Biology Reference
In-Depth Information
fork, the newly replicated strand is transiently unmethylated. By nicking the
unmethylated strand at these hemimethylated GATC sites, MutH provides an
entry point for MutL-dependent loading of helicase II (also known as UvrD)
and single-strand binding (SSB) protein. Therefore, the primary role of MutH is
to ensure that repair is targeted exclusively to the newly synthesized strand.
Mismatch repair is bidirectional; therefore, mismatches and indels are
repaired regardless of whether MutH nicks the newly synthesized strand 5
0
or 3
0
to the lesion.
34-36
If the nick occurs 5
0
to the mismatch, MutL recruits the
exonucleases RecJ or ExoVII to remove the newly synthesized strand.
37,38
Conversely, ExoI, ExoVII, or ExoX are recruited when MutH nicks 3
0
to the
mismatch.
37-41
5
0
polarity is
recruited, and likely oriented, by MutL to unwind DNA toward the mis-
match.
42-46
Once the erroneous strand has been removed, the SSB protein stabilizes
the gap so that polymerase III and DNA ligase can resynthesize the strand
correctly.
40
Importantly, the
b
subunit of polymerase III—commonly referred
to as
b
, sliding, or processivity clamp, owing to its role in enhancing the
processivity of polymerase III—has binding sites for MutS and MutL, and it
has been suggested that these interactions may be important to sense DNA
lesions and recruit mismatch repair proteins to the sites of damage.
47-49
In both cases, the uvrD helicase, with 3
0
!
B. Strand Discrimination in Mismatch Repair in
Organisms Lacking MutH
While homologs of MutS and MutL are found in all domains of life, the
dam methylase
and
mutH
genes are only found in a subset of
g
-proteobacteria
(
Table I
and
Fig. 1
). A strand discrimination endonuclease has not been
identified in other organisms, but the excision step can be recreated
in vitro
using purified proteins, as far as a nicked DNA is supplied to the reaction.
50
In humans, five paralogs of MutS and four paralogs of MutL have been
identified (
Table I
). MutS paralogs associate to form heterodimers with spe-
cialized functions. Studies in yeast, mice, and humans indicated that MutS
a
(formed by the association of the
M
ut
Sh
omologs MSH2 and MSH6)
is primarily responsible for correcting single-base mismatches or small
indels (
Table I
),
51-53
whereas MutS
b
(formed by the association of MSH2
and MSH3) is primarily responsible for correcting larger indels (up to 16
nucleotides).
54
The other homologs of MutS are involved in mitochondrial
transactions and meiotic recombination.
3
Likewise, three MutL heterodimers
formed by the association of hMLH1 with hPMS2 (hMutL
a
), hPMS1
(hMutL
b
), and hMLH3 (hMutL
g
) have been identified (
Table I
).
55-58
Of
these, MutL
a
is the only MutL heterodimer required for mismatch repair.
56
MutL
g
has a defined role in meiotic recombination
59
and, while no function
