Biomedical Engineering Reference
In-Depth Information
in vitro
,.but.
in vivo
.Mre11.is.primarily.found.within.the.MRN.complex.(Williams.et.al..2008)..
Mre11.is.capable.of.DNA.binding,.speciically.binding.to.DSB.ends.in.a.process.known.as.
synapsis.(Williams.et.al..2008)..Mre11.also.contains.DNA.endonuclease.and.exonuclease.
activities. against. both. single-stranded. DNA. and. double-stranded. DNA;. however,. it. has.
not.been.fully.determined.to.what.extent.this.activity.is.utilized.in.DNA.damage.repair.
(de.Jager.et.al..2001a,.Paull.and.Gellert.1998,.1999).
Rad50.is.a.150.kDa.protein.containing.both.a.Walker.A.and.Walker.B.domain.at.the.5′.
and. 3′. ends,. respectively. (Hopfner. et. al.. 2000,. 2001).. Between. the. Walker. A/B. domains.
is.a.large.domain.that.folds.within.itself.to.form.an.antiparallel.coiled-coil.domain.with.
a. CXXC. zinc-binding. “hook”. at. the. end. (Hopfner. et. al.. 2002).. The. hooks. interact. with.
each.other.by.coordinating.with.a.Zn
2+
.cation..The.coiled-coil.folding.brings.the.Walker.
A. and. Walker. B. domains. together. to. form. a. globular,. ATP-dependent. ATPase. domain.
(Hopfner.et.al..2000)..This.ATPase.domain.may.help.regulate.the.MRN-DNA.interaction..
Like.Mre11,.Rad50.commonly.exists.as.a.dimer..Crystal.structure.analysis.of.the.Mre11/
Rad50.interaction.indicates.that.Mre11.interacts.with.the.globular.region.of.Rad50,.creat-
ing.a.complex.where.the.DNA-interacting.regions.are.in.a.large.globular.domain,.and.the.
Rad50.coiled-coil.domains.protrude.outward.from.a.potential.DNA/MRN.interaction.(de.
Jager. et. al.. 2001b).. Current. studies. suggest. that. the. coiled-coil. domains. are. responsible.
for.latching.one.MRN.complex.to.another,.thus.tethering.the.damaged.ends.of.two.DNA.
molecules.until.repair.can.take.place.(Hopfner.et.al..2002,.Wiltzius.et.al..2005)..X-ray.crys-
tallography.analysis.of.Rad50.has.revealed.that.the.coiled-coil.hook.region.of.Rad50.has.
a. range. of. up. to. 120.nm.. This. allows. the. MRN. complex. to. reach. connect. double-strand.
breaks.in.close.proximity.to.each.other.(Hopfner.et.al..2002).
Nbs1.is.a.65-85.kDa.protein.containing.two.BRCA1.C-terminus.(BRCT).domains.and.one.
forkhead-associated.(FHA).domain.at.the.N-terminus.in.addition.to.an.Mre11-interacting.
domain. at. the. C-terminus. (Lloyd. et. al.. 2009,. Williams. et. al.. 2009).. Nbs1. also. contains. a.
nuclear.localization.signal.that.allows.the.MRN.complex.to.enter.the.nucleus..The.FHA.
and. BRCT. domains. act. as. phosphopeptide-interacting. domains.. This. allows. the. MRN.
complex. to. interact. with. a. variety. of. DNA. damage. response. proteins. (Lloyd. et. al.. 2009,.
Williams.et.al..2009)..Nbs1.also.features.a.number.of.ATM.phosphorylation.targets,.allow-
ing.additional.communication.avenues.between.the.MRN.complex.and.other.DNA.dam-
age.response.kinases.(Falck.et.al..2005,.You.et.al..2005).
The.MRN.complex.is.composed.of.three.proteins.(Mre11,.Rad50,.and.Nbs1).that,.when.
combined,.are.responsible.for.DSB-sensing.protein.recruitment.to.the.damage.site,.DNA.
binding,.and.initiation.of.the.appropriate.damage.response.pathway.(Lamarche.et.al..2010,.
Stracker.and.Petrini.2011,.Williams.et.al..2010)..The.fact.that.one.complex.can.play.such.a.
signiicant. role. in. a. fundamental. cellular. duty. makes. the. MRN. complex. an. interesting.
protein.to.study.from.a.nanobiological.perspective..As.evidence.of.its.importance,.MRN.
has. known. roles. in. disease. prevention.. Mutations. in. the.
NBS1
. gene. cause. Nijmegen.
breakage. syndrome. (NBS;. Carney. et. al.. 1998).. Symptoms. of. this. disease. include.
microcephaly,.immunodeiciency,.and.an.increased.predisposition.to.a.variety.of.cancers.
such.as.leukemia.and.lymphoma.(Antoccia.et.al..2006)..Based.on.mouse.model.studies,.it.is.
believed.that.the.immunodeiciency.is.the.result.of.an.improper.class.switch.recombination.
in.B.lymphocytes.(Reina-San-Martin.et.al..2005)..A.deletion.mutation.of.the.RAD50.gene.
has.been.found.in.one.human.patient.(Waltes.et.al..2009)..This.patient.displayed.symptoms.
similar.to.NBS,.so.the.disease.was.named.“NBS-like.disorder.”.Mutations.in.the.
MRE11
.
gene. cause. ataxia-telangiectasia-like. disorder. (A-TLD;. Stewart. et. al.. 1999).. Much. like.
ataxia-telangiectasia. (A-T),. A-TLD. is. characterized. by. neurodegeneration. and. ataxia..
Unlike. A-T,. A-TLD. patients. do. not. have. any. facial. abnormalities.. There. have. been. two.
