Biomedical Engineering Reference
In-Depth Information
dsDNA,.allowing.for.a.general.synapse.between.the.Mre11,.while.Rad50.is.required.to.
bridge.the.DNA.ends.(Williams.et.al..2008).
The.9-1-1.complex.structurally.resembles.a.doughnut-shaped.homotrimeric.complex.like.
the.proliferating.cell.nuclear.antigen.(PCNA)..The.9-1-1.complex.serves.as.a.sliding.clamp.
that.encircles.DNA.and.loads.other.enzymes.on.stalled.replication.forks.to.accommodate.
speciic. checkpoint,. repair,. and. tolerance. responses. to. DNA. damage.. The. 9-1-1. complex.
contains.an.elliptical.hole.that.is.of.suficient.size.to.accommodate.the.presence.of.double-
stranded.DNA.that.is.2.4.nm.in.diameter..The.elliptical.hole.is.formed.around.interactions.
between.Rad9-Hus1,.Rad9-Rad1,.and.Rad1-Hus1.(Dore.et.al..2009)..Like.the.9-1-1.complex,.
PCNA. is. also. a. processivity. factor. that. resembles. a. doughnut.. PCNA. encircles. DNA. at.
sites.of.DNA.replication.and.DNA.repair.and.essentially.threads.DNA.through.a.central.
hole.that.is.∼3.4.nm.in.diameter.(Ivanov.et.al..2006)..Both.PCNA.and.the.9-1-1.complex.are.
thus.nanopore-based.structures.vital.to.DNA.integrity..These.nanopores.serve.as.docking.
and.coordination.stations.to.attract.and.stimulate.potential.response.proteins..Signaling.
from.the.9-1-1.nanopore.requires.Rad17,.which.not.only.forms.a.complex.with.replication.
factor.C.to.assist.the.9-1-1.complex.in.loading.onto.and.around.the.DNA.but.is.also.critical.
for. the. ATM. and. Rad3-related. (ATR)-dependent. activation. (Lee. and. Dunphy. 2010).. The.
MRN.and.9-1-1.complexes.initially.detect.the.DNA.lesions.or.stalled.replication.fork.and.
then.trigger.or.recruit.the.phosphatidylinositol.3-kinase-like.protein.kinase.(PIKK).family.
members—ATM.(ataxia.telangiectasia.mutated).and.ATR.onto.chromatin.
6.1.2 Nano-Features of Signals from DNA Damage
ATM. is. recruited. by. the. MRN. complex. to. double-strand. breaks,. where. inactive. dimeric.
ATM. dissociates. and. becomes. active. monomeric. ATM. by. autophosphorylation. at. serine.
1981..Autophosphorylation.of.ATM.also.activates.its.kinase.activity.for.use.in.downstream.
signaling.(Cortes.et.al..2003)..Some.controversial.data.in.mouse.models.indicated.that.auto-
phosphorylation.at.serine.1981.is.dispensable.for.ATM.activation.(Daniel.et.al..2008,.Pellegrini.
et.al..2006)..However,.recent.data.have.demonstrated.that.autophosphorylation.at.serine.1981.
is. required. for. ATM. monomerization. and. retention. on. DNA. lesions. through. association.
with. mediator. of. DNA. damage. check. point. protein. 1. (MDC1;. So. et. al.. 2009).. Following.
the. recognition. of. DNA. damage. by. sensor. proteins,. ATM. and. ATR. kinases. immediately.
activate.and.phosphorylate.mediators.that.function.as.recruiters.for.additional.ATM/ATR.
substrates.or.as.scaffolds.to.mediate.DNA.damage.response.(DDR).complex.formation.
Several.mediators.of.the.DNA.damage.response.have.been.discovered,.such.as.Histone.
H2A.x.(H2AX),.MDC1,.breast.cancer.1.(BRCA1),.Claspin,.and.p53-binding.protein.1.(53BP1)..
Upon.DNA.damage,.the.histone.variant.H2AX.can.be.phosphorylated.by.all.three.PIKK.
members,.ATM,.ATR,.and.DNA-dependent.protein.kinese.catalytic.subunit.(DNA-PKc),.at.
the.serine.139.site..However,.ATM.seems.to.be.the.major.kinase.that.actives.the.formation.
of.phosphorylated.H2AX,.known.as.γ-H2AX..DNA.is.tightly.packaged.into.higher.order.
chromatin.structures.termed.nucleosomes,.consisting.of.∼146.bp.of.DNA.wrapped.around.
an.octamer.of.core.histone.proteins;.condensed.nucleosomes.form.solenoid.structures.with.
an.approximate.diameter.of.30.nm,.representing.a.DNA-protein.complex.on.the.nanoscale.
(Downs. et. al.. 2007).. As. a. DNA-damage-mediating. protein. and. histone. variant,. γ-H2AX,.
binding.in.foci.at.sites.of.double-strand.breaks.(DSBs).likely.facilitates.recruitment.of.DNA.
repair.machinery.to.otherwise.inaccessible.DNA.(Huyen.et.al..2004)..Moreover,.γ-H2AX.foci.
formation.has.been.proposed.to.act.as.a.highly.sensitive.biological.dosimeter.of.exposure.to.
ionizing.radiation.with.the.number.of.foci.being.proportional.to.the.number.of.DSBs.(Pilch.
et.al..2003)..Thus,.it.may.be.possible.to.develop.nanobiosensors.based.on.the.formation.of.
