Biology Reference
In-Depth Information
ATPase chromatin-remodeling enzyme.
7
MDC1 then recruits RNF8, which
ubiquitylates H2AX. In the best described model, this signals a transition from
early to later recruitment of repair factors. The BRCA1 complex was previously
reported to be dependent on
g
-H2AX it is now known to fall later in the
pathway and be dependent upon ubiquitylation of H2AX as well. Once
BRCA1 is recruited, it is further responsible for the maintenance of H2AX
ubiquitylation by means of its own E3 ligase function.
21
This pathway then
remains intact to continue to recruit and maintain the damage response.
22,23
As mentioned earlier, not only are histone variants and modifications
important in the process of HRR, but histone remodeling also plays a central
role in allowing access to DNA. There are multiple enzymes that play a role in
histone remodeling in HRR. These include INO80, SWI/SNF, and SWR1.
Each of these proteins is recruited to the break in a DSB-dependent manner
and function to allow HRR to occur.
The ATPase motifs of the chromatin-remodeling proteins are in the
SWI/SNF family. Within this family are several subdivisions including the
INO80 family, which is present from yeast to mammals and includes INO80
and SWR1 in
S. cerevisiae
and INO80, Snf2-related CBP activator protein,
and p400 in mammals. This family of proteins has been most described in
transcription, but also plays a significant role in DSB repair and HRR.
24
Both INO80 and SWR1 contain an Arp4 subunit that directly interacts with
g
-H2AX and is required for recruitment to the DSB.
25,26
Both INO80 and
SWR1 are required for HRR. SWR1 functions in concert with the HAT NuA4
(Tip60 in mammals) to introduce H2AZ into the chromatin around the break,
27
and when H2AZ is deleted, less single-stranded DNA is produced, though the
kinetics of HRR are not significantly altered.
15
This process allows chromatin
eviction to occur more readily and promotes proper processing of DNA to yield
access for HRR.
In the last few years, the role of INO80 and chromatin eviction has become
convoluted. It is clear that INO80 plays a major role in chromatin dynamics
around a DSB because when the Arp8 subunit of INO80 is deleted, signifi-
cantly less ssDNA is observed at the ends of the DSB.
15
Several studies have
indicated that INO80 is responsible for histone eviction surrounding the DSB
during DNA processing.
16,28
This is further corroborated by data indicating
that the RSC complex also plays a role in chromatin remodeling surrounding
the DSB.
29
On the other hand, some data indicate that H2B is not lost during
DNA processing, even at the point at which RAD51 is bound to ssDNA.
30
An interesting point is that transcriptional histone eviction leads to five- to
tenfold decreases in ChIP signal whereas in DSB repair, only two- to fourfold
decreases are seen.
31
Taking all of this into account, it is clear that chromatin is
significantly remodeled around a DSB due to INO80, but the amount of true
histone eviction that occurs is still cloudy.
