Molecular Motors - The Nanobiotechnology - page 98

turns.since.the.turning.radius.in.which.these.ilaments.can.be.guided.is.restricted..The.
lexural.rigidity.of.a.Taxol-stabilized.microtubule.has.been.estimated.to.be.2.2.×.10 −23 .nm 2 ..
When.examining.the.minimum.turning.radius.of.a.non-stabilized.microtubule,.the.value.
is.found.to.be.around.0.6.μm..The.turning.radius.of.actin.is.much.more.lexible.to.0.2.μm..
The. more. rigid. structure. of. taxol-stabilized. microtubules. and. their. higher. persistence.
length.means.that.channel.width.in.a.device.can.be.quite.wide.(19.μm.for.microtubules.
versus.0.9.μm.for.actin.microilaments).while.maintaining.directionality.(Nitta.et.al.,.2008)..
Microtubules.have.an.overall.negative.charge.due.to.aspartate.and.glutamate.amino.acids.
in. the. α-. and. β-tubulin. C-terminal. tail. regions.. Electric. ields. have. been. used. to. take.
advantage.of.this.feature.and.to.align.microtubules.along.a.positive.electrode..However,.
this.method.has.several.disadvantages..This.includes:.polarization.of.the.buffer.solution.
that.may.hinder.device.performance,.generation.of.oxygen.at.the.electrode,.that.may.speed.
the.degradation.of.proteins,.and.adds.competition.with.motor.protein.binding.that.utilizes.
the.charged.tails.of.tubulin.in.protein-protein.interactions.thus.affecting.processivity.and.
speed.(Lakämper.and.Meyhöfer,.2005).
Other.methods.of.directional.guidance.include.the.use.of.magnetic.ields.that.play.on.
the. dipole. moment. within. the. microtubule. ilament. or. low. ields.. For. magnetic. ields,.
the. forces. generated. by. inherent. features. of. microtubules. are. fairly. low.. The. introduc-
tion. of. superparamagnetic. beads. (Uchida. et. al.,. 1998). could. be. used. to. generate. higher.
force,. though. these. beads. are. fairly. large. in. size. ∼2.8.μm,. which. is. not. practical. when.
working. with. a. microtubule. ilament. with. a. diameter. of. 25.nm.. Incorporation. of. iron.
oxide.nanoparticles.onto.microtubules.is.an.alternative.approach.(Hutchins.et.al.,.2007;.
Cho.et.al.,.2009)..Nanoparticles.have.unique.optical,.electric,.and.magnetic.properties..By.
attaching. a. magnetic. nanoparticle. through. the. use. of. biotin/streptavidin. bonds,. mag-
netic.ield.guidance.is.possible..How.this.impacts.the.ability.of.microtubules.to.interact.
with.kinesin.or.Klps.has.not.been.fully.tested..By.limiting.the.number.of.nanoparticles.
attached. to. the. microtubule,. similar. to. strategies. of. “speckling”. for. imaging. microtu-
bule.dynamics.in.cells.(Waterman-Storer.and.Salmon,.1998),.or.by.providing.a.tether.that.
will. not. block. motor. attachment. sites,. limitations. to. this. strategy. might. be. bypassed..
Understanding. how. the. cell. handles. traficking. around. obstacles. that. may. be. ixed. or.
other.motors.traveling.along.the.same.microtubule.protoilament.in.similar.or.opposite.
directions. remains. challenging. (Verhey. and. Hammond,. 2009);. however,. advances. in.
super. resolution. microscopy. may. soon. provide. insights. (Kural. et. al.,. 2005).. Flow. ields.
provide.another.method.to.help.orient.microtubules.(Limberis.et.al.,.2001)..When.using.
a. low. of. the. same. rate. at. which. the. microtubules. are. moving,. for. example,. 1.μm/s. for.
kinesin,.this.results.in.a.drag.force.of.0.1.pN..This.amount.is.negligible.when.looking.at.
the.forces.exerted.collectively.by.multiple.bound.kinesins..A.single.kinesin.generates.a.
force.of.5.pN.through.ATP.hydrolysis..This.is.important.to.note.because.it.indicates.that.
the.low.ield.will.not.signiicantly.hinder.motor.function.(Hess.and.Vogel,.2001).
4.4.5 Tracks
The.primary.dynamic.tracks.used.in.nature.by.motor.proteins.are.microtubules.and.actin.
microilaments,. each. utilizing. distinct. motor. proteins. and. relecting. unique. parameters.
and.capabilities..In.considering.track.features,.it.is.important.to.consider.(1).its.composition.
and. dynamics;. (2). the. need. for. uniformity. or. polarity. and. how. to. accomplish. this;. (3).
design.features.of.branching,.curvature,.and.length;.and.(4).the.need.for.free.ends.or.end.
complexes.. If. tracks. are. to. be. used. as. the. shuttle. for. cargo. the. helicity. of. the. structure.
is. an. important. consideration.. Actin. ilaments. are. helical,. such. that. every. 72.nm,. the.