MCM Proteins in DNA Replication

Abstract

The MCM proteins are essential replication initiation factors originally identified as proteins required for minichromosome maintenance in Saccharomyces cerevisiae. The best known among them are a family of six structurally related proteins, MCM2-7, which are evolutionally conserved in all eukaryotes. The MCM2-7 proteins form a hexameric complex. This complex is a key component of the prereplication complex that assembles at replication origins during early G1 phase. New evidence suggests that the MCM2-7 proteins may be involved not only in the initiation but also in the elongation of DNA replication. Orchestration of the functional interactions between the MCM2-7 proteins and other components of the prereplication complex by cell cycle-dependent protein kinases results in initiation of DNA synthesis once every cell cycle.

Perpetuating the double helix: molecular machines at eukaryotic DNA replication origins

“…But there is also evidence that the six eukaryotic MCMs can form a heterohexamer even when not associated with the chromatin (Ref. 48; our unpublished observations). If such a hexameric (or dodecameric) form of the MCMs were recruited to the origins, the ring would have to be momentarily open to engage the DNA (Fig.…”

“…3A). This model seems plausible because MCM proteins form different subcomplexes in vivo, (48) and the binding of Xenopus Mcm2, Mcm4 and Figure 2. Possible mechanisms of replicative helicase action.…”

“…However, they are not redundant because all are essential in yeast (reviewed in Ref. 48). They are required for initiation as well as elongation of DNA replication because inactivation of any of the six yeast proteins during G 1 prevented entry into S phase and inactivation during S phase impaired progression of the existing replication forks.…”

The mechanism and regulation of deadenylation: Identification and characterization of Xenopus PARN

“…Peptide sequence data were obtained from purified p62 xPARN by mass spectrometry (see Fig+ 3 for peptide positions)+ As previously reported, these peptides were highly homologous to the human poly(A) ribonuclease (hPARN) amino acid sequence (Korner et al+, 1998)+ Taking advantage of this similarity, an hPARN cDNA probe was used to screen a Xenopus oocyte cDNA library at low stringency+ A 1+53-kb clone was obtained that was 75% identical to the 59 coding region of the human cDNA, but which lacked the 39 portion and did not contain an in-frame stop codon+ To obtain a fulllength clone, the partial sequence was used to screen a Xenopus kidney l ZAP II cDNA library+ Two clones were obtained from this library from a screen of ;600,000 plaques+ The larger of the clones (2 kb) overlapped the original clone and contained all of the coding region except for the first 16 nt+ The predicted molecular weight of full-length xPARN is 72+8 kDa+ The deduced amino acid sequences for xPARN and hPARN are shown in Figure 3+ Although the two proteins share 72% overall identity, there is significantly more heterogeneity in the C-terminal region than elsewhere, suggesting a possible divergence of function+ No other proteins of known function are similar to human or Xenopus PARN, but related hypothetical proteins from several species (Arabidopsis thaliana, Schizosaccharomyces pombe, and Caenorhabditis elegans) are present in the nonredundant database+ In addition to the conserved tripartite RNase D exonuclease domains (Korner et al+, 1998), xPARN also contains motifs that were not reported for the human enzyme+ One of these is a region that is similar to a functionally undefined domain conserved among helicases known as minichromosome maintenance proteins (MCM; amino acids 132-152)+ Although the conserved region falls outside the Walker A and Walker B motifs known to be required for nucleotide binding, other conserved regions, which may confer nucleotide specificity, have not been characterized (Tye, 1999)+ The xPARN sequence also contains a putative RNA recognition motif (RRM; amino acids 446-502) that may contribute to its poly(A)-binding activity (Siomi & Dreyfuss, 1997)+ …”

The malarial CDK Pfmrk and its effector PfMAT1 phosphorylate DNA replication proteins and co-localize in the nucleus

“…The MCM2-7 proteins form a hexameric complex that assembles at replication origins during early G1 phase [20]. Coordination of the functional interactions between the MCM2-7 proteins and other components of the pre-RC is mediated by CDKs [19,21]. Subsequently, loading of DNA polymerase occurs and is mediated by the replication factor complex (RFC) [22].…”