosome Maintenance (MCM) Helicase N-terminal Domains

“…Although the mtMCM appears to form double hexamers (10 -13), the enzymes from S. solfataricus (16), A. fulgidus, and Aeropyrum pernix (18) are hexamers. Electron micrograph reconstruction of mtMCM revealed ring-shaped hexamers (20) or heptamers (21), but no dodecamers could be observed. The observation that the M. thermautotrophicus enzyme forms only single ring structures at low concentrations (such as those used for electron microscopy) may suggest that the double hexamers are formed by nonspecific hydrophobic or ionic interactions involving the two hexamers.…”

“…The crystal structure of the N-terminal portion of the mtMCM protein suggested a dodecameric structure (19). However, electron microscope reconstructions of fulllength mtMCM revealed hexameric (20) or heptameric (21) structures, but not larger multimers.…”

A Novel Zinc Finger Is Required for Mcm10 Homocomplex Assembly

“…Consistent with this notion, the Mcm10 -43 mutant allele defective in Mcm10 selfinteraction is also defective in interacting with Mcm7, a subunit of the MCM2-7 complex known to interact with Mcm10 (43). A number of studies have suggested that the MCM2-7 complex functions as a double hexameric structure (45,46). Although it is conceivable that the ϳ800-kDa Mcm10 homocomplexes may assume a similar configuration that facilitates their interactions with the MCM2-7 complex, the stoichiometry of the Mcm10 homocomplexes remains to be determined.…”

Cell‐cycle‐dependent regulation of DNA replication and its relevance to cancer pathology

“…These and other observations have led to an alternative model in which the MCM complex functions like a molecular rotary pump that is tethered during S phase and uses ATP hydrolysis to spool DNA through its pore, thereby unwinding it from a distance [7]. Structural studies have confirmed that the central pore of the MCM heterohexamer is large enough to accommodate the DNA double helix [8,9].…”