Biology Reference
In-Depth Information
Moreover, PCNA apparently disrupts MutS-MutL-heteroduplex com-
plexes,
154
reinforcing the idea that the role of the processivity clamp is likely
transferring information between sequential mismatch repair steps. Therefore,
it has been proposed that MutS
a
also exerts its effect on the endonuclease
activity of MutL
a
through the direct interaction between the two proteins.
139
Despite being critical for mismatch repair, the interaction between MutS
and MutL is poorly characterized.
69,71,155,156
MutS-MutL complexes form
only transiently in the presence of heteroduplex DNA and ATP.
25,69,155
How-
ever, the two subunits of the MutS and MutL dimers have distinct nucleotide
affinities and intrinsic ATPase activities, in turn allowing for a significant
number of nucleotide-bound states and conformations that have obscured
the detailed characterization of the MutS-MutL-heteroduplex complex.
1,3,52
The formation of these complexes has been studied in the context of three
different models—translocation, molecular switch, and stationary—that are at
odds as to whether MutS-MutL complexes stay bound to the mismatch or
move away from it to signal nicking of the newly replicated strand.
69,157-162
Whether moving or stationary, all models agree that ATP binding and hydro-
lysis regulate the formation of the MutS-MutL complex and, in turn, the
activation of downstream effectors in the repair response. ATP binding, but
not hydrolysis, by yMutS
a
is required to form a ternary MutS-MutL-DNA
complex.
69
Conversely, nucleotide binding by MutL does not seem to be
critical for the interaction with MutS,
28,156
even though the ATPase domain
of MutL mediates the interaction.
70,71,86
It is plausible that the interaction with
MutS affects ATP or DNA binding to MutL, indirectly regulating its endonu-
clease activity. However, probing this idea necessitates a detailed analysis of the
MutS-MutL-heteroduplex complex.
V. Concluding Remarks
Six years ago, Modrich and coworkers discovered that eukaryotic MutL
a
is
a latent endonuclease that preferentially nicks the discontinuous strand of a
mismatched DNA.
67
The subsequent characterization of the endonuclease
activity of other MutL homologs has provided important clues as to how
organisms lacking the
mutH
gene—both eukaryotes and prokayotes—target
the newly replicated strand during mismatch repair. We now understand how
MutL nicks DNA, how this activity is regulated by ATP binding, and how it is
stimulated by the interaction with the processivity clamp. These findings have
profoundly advanced our understanding of MutH-independent mismatch re-
pair, but an important question remains unanswered: How does MutS affect
the endonuclease activity of MutL and, in turn, impose the mismatch depen-
dency on the reaction? As we move forward, it is imperative to understand how
