Mini-chromosome Maintenance (MCM) proteins play an essential role in both initiation and elongation phases of DNA replication in Eukarya. Genes encoding MCM homologs are present also in the genomic sequence of Archaea and the MCM-like protein from the euryarchaeon Methanobacterium thermoautotrophicum (Mth MCM) was shown to possess a robust ATP-dependent 3-5 DNA helicase activity in vitro. Herein, we report the first biochemical characterization of a MCM homolog from a crenarchaeon, the thermoacidophile Sulfolobus solfataricus (Sso MCM). Gel filtration and glycerol gradient centrifugation experiments indicate that the Sso MCM forms single hexamers (470 kDa) in solution, whereas the Mth MCM assembles into double hexamers. The Sso MCM has NTPase and DNA helicase activity, which preferentially acts on DNA duplexes containing a 5-tail and is stimulated by the single-stranded DNA binding protein from S. solfataricus (Sso SSB). In support of this functional interaction, we demonstrated by immunological methods that the Sso MCM and SSB form protein⅐protein complexes. These findings provide the first in vitro biochemical evidence of a physical/functional interaction between a MCM complex and another replication factor and suggest that the two proteins may function together in vivo in important DNA metabolic pathways. Mini-chromosome Maintenance (MCM)1 genes (MCMs 2-7) were originally identified in budding and fission yeast by a genetic analysis of mutants that were unable to efficiently replicate mini-chromosomes (1, 2). Homologs of the six yeast MCM genes were subsequently identified in various other eukaryotic organisms, from Drosophila melanogaster to Homo sapiens and found to code for proteins (ranging in length from 776 to 1017 amino acidic residues), which are evolutionarily conserved especially in the central third of their polypeptide chain (3, 4). In fact, this region contains the four sequence motifs typically found in DNA helicases, including the Walker A and B boxes that are critical for nucleotide binding and hydrolysis (5). The MCM proteins are relatively abundant in proliferating cells and were purified from cell extracts of various organisms either as hetero-hexameric complexes containing all six polypeptides or as sub-assemblies of various subunit composition (such as MCM 2/4/6/7 and MCM 4/6/7 (6 -10)). However, among all these multimeric complexes only the MCM 4/6/7 hexamer was demonstrated to have a weak and nonprocessive DNA helicase activity (11-13). The MCM 4/6/7 complex is dis-assembled in vitro upon addition of MCM 2 or MCM 3/5, and this causes inhibition of its DNA unwinding activity (13,14). Based on these findings, it was proposed that the MCM 4/6/7 assembly could act as DNA unwinding factor at the replication origins, whereas the other MCM subunits could play regulatory functions. However, due to the limited processivity of their DNA unwinding activity the MCM proteins were considered poor candidates for the helicase associated with the DNA replication fork. In addition, several genetic studie...
Cdc6 proteins play an essential role in the initiation of chromosomal DNA replication in Eukarya. Genes coding for putative homologs of Cdc6 have been also identified in the genomic sequence of Archaea, but the properties of the corresponding proteins have been poorly investigated so far. Herein, we report the biochemical characterization of one of the three putative Cdc6-like factors from the hyperthermophilic crenarchaeon Sulfolobus solfataricus (SsoCdc6-1). SsoCdc6-1 was overproduced in Escherichia coli as a His-tagged protein and purified to homogeneity. Gel filtration and glycerol gradient ultracentrifugation experiments indicated that this protein behaves as a monomer in solution (molecular mass of about 45 kDa). We demonstrated that SsoCdc6-1 binds single-and double-stranded DNA molecules by electrophoretic mobility shift assays. SsoCdc6-1 undergoes autophosphorylation in vitro and possesses a weak ATPase activity, whereas the protein with a mutation in the Walker A motif (Lys-59 3 Ala) is completely unable to hydrolyze ATP and does not autophosphorylate. We found that SsoCdc6-1 strongly inhibits the ATPase and DNA helicase activity of the S. solfataricus MCM protein. These findings provide the first in vitro biochemical evidence of a functional interaction between a MCM complex and a Cdc6 factor and have important implications for the understanding of the Cdc6 biological function.
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