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It is worth noting that in addition to a direct contribution to DNA DSB
repair activity at chromatin flanking the break, SWI/SNF has indirect effects as
well. Both SWI/SNF complexes are involved in regulating transcription of a
large number of genes, and some of these are important for DNA damage
responses (e.g., Refs. 91,92 ). It would be very valuable to identify separation of
function mutants of SWI/SNF subunits in order to study the effects on DNA
damage responses that are specifically due to remodeling activity at DNA DSBs
in vivo . Moreover, it will be of great interest to determine what, if any,
specialization exists between PBAF and BAF in their DNA repair and genome
stability functions.
III. INO80
A. Subunit Composition
The INO80 complex was first purified from S. cerevisiae by immunopre-
cipitation of a tagged version of the catalytic subunit, Ino80, which had been
identified as important for transcription of inositol biosynthesis genes. 93,94 Gel
filtration analysis identified Ino80 as part of a high-molecular-weight complex
and using mass spectroscopy it was found to contain 15 subunits: Ino80, Rvb1,
Rvb2, Arp4, Arp5, Arp8, actin, Taf14, Nhp10, and the I no- E ighty-associated
S ubunits Ies1-Ies6 (see Table III ). 94 Seven of these subunits are conserved in
the human INO80 complex, which also contains some metazoan-specific sub-
units (see Table III ). 95 The catalytic subunit, Ino80, shares significant similarity
with the Snf2/Swi2 family of ATPase-dependent chromatin remodelers and
mutation of a conserved lysine required for ATPase activity failed to comple-
ment ino80 mutant phenotypes. 93,94 The characteristic features of the catalytic
subunits of the INO80 subfamily of remodelers are the presence of an insertion
within the ATPase domain and the presence of two conserved motifs, the TELY
motif near the N terminus and the GTIE motif near the C terminus.
Unique to the INO80 subfamily of chromatin-remodeling complexes
(which includes Swr1) is the presence of two other proteins with ATPase
activity, Rvb1 and Rvb2. These two subunits are homologous to the RuvB
bacterial helicase, a hexameric AAA
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family protein, and appear to be associ-
ated with the INO80 complex with a stoichiometry of 6:1. 94 Temperature
degron strains of ino80 and rvb2 show significant overlap in the genes that
are misregulated. 96 The Nhp10 subunit is an HMGB-like protein and therefore
could be capable of binding to structured DNA or nucleosomes, and while the
Ies1-6 subunits have not been well characterized, the fact that Ies2 and Ies6
are conserved in humans suggests that their function is important for INO80
complex activity. In common with other remodelers, INO80 also contains Arps
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