Biology Reference
In-Depth Information
SWR1 is closely related to INO80 that is recruited to the DSB site in a
g
H2AX-dependent manner.
329,396
However, its involvement in DSB repair is
different than the role of INO80. SWR1 is known for exchanging histone H2A
with the H2AZ variant.
374,397
SWR1 is also implicated in regulating
g
H2AX
levels. Inactivation of SWR1 ceases H2AZ incorporation at nucleosomes sur-
rounding the DSB; however, this restores
g
H2AX levels and checkpoint adap-
tation, which functions antagonistically to INO80.
373
Other than the acetylation of histones H4 and H2AX by TIP60 following
DNA damage,
356-359
the
Drosophila
TIP60 has been shown to exchange acety-
lated histone H2Av (H2AX homolog) with unmodified H2Av using the domino/
p400 ATPase.
358,398
The conjunctional action of TIP60/p400 that leads to nucle-
osomal destabilization at the DSB is required for efficient RNF8-mediated
ubiquitination of histones and recruitment of BRCA1 and 53BP1 for DSB
response.
398
Furthermore, the exchange of acetylated H2Av with unmodified
H2Av might be critical for attenuation of DSB signal propagation.
362
RAD54 appears to have the ability to remodel chromatin.
399
RAD54 from
budding yeast,
Drosophila
, and humans has been shown to possess ATP-driven
chromatin-remodeling activity
in vitro
using assembled mononucleosomes and
nucleosomal arrays.
285,400-402
The RAD51 NPF stimulates chromatin-
remodeling activity as well as the dsDNA-dependent ATPase activity of
RAD54.
250-252
Strand exchange by RAD51 is greatly enhanced by RAD54 in
chromatinized substrates,
285,400
even though for the initial homology, capture
by RAD51 NPF on a chromatin substrate RAD54 is not required.
392
Interest-
ingly, a specific protein-protein interaction between the amino terminus
of histone H3 and RAD54 has also been reported, implying a specific role
in RAD54-mediated chromatin remodeling.
403
VII. Postsynaptic Removal of RAD51
Postsynaptic RAD51 turnover plays a critical role in regulating the HR
repair pathway (
Table I
). The deproteinization step in conventional strand
exchange studies
in vitro
obliterates the possible analysis of RAD51 dissocia-
tion from dsDNA.
404
Furthermore, unlike RecA that dissociates from dsDNA
upon ATP hydrolysis,
99
RAD51 remains bound to heteroduplex DNA, proba-
bly due to its intrinsic slow ATP hydrolysis. These results suggest that eukary-
otes accessory proteins have evolved to facilitate RAD51 removal from the
nascent heteroduplex to allow the 3
0
-end to prime DNA synthesis.
85,404
RAD54
has been shown to dissociate RAD51 from dsDNA.
198,251
Recent studies on the
nematode
C. elegans
identified two more gene products, a helicase HELQ-1
(homologous to human HEL308) and the single RAD51 paralog RFS-1, that
are essential for postsynaptic RAD51 turnover during meiotic HR.
405
Both
