Biology Reference
In-Depth Information
activity.
42,43
MutL also interacts with several single-strand exonucleases
87
and it
has been shown to interact with the processivity clamp (
b
) and its clamp
loader.
47,88
Beyond mismatch repair, MutL activates the very short patch repair
pathway through its interaction with the endonuclease vsr, the base excision
repair pathway through its interaction with the MutY glycosylase, and nucleo-
tide excision repair through its interaction with the UvrB nuclease.
85,89-91
Given the extent of their interactomes, it is not surprising that MutL
proteins are classified as molecular matchmakers.
31
However, the discovery
of the endonuclease activity of MutL
a
and later on in bacterial MutL homologs
from organisms lacking the
mutH
gene
67,92-96
has brought MutL from behind
the scenes to center stage.
II. MutL is a Multidomain Protein
MutL is composed of two structured domains connected by a flexible
linker. The N-terminal region of the protein (
330 residues) encompasses an
ATPase domain that is highly conserved from
E. coli
to humans.
97,98
The
C-terminal region (
200 residues) mediates either homo- (in prokaryotes) or
hetero- (in eukaryotes) dimerization. Despite the limited sequence conservation of
this domain, secondary structure prediction and structural studies have revealed
that this region of the protein is structurally conserved for most MutL homo-
logs.
42,96,99
The metal-binding motif associated with the endonuclease activity
found in some MutL proteins is located in this region of the protein.
67,93-96
The linker connecting these two conserved regions (100-150 residues) is variable
in sequence and susceptible to protease degradation, reinforcing the idea of its
flexibility and lack of defined structure.
42,97
ATP binding at the N-terminal region of the protein triggers the engage-
ment of the ATPase domains and a substantial conformational change that has
been elegantly monitored by size-exclusion chromatography using purified
E. coli
MutL (
Fig. 2
).
42,97
Similar conformational changes have also been
observed for eukaryotic MutL
a
.
100,102-104
Importantly, atomic force microsco-
py has demonstrated that ATP binding induces asymmetric conformational
changes in human and yeast MutL
a
that yield four major conformations of
the heterodimer.
100
Comparison of the ATPase activities of yeast MLH1 and PMS1 (the yeast
homolog of human PMS2) has revealed important differences between the two
protomers of MutL
a
. Yeast MLH1 binds ATP with higher affinity than yeast
PMS1.
102
Additionally, the effect of replacing key residues in the ATPase site of
yMLH1 is more deleterious than generating the equivalent mutations in the
ATPase site of yPMS1—an effect that is also seen for human MLH1 and
PMS2.
102-104
Importantly, mutations in the ATPase site of hMLH1 disrupt
