Biomedical Engineering Reference
In-Depth Information
be. limited. to. the. cytoplasm. of. cells,. but. growing. evidence. indicates. a. nuclear. presence.
and. suggests. an. expanded. repertoire. of. functions. in. chromatin. remodeling. and. RNA.
processing.(Vartiainin,.2008;.Skarp.and.Vartiainin,.2010)..In.addition.to.microtubules.and.
actin.microilaments,.the.cells.of.higher.eukaryotes.also.contain.a.family.of.related.pro-
teins. and. compositions. termed. intermediate. ilaments. that. are. ∼10.nm. in. diameter.. In.
addition. to. contributing. to. cell. shape. in. the. cytoplasm,. intermediate. ilaments. are. pri-
mary. elements. of. the. nuclear. lamins. that. deine. the. nuclear. compartment. (for. review,.
see.Herrmann.and.Strelkov,.2011)..Once.thought.to.be.static,.intermediate.ilaments.are.
now. known. to. be. dynamic,. motile. structures. that. exhibit. cross-talk. with. microtubules.
and.actin.through.associated.microtubule.motors.(Helfand.et.al.,.2004)..The.microtubule.
motors.dynein.and.kinesin.contribute.to.this.cross-talk.as.motile.architects,.fasteners,.and.
regulators.of.dynamic.parameters..They.are.also.users.of.the.architectures.they.help.to.
design.and.build..Perhaps,.the.most.marveled.feature.of.the.microtubule.and.actin.cyto-
skeletons. is. their. ability. to. rapidly. disassemble. and. reorganize. their. architectures. into.
functionally. speciic. arrangements.. Development. and. specialization. in. complex. organ-
isms. would. not. be. possible. without. the. implementation. of. these. dynamic. cellular. ila-
ments. and. associated. proteins.. While,. here,. we. can. only. provide. a. concise. overview. of.
actin.microilaments.and.microtubules,.we.encourage.the.reader.to.explore.in.depth.their.
true.complexity.(Rieder.and.Khodjakov,.2003;.Chhabra.and.Higgs,.2007;.Morrison.2007;.
Ishikawa.and.Marshall.2011).
4.3.2 Actin Microfilaments
Actin. microilaments. participate. in. a. wide. array. of. functions. at. the. cellular. and. tissue.
level..These.roles.are.often.integrated.alongside.microtubules.such.as.in.cell.division,.cell.
motility,.and.maintenance.or.alteration.of.cell.conformation.(Yarm.et.al.,.2001);.however,.
unique.roles.in.muscle.contraction.as.well.as.novel.nuclear.roles.are.also.present.(Pollard.
and. Borisy,. 2003;. Vartiainen,. 2008).. Microilaments. are. long. polymers,. roughly. 7.nm. in.
diameter,. that. are. composed. of. intertwined. actin. subunits. that. spiral. around. the. axis.
of. the. linear. ilament. to. form. a. double. helix.. Force. measurements. on. single. actin. ila-
ments.indicate.that.they.are.sturdy,.withstanding.tensile.forces.up.to.500.pN.(Kishino.and.
Yanagida,.1988)..Actin.ilaments.exhibit.structural.polarity.with.the.“barbed”.or.positive.
end. of. the. actin. subunit. focused. toward. the. cell. membrane. and. the. “pointed”. or. nega-
tive. end. oriented. toward. the.cell. interior. (Hild. et.al.,. 2010)..A. process. called. “treadmill-
ing,”. present. also. in. microtubules,. allows. actin. microilaments. to. generate. force. while.
retaining.length.(Dominguez,.2009)..In.treadmilling,.elongation.at.one.end.of.the.ilament.
is. balanced. by. subunit. dissociation. at. the. opposite. end,. similar. to. a. conveyor. belt,. with.
net.force.in.the.direction.of.elongation..Associated.proteins.can.enable.actin.ilaments.to.
adopt.more.complex.networks.through.the.formation.of.branched.structures.and.regulate.
force-generating. characteristics.. For. example,. the. reversible. binding. of. tropomyosin. can.
block.myosin.association.with.actin.ilaments..Tropomyosin.removal.by.binding.of.Ca
2+
.
ions.occurs.through.a.conformational.change.(Narita.et.al.,.2001;.Bacchiocchi.and.Lehrer,.
2002;.for.review,.see.Gunning.et.al.,.2008)..Within.cells,.the.dynamic.parameters.of.actin.
ilaments.and.microtubules.can.be.harnessed.as.motors..The.pathogenic.bacteria.Listeria.
exploits.the.fundamental.forces.of.actin.polymerization.for.self-propulsion.that.is.visual-
ized. with. luorophores. as. “comet. tails”. in. cells. (Lambrechts. et. al.,. 2008).. Thus. the. actin.
“tracks”. that. bind. molecular. motors. can. themselves. be. used. to. promote. motility. in. an.
entirely.different.manner.
