Biomedical Engineering Reference
In-Depth Information
delayed. single-strand. break. repair. and. hypersensitivity. to. ionizing. radiation. (Yelamos.
et. al.. 2008).. Activation. of. PARP1. and. PARP2. by. strand. breaks. immediately. triggers. the.
synthesis.of.poly.ADP-ribose.chains.that.recruit.DDR.proteins.to.the.damage.site..PARP1.
and.PARP2.targets.include.histones.and.DNA-repair.proteins,.among.others,.to.promote.
repair.. Both. PARP1. and. PARP2. also. interact. with. a. number. of. single-strand. break-. and.
base.excision-repair.proteins.(X-ray.repair.cross-complementing.protein.1.[XRCC1],.DNA.
polymerase. β,. and. DNA. ligase. III),. which. are. thought. to. stimulate. activities. in. each. of.
their. respective. repair. pathways.. PARP1. also. mediates. the. accumulation. of. the. MRN.
complex. on. DNA. lesions. to. facilitate. ATM. activation. and. signaling. (Haince. et. al.. 2007,.
2008)..However,.ATM.and.PARP1/PARP2.have.independent.functions.in.the.DNA.damage.
response. pathway. due. to. the. synthetic. lethality. of. PARP1/PARP2. deletion. in. the. ATM.
deiciency.mouse.model.(Huber.et.al..2004)..The.heart.of.PARP1.and.PARP2's.activity.is.
the. synthesis. of. poly. ADP-ribose. chains. using. NAD
+
. to. catalyze. the. addition. of. ADP-
ribose. to. a. growing. chain,. with. this. activity. being. stimulated. upon. binding. of. strand.
breaks..The.structure.of.PARP1.in.complex.with.a.DNA-double-strand.break.has.recently.
been. determined,. and. damage. identiication. was. found. to. occur. through. a. sequence-
independent.mode.of.action.(Langelier.et.al..2011)..PARP1.uses.a.phosphate.backbone.grip.
and.a.base-stacking.loop.to.interact.with.the.phosphate.backbone.and.expose.nucleotides.
found. at. the. double-strand. break.. The. phosphate. backbone. grip. is. envisioned. to. bind.
∼1.nm. of. uninterrupted. DNA. (three. nucleotides). with. the. base-stacking. loop. being. a.
lexible.component.that.allows.for.interaction.with.a.range.of.DNA.structures.found.at.the.
end.of.damaged.DNA.strands..Analysis.of.PARP1.structures.in.the.absence.or.presence.
of.DNA.suggests.that.the.base.stacking.loop.will.reposition.itself.∼1.nm,.away.from.the.
main. structure. to. facilitate. interaction. with. nucleotides.. Structural. data. on. PARP1. thus.
highlights.important.nanoscale.features.used.for.damage.recognition.
6.2 Nano-Features of Strand Break Repair
If.a.cell.is.in.the.G2,.S,.or.meiotic.phase,.the.cell.commonly.uses.homologous.recombination.
(Jazayeri. et. al.. 2006). to. repair. strand. breaks.. The. key. mechanisms. of. homologous.
recombination.are.the.creation.of.long.3′.overhang.regions.at.the.damaged.ends.followed.
by.base.pairing.these.overhangs.with.their.corresponding.strands.of.the.sister.chromatid.
(Aylon.et.al..2004,.Ira.et.al..2004)..MRN.plays.a.key.role.here.along.with.the.protein.C-terminal.
binding.protein.interacting.protein.(CtIP)..CtIP.is.phosphorylated.by.ATM,.allowing.CtIP.
to. bind. to. the. MRN. complex. and. to. initiate. a. 5′. end. resectioning. (You. and. Bailis. 2010)..
Following.the.initial.resectioning,.a.series.of.other.helicases.and.nucleases.are.recruited.to.
the.resectioned.termini.(Gravel.et.al..2008)..These.proteins.then.convert.the.end.to.a.long.
single-stranded. DNA. sequence,. which. offers. a. template. to. bind. to. the. sister. chromatid.
and.repair.the.damage..Strand.invasion.is.followed.by.a.homology.search.to.identify.base-
pairing.partners..The.base-pairing.interface.found.in.the.intact.sister.chromatid.provides.a.
template.for.the.synthesis.of.a.new.DNA.strand.
6.2.1 Nano-Features of the MrN Complex used in Strand Break repair
Mre11. is. a. 70-90.kDa. protein. containing. an. N-terminal. phosphodiesterase. domain,. two.
C-terminal.DNA-binding.domains,.and.binding.domains.for.Nbs1.and.Rad50.(D'Amours.
and. Jackson. 2002,. Hopkins. and. Paull. 2008,. Williams. et. al.. 2007).. Mre11. self-dimerizes.
