Biology Reference
In-Depth Information
specifically on chromosome arms but not at a centromere.
65
As expected, for a
strain with defective sister chromatid cohesion,
rsc2
deletion strains lost chro-
mosomes with greater frequency than for a WT strain.
64,65
An interaction
between the cohesin complex and the RSC complex was found by
co-immunoprecipitation, but conflicting data exist on whether the role of
RSC in sister chromatid cohesion is in loading of cohesin or in establishing
cohesion.
64,65
The contribution of RSC to damage-induced cohesion has also
been investigated, and enrichment of Mcd1 at an
20 kb domain surrounding
a DSB was reduced in an
rsc2
deletion strain.
75
It is yet to be determined if this
reduction is sufficient to lose cohesion of the sister chromatids at the break site,
which could result in either decreased or inappropriate recombination.
10. PBAF: T
HE
H
UMAN
H
OMOLOG OF
RSC
As described earlier, RSC is a member of the SWI/SNF subfamily of
chromatin-remodeling enzymes. In yeast, RSC is very similar to, and even
shares, some subunits with SWI/SNF (
Table I
). In humans, there are also
two complexes in the SWI/SNF subfamily, termed BAF and PBAF (or SWI/
SNF-A and SWI/SNF-B). Notably, however, the overlap between the two
complexes is far more substantial in humans with only a few defining subunits
(
Table I
). The BAF180 subunit of PBAF contains six bromodomains followed
by two BAH domains and appears to be a fusion of the budding yeast Rsc1,
Rsc2, and Rsc4 subunits, suggesting that PBAF is the homologous complex to
RSC. Frequently, studies are performed using cell lines with mutations or
knockdowns of shared subunits, which provide insights into the role of ''SWI/
SNF,'' as it is not possible to make any distinction between the two complexes.
To add to this complexity, the presence of BRG1 and hBRM in BAF are
mutually exclusive and are not likely to be functionally redundant (for review
see Ref.
82
). Therefore, loss of BRG1 affects PBAF and a subset of BAF
complexes, while loss of hBRM affects solely a subset of BAF complexes.
This overlap makes it difficult to determine whether PBAF has unique func-
tions in mediating DNA DSB repair in mammalian cells, but what is clear from
the studies described later is that when subunits common to both complexes
are impaired, DNA DSB responses are affected.
After treatment with IR, expression of an ATPase-defective dominant
negative version of BRG1 resulted in defective H2AX phosphorylation, even
though ATM activation and foci formation appeared to be normal. This was also
apparent when cells with both BRG1 and hBRM knocked down were treated
with IR.
83
While not directly in response to DNA DSBs, another study also
found defective H2AX phosphorylation in cells depleted of either hSNF5 or
BRG1 in response to UV irradiation in an ATM-dependent manner.
84
These
reports are consistent with the data from yeast, suggesting a conserved function
in promoting H2A(X) phosphorylation at sites of damage. However, in another
