Biology Reference
In-Depth Information
H2AX(Y142) is also constitutively phosphorylated by the Williams-Beuren
syndrome transcription factor (WSTF) kinase.
330
Following that advent of a
DSB, H2AX(Y142) is dephosphorylated by the EYA1/EYA3 phospha-
tases.
331,332
Interestingly, MDC1 interaction with
g
H2AX depends on a de-
phosphorylation of H2AX(Y142), and the relative amount of dephosphorylation
determines the recruitment of either proapoptotic or DNA repair factors to the
damaged site.
331
This has suggested that the H2AX(Y142) phosphorylation-
dephosphorylation is a ''molecular switch'' in response to DSB damage.
333
An essential role for histone ubiquitination during the DSB repair response
emerged after the discovery of the E3 ubiquitin ligase RNF8 at DSB foci.
334-336
RNF8 recruitment to DSB is mediated by the interaction between its FHA
domain with the phosphorylated motifs of MDC1.
334-336
RNF8 was shown to
catalyze the ubiquitinylation of histone H2A and H2AX upon DSB formation,
and knockdown of RNF8 or disruption of FHA domains leads to failure to
recruit the checkpoint-activating proteins BRCA1 and 53BP1 to the
DSB.
334,335
Furthermore, depletion of the E2 ubiquitin adapter UBC13 com-
promises RNF8 function.
335,336
UBC13 is an essential ubiquitin adapter for HR
that is also recruited to DSBs.
337,338
However, RNF8-mediated ubiquitination is
not sufficient to sustain the damage signal at DSB foci and another ubiquitin E3
ligase, RNF168, that acts with UBC13 to amplify the ubiquitination signal via
K63-linked ubiquitination of H2A and H2AX.
339,340
Another nonproteolytic E3
ligase, HERC2, is recruited to damage-induced foci and also forms a complex
with RNF8 and RNF168 to extend their retention at the repair site.
341
The K63-
linked polyubiquitinated histones function as substrates for BRCA1 A-complex
binding that includes BRCA1/BARD1, ABRAXAS, RAP80, and BRCC36
through the ubiquitin-interaction motif (UIM) of RAP80.
342
ABRAXAS is
thought to recruit RAP80 to the DSB site.
343
Just as ubiquitination of H2AX
is critical for DSB repair factor recruitment, deubiquitination is also tightly
regulated during the damage response. In fact, the deubiquitinating enzymes
BRCC36 and OTUB1 are simultaneously recruited damage-induced foci.
344,345
BRCC36 is part of the BRCA1 A-complex and OTUB1 physically binds to
UBC13 and inhibits RNF8- and RNF168-mediated polyubiquitination.
344,345
SUMOylation also appears to be essential for effective DSB damage
response. Recently it was shown that SUMO1, SUMO2, and SUMO3 are
recruited to damage-induced foci along with the E3 ligases PIAS1 and
PIAS4.
346,347
SUMOylation is critical for productive assembly of BRCA1,
53BP1, and RNF168 at damage-induced foci, and PIAS-mediated
SUMOylation of BRCA1 leads to increased ubiquitin ligase activity of the
BRCA1-BARD1 heterodimer
in vitro
.
346,347
Histone H2B(K120) has recently been shown to be ubiquitinylated by the
E3 ligase RNF20-RNF40 heterodimer following DSB damage. The H2B
(K120ub) modification leads to the recruitment of chromatin-remodeling
factor SNF2h to the DSB.
348,349
