| Literature References | 1. KUBOTA, Y., MIMURA, S., NISHIMOTO, S., MASUDA, T., NOJIMA, H. AND TAKISAWA, H.
Licensing of DNA replication by a multi-protein complex of MCM/P1 proteins in
Xenopus eggs.
EMBO J. 16 3320-3331 (1997).
2. STILLMAN, B.
Initiation of chromosomal DNA replication in eukaryotes. Lessons from lambda.
J.BIOL.CHEM. 269 7047-7050 (1994).
3. DIFFLEY, J.F.
Once and only once upon a time: specifying and regulating origins of DNA
replication in eukaryotic cells.
GENES DEV. 10 2819-2830 (1996).
4. DAHMANN, C., DIFFLEY, J.F. AND NASMYTH, K.A.
S-phase-promoting cyclin-dependent kinases prevent re-replication by
inhibiting the transition of replication origins to a pre-replicative state.
CURR.BIOL. 5 1257-1269 (1995).
5. APARACIO, O.M., WEINSTEIN, D.M. AND BELL, S.P.
Components and dynamics of DNA replication complexes in S.cerevisiae:
redistribution of MCM proteins and Cdc45p during S phase.
CELL 91 59-69 (1997).
6. MACKAY, J.P. AND CROSSLEY, M.
Zinc fingers are sticking together.
TRENDS BIOCHEM.SCI. 23 1-4 (1998).
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| Documentation | Mini-chromosome maintenance (MCM) proteins are a family of eukaryotic
replication factors required for the initiation of DNA replication. All
eukaryotes contain six orthologous MCM proteins, designated MCM2-7. Studies
in Xenopus eggs have showed them to form hexamers, where each class is
present in equal stoichiometry [1]. The initiation of DNA synthesis in
eukaryotes requires the binding of origin recognition complex (ORC) - a
complex of six subunits - to the autonomously replicating sequences (ARS)
of replication origins [2], the recruitment of CDC6 and binding of the MCM
protein complex to the ARS to form the prereplicative complex (pre-RC) [3].
DNA synthesis is subsequently initiated by the activation of pre-RC by CDC7
and CDC28 protein kinases [4].
MCM proteins associate with chromatin during G1 phase and dissociate again
during S phase, remaining unbound until the end of mitosis [5]. Periodic
chromatin association of the MCM complex ensures that DNA synthesis from
replication origins is initiated only once during the cell cycle, avoiding
over-replication of parts of the genome. Elongation of replication forks
away from individual replication origins results in displacement of the
MCM-containing complex from chromatin. Budding yeast MCM proteins are
translocated in and out of the nucleus during each cell cycle. However,
fission yeast MCMs, like those in metazoans, are constitutively nuclear.
The six classes of MCM protein together share a conserved 200 amino acid
residue domain, while sequences within the same class show more extensive
similarity outside this region. The conserved central domain is similar to
the A motif of the Walker-type NTP-binding domain; it also shares similarity
with ATPase domains of prokaryotic NtrC-related transcription regulators.
The ATP-binding motif is thought to mediate ATP-dependent opening of double-
stranded DNA at replication origins. In addition to the central region, MCM2,
4, 6 and 7 contain a zinc-finger-type motif thought to have a role in
mediating protein-protein interactions [6]. Moreover, a conserved alpha-
helical structure in the C-terminal region has been noted; this comprises a
conserved heptad repeat and a putative four-helix bundle. Most of the MCM
proteins contain acidic regions, or alternately repeated clusters of acidic
and basic residues.
In addition to its role as a replication factor, the MCM6 protein has DNA
helicase activity when complexed as a hexamer (containing two molecules each
of MCM4, MCM6 and MCM7), suggesting that this complex is involved in the
initiation of DNA replication as a DNA-unwinding enzyme. Xenopus MCM6
exists in two forms, maternal and zygotic, suggesting that specific
forms of MCM6 may be used at different developmental stages.
MCMPROTEIN6 is a 5-element fingerprint that provides a signature for the
mini-chromosome maintenance (MCM) protein 6. The fingerprint was derived
from an initial alignment of 6 sequences: the motifs were drawn from
conserved regions spanning the full alignment length, focusing on those
sections that characterise the MCM6 proteins but distinguish them from the
rest of the MCM family. Two iterations on SPTR40_18f were required to reach
convergence, at which point a true set comprising 9 sequences was identified.
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