Biology Reference
In-Depth Information
and Exo1 to initiate HR,
24
but is also present at other types of lesion such as
stalled replication forks. Activation of Mec1 and Tel1 results in amplification of
the DNA damage signal cascade and leads to recruitment and retention of
many repair and checkpoint proteins near the site of the lesion. One of the
consequences is arrest of the cell cycle by activation of checkpoints, permitting
repair of DNA damage before cell division or replication. In
S. cerevisiae
, the
major checkpoint activated in response to DNA damage is at the G2/M
boundary and involves the upregulation of the ribonucleotide reductase
(
RNR
) genes and phosphorylation of Rad53 in an Mec1-dependent manner.
Any of the steps of the DNA damage response, that is, damage recognition,
resection, H2A phosphorylation, checkpoint activation, or binding and reten-
tion of downstream effectors, could conceivably be affected by chromatin
structure and hence requires the action of chromatin-remodeling complexes.
This review focuses on the roles of two ATP-dependent chromatin remo-
delers, namely RSC (PBAF or SWI/SNF-B in mammalian cells) and INO80 in
DSBR.
II. RSC
A. Subunit Composition and Structure
The RSC complex is the most abundant ATP-dependent chromatin-
remodeling complex in budding yeast (
1000-2000 molecules per cell)
25
and
is composed of 16 subunits: Sth1 (the catalytic subunit),
26,27
Sfh1,
28
Rsc3,
29,30
Rsc30,
29
Rsc4,
31,32
Rsc58,
33
Rsc6,
34
Rsc7(Npl6),
35
Rsc8(Swh3),
36
Rsc9, Arp7
(Rsc11),
37
Arp9 (Rsc12),
37
Rsc14 (Ldb7), Htl1,
38-40
Rtt102, and either Rsc1 or
the highly similar Rsc2 subunit
41
(see
Table I
). The catalytic subunit, Sth1, is
essential for viability and its ATPase domain is closely related to that of the
Swi2/Snf2 subunit of the SWI/SNF complex. Three other RSC subunits are
also found in the related SWI/SNF complex, namely Rtt102, Arp7, and Arp9
(see
Table I
). Another three subunits are homologous to subunits of SWI/SNF,
namely Sfh1, Rsc8, and Rsc6 (see
Table I
), suggesting that these form the core
of the complex with Sth1, while the remaining subunits are specific to RSC.
Two isoforms of the RSC complex exist, containing either the Rsc1 subunit
or the highly similar Rsc2 subunit
41
(46% identity and 64% similarity). Simul-
taneous deletion of both
rsc1
and
rsc2
is lethal, whereas single deletion of either
is viable, suggesting that there is some functional redundancy between the two
RSC isoforms. Rsc2 is approximately 10-fold more abundant than Rsc1, but
both contain the same domain organization, that is, two bromodomains (likely to
be acetyl-lysine-binding domains) separated by a weak nonspecific DNA-
binding motif, termed an AT-hook, followed by a Bromo-Adjacent Homology
(BAH) domain (important
for nucleosome binding).
41,123
Several other
