Biology Reference
In-Depth Information
exclusively detected in the presence of manganese and its endonuclease activity
is also strongly stimulated by ATP, but not other adenosine compounds.
96
Interestingly, the addition of Zn
2
þ
and Co
2
þ
to reactions containing Mn
2
þ
ions stimulates the endonuclease activity of
B. subtilis
MutL significantly,
reinforcing the idea that two different metal ions with distinct roles are re-
quired for the endonuclease activity of MutL.
96
However, in the presence of
metal ion mixtures, cutting of the two strands of the duplex is favored over
nicking, presumably due to the presence of the two active sites in the
B. subtilis
MutL homodimer.
96
The endonuclease activity of
N. gonorrhoeae
MutL is more promiscuous
than that of
B. subtilis
or eukaryotic MutL homologs and can be activated by
both Mn
2
þ
and Mg
2
þ
, and to a lesser extent by Ca
2
þ
.
92
In contrast to what has
been reported for human, yeast, and
B. subtilis
MutL, ATP inhibits the
endonuclease activity of full-length
N. gonorrhoeae
MutL.
67,94,96
ATP had
also been shown to inhibit the endonuclease activity of
Thermus thermophilus
and
Aquifex aeolicus
MutL,
93
though inhibition was later ascribed to a concen-
tration effect.
95
It is cumbersome that mutation of key residues in the endo-
nuclease motif of
N. gonorrhoeae
MutL (D491N or E502Q,
D
QHAX
2
E
X
4
E)
barely affects the endonuclease activity of the protein.
92
However, the double
mutation of Asp491 and Glu502 radically reduces the endonuclease activity of
the protein, indicating that this motif is indeed responsible for this activity.
92
While some of the apparently conflicting results obtained with
N. gonorrhoeae
MutL have been partly resolved in studies using
A. aeolicus
and
T. thermo-
philus
MutL,
93,95,116
detailed mechanistic studies are still required to under-
stand the catalysis of this reaction.
IV. Regulation of the Endonuclease Activity of MutL
In vivo
, the nicking activity of MutL must be tightly regulated to ensure its
mismatch dependency. Modrich and coworkers found that PCNA and RFC
stimulate the activity of MutL
a
in both homo- and heteroduplexes.
67
This
situation is reminiscent of the MutH activation by MutL, where MutH stimu-
lates ATP hydrolysis by MutL and this, in turn, can activate the endonuclease
activity of MutH even in the absence of a mismatch.
97,140
However, it is the
formation of a MutS-MutL-heteroduplex complex that fully activates the
endonuclease activity of MutH.
25
Similarly, it is the presence of MutS
a
that preferentially stimulates the endonuclease activity of MutL on heterodu-
plex DNA.
67
In this section, we discuss how DNA and ATP binding, as well
as the interaction with other repair factors, regulate the endonuclease activity
of MutL.
