Biology Reference
In-Depth Information
appear to be the result of gene duplication events of an ancestral
RecA/RAD51
gene.
220
However, these paralogs show a high degree of evolutionary diver-
gence from RAD51 as well as from each other.
230
XRCC2 and XRCC3
(X-ray repair cross complementing) were first identified in their ability to com-
plement the extreme sensitivity of irs1 and irs1SF hamster cell lines.
231-233
Homology searches further identified RAD51B,
234-236
RAD51C,
237
and
RAD51D.
236,238
Yeast two- and three-hybrid analyses, co-immunoprecipitation
techniques, and biochemical studies have indicated interaction among the
five RAD51 paralogs.
86,239
For example, RAD51B forms a stable heterodimer
with RAD51C,
237,240,241
while RAD51 interacts with XRCC3 and weakly with
RAD51C.
239,242
The latter interaction is improved in the presence of XRCC3.
239
RAD51D forms a stable complex with XRCC2.
243
In HeLa cells, two discrete
complexes containing XRCC3 and RAD51C and the other containing RAD51B,
RAD51C, RAD51D, and XRCC2 were found.
244,245
Knockout mutants of the
RAD51 paralogs in chicken DT40 are viable yet exhibit increased sensitivity to
cross-linking agents and IR as well as reduced RAD51 foci formation upon
damage induction.
246,247
These phenotypes can be partially corrected by over-
expression of RAD51.
246
Compared to RAD51, the RAD51 paralogs display weaker DNA-
stimulated ATPase activities.
143,241,243,248
RAD51B and RAD51C bind to
ssDNA, dsDNA, and 3
0
-tailed dsDNA.
241
Moreover, RAD51C has an apparent
strand exchange activity perhaps by destabilizing dsDNA.
241
RAD51D prefer-
entially binds to ssDNA.
243
RAD51B-RAD51C heterodimer possesses
in vitro
recombination mediator activity for RAD51-catalyzed strand exchange by
suppressing the inhibitory effect of RPA.
143
Furthermore, RAD51B-
RAD51C is able to suppress the anti-recombinogenic activity of BLM during
RAD51-mediated D-loop formation (R. Amunugama and R. Fishel, unpub-
lished data). XRCC2 has been shown to enhance the ATP-processing activity of
RAD51 by facilitating adenosine diphosphate (ADP) to adenosine triphosphate
(ATP) exchange by reducing the affinity for ADP.
248
The
in vivo
complexes
RAD51B, RAD51C, RAD51D, and XRCC2 and RAD51C-XRCC3 bind to
ssDNA, 3
0
-and5
0
-tailed dsDNA, forked DNA structures, and Holliday
junctions.
244,245,249
C. RAD54
RAD54
is a highly conserved eukaryotic gene of the
RAD52
epistasis group
(
Table I
; Refs.
250-252
). RAD54 homologs have been identified in a number of
eukaryotes including yeast, Drosophila, plants, zebrafish, chicken, mice, and
humans.
253-259
A RAD54 homolog has been identified in the archaebacterium
Sulfolobus solfataricus
, but not in eubacteria.
251,260
RAD54 homologs of
budding yeast and humans share a 66% similarity and 48% homology.
253,259
Budding yeast
Rad54
was first discovered in a genetic screen to isolate mutants
