Biology Reference
In-Depth Information
Other histone modifications include constitutively methylated histone H3
(K79) (H3(K79me)), which has been shown to provide an interacting domain
for 53BP1 upon relaxation of higher-order chromatin structure during a
DSB.
350
Furthermore, 53BP1 also interacts with the dimethylated form of
histone H4(K20) (H4(K20me2)) via its tandem tudor domains.
351
HAT acety-
lation of histones is common during DNA replication and repair.
300,352
Acety-
lation is found not only on the histone tails but also on the core regions. For
example, histones H3(K9) as well as H3(K56) are acetylated by GCN5/KAT2A
and p300 in response to DSB damage in human cells.
353,354
It is believed that
H3(K56ac) influences the mobility of nucleosomes by neutralizing the positive
charge on the lysine and modelizing the entry-exit region of the nucleo-
some.
352,355
Finally, TIP60/Esa1 has been found to acetylate H4 and H2AX
during DSB repair.
356-359
B. ATP-Dependent Chromatin Remodeling
A yeast system that employed a galactose-inducible
HO
endonuclease-
provoked single DSB at a defined position in the ''mating-type'' (
MAT
)locus
followed by chromatin immunoprecipitation allowed monitoring of chromatin
remodeler and HR repair machinery recruitment at the recipient and donor sites
in real time.
360,361
Once
HO
endonuclease cleaves the specific site within the
MAT locus, which is otherwise protected by highly positioned nucleosomes, the
break can be repaired by either NHEJ or HR.
362
The latter pathway is employed
if the donor sequence
HMRa
or
HML
is present.
360,363
These studies have
identified several chromatin remodelers recruited to the DSB site including
INO80, SWI/SNF, RSC, SWR1, RAD54, and TIP60
155,282,313,328,356,358,364-377
.
Inositol auxotroph 80 (INO80) is a multisubunit chromatin-remodeling
complex which was first characterized in a budding yeast mutant strain that
exhibited defective transcription activation following inositol depletion (
Tabl e I
;
Ref.
378
). Relatively widely studied compared to other ATP-dependent chroma-
tin remodelers, it is composed of several subunits that are shared by yeast and
other eukaryotes.
362
These core subunits include INO80 ATPase, two AAA
รพ
ATPases (Rvb1 and Rvb2 in yeast, RuvB-like 1 and RuvB-like 2 in humans), actin
and actin-related proteins Arp4, Arp5, and Arp8, and INO80 subunits (Ies2 and
Ies6).
379
In addition, the yeast Ino80 complex contains unique polypeptides
Taf14, HMG, Nhp10, and Ies1, Ies3, Ies4, and Ies5, whereas the human homo-
log contains YY1, Uch37, and NFRKB.
379
The Ino80ATPase subunit of INO80
appears to be essential for its cellular function and for ATP-dependent chromatin
remodeling
in vitro
.
380-382
Ino80 is recruited to the HO endonuclease DSB
within an hour.
313,328,364
In yeast, deletion of Ino80ATPase, Arp5, or Arp8
subunit leads to increased sensitivity to DSB-inducing agents.
313,328,364
Recruit-
ment of Ino80 to the DSB is dependent on a specific interaction between the
a
