Biology Reference
In-Depth Information
array of nucleosomes resembling ''beads on a string''.
298
This array is further
compacted into a 30-nm filament solenoid through internucleosomal interac-
tions and interactions with linker histones H1 or H5.
299
In order to perform DNA replication, repair, and transcription, the chro-
matin must be dynamic. The dynamic character is achieved by posttranslational
modifications (PMTs) of amino acid residues on both the solvent-exposed
histone tails and the core regions.
300
The PMTs include phosphorylation,
methylation, acetylation, ubiquitination, SUMOylation, ADP-ribosylation,
and proline-isomerization.
300,301
Collectively, these modifications have been
termed the ''histone code''.
301-303
Once the histones are modified, their
affinity for DNA may change. For example, biochemical studies have shown
that nucleosomes containing acetylated histone are more mobile than unmo-
dified nucleosomes.
304,305
These nucleosomes can then be evicted or
pushed from the region of DNA that needs to be replicated, repaired, or
transcribed by chromatin remodelers using the free energy of ATP binding
and/or hydrolysis.
306-312
The initial chromatin-mediated response to a DSB is the phosphorylation of
the C terminus of either major H2A variant in budding yeast on S129 (often
referred to as H2AX for simplicity
313
) or the H2AX variant in mammals on
S139.
314-316
H2AX constitutes approximately 10% of the nucleosomal H2A
complement and the phosphorylated form is referred to as
g
H2AX.
315
The
phosphorylation is ATM and MDC1 mediated, and in yeast it spans several
kilobases from the DSB, while in mammals,
g
H2AX spreads to megabase
distances flanking the DSB.
314,317,318
MDC1 directly interacts with
g
H2AX via
itsC-terminalBRCTmotifs.
319
Upon DSB formation, MDC1 is phosphorylated
by casein kinase 2 (CK2) on its N-terminal S-D-T triamino acid repeats and these
phospho-domains interact with FHA and BRCT repeats of NBS1 in the MRN
complex, which facilitates its recruitment to the DSB site.
320-322
Phosphorylated
MDC1 once recruited to the DSB site functions as a positive feedback regulator
by binding to
g
H2AX via its BRCT domain and to ATM through its FHA domain,
respectively, to facilitate ATM-mediated additional phosphorylation of H2AX to
amplify the DNA damage signal.
323
Sustaining
g
H2AX flanking, a DSB is critical
for the recruitment of downstream repair factors.
324
g
H2AX functions as a molecular beacon to recruit the cohesion complex
that is involved in linking sister chromatids during the postreplicative phase of
the cell cycle.
325-327
This process prevents LOH during mitotic HR by allowing
the broken strand to use the sister chromatid as the donor template. Studies on
budding yeast indicate that
g
H2AX recruits ATP-dependent chromatin-
remodeling factor INO80 through direct interaction.
313,328
Furthermore,
ssDNA formation that functions as the substrate for RAD51 NPF assembly is
compromised in
arp8
INO80 subunit mutants in yeast.
328
The histone acetyl
transferase (HAT) NuA4 has also shown to interact with
g
H2AX and appears to
acetylate histone H4 following DSB formation.
329
