Biomedical Engineering Reference
In-Depth Information
7.6 Tobacco Mosaic Virus
7.6.1  general Properties, Structure, and Assembly
Perhaps.no.virus.has.played.a.larger.role.in.the.development.of.modern.virology.than.the.
tobacco.mosaic.virus.(TMV)..The.intensive.study.of.TMV.has.resulted.in.several.signii-
cant.breakthroughs.in.molecular.biology..Investigation.into.the.causes.of.tobacco.mosaic.
syndrome.dating.back.to.1886.resulted.in.the.identiication.of.an.infectious.agent.that.was.
small.enough.to.pass.through. candle-ilters. capable. of.removing.bacteria.from. solution..
This.was.the.irst.instance.of.the.identiication.of.a.“ilterable.agent”.known.to.cause.dis-
ease,.an.agent.that.would.eventually.be.called.a.virus..It.was.the.irst.virus.to.be.puriied,.
crystallized,.and.shown.to.be.composed.of.ribonucleoprotein;.the.irst.virus.to.be.imaged.
in.an.electron.microscope;.and.the.irst.virus.shown.to.have.intrinsically.infective.RNA..
Later,.TMV.mutants.were.an.integral.part.of.the.research.that.proved.that.the.genetic.code.
is.nonoverlapping.(Creager.et.al..1999,.Klug.1999,.Knipe.et.al..2007)..Part.of.the.reason.that.
TMV.is.so.well.studied.is.because.it.is.easily.produced.in.large.quantities.from.infected.
tobacco.plants.and.that.it.can.be.assembled.in.vitro.from.puriied.protein.and.RNA;.also.as.
puriied.virions.are.remarkably.stable,.they.can.remain.infective.for.up.to.50.years.
TMV.is.a.rod-shaped,.nonenveloped.virus.in.the.family. Virgaviridae ..It.infects.tobacco,.
in. addition. to. more. than. 200. other. plant. species. and. is. a. Biosafety. level. 1. pathogen.
(Scholthof. 2004).. The. assembled. capsid. is. 300.nm. in. length,. 18.nm. in. width,. and. has. a.
central.channel.with.a.diameter.of.4.nm.(Knez.et.al..2003)..Fully.assembled,.the.rigid.rod.
structure.is.composed.of.a.helical.array.of.2130.capsid.protein.subunits.wrapped.around.
a.single-stranded,.(+).sense.RNA.molecule,.6400.nucleotides.in.length.(Creager.et.al..1999)..
TMV. lends. itself. to. engineering. efforts. owing. to. its. highly. repeatable,. one-dimensional.
structure,.in.addition.to.the.open.ends.of.the.tubular.capsid..The.outer.and.inner.surfaces.
of.the.TMV.capsid.have.been.shown.to.be.hydrophilic,.allowing.researchers.a.multitude.
of. approaches. for. interior. and. exterior. capsid. modiications. (Bhyravbhatla. et. al.. 1998)..
Moreover,.TMV.capsids.can.remain.stable.at.temperatures.up.to.90°C,.and.can.withstand.
ranges.of.pH.between.3.5.and.9.(Perham.and.Wilson.1978).
At.physiological.conditions,.TMV.capsid.proteins.aggregate.into.two-layer.disks,.with.
each. layer. comprising. 17. individual. capsid. protein. molecules.. Self-assembly. of. TMV.
begins. when. one. such. disk. interacts. with. a. speciic. sequence. of. the. genomic. RNA,. the.
origin.of.assembly.(O A ),.approximately.300.nucleotides.in.length.(Turner.and.Butler.1986)..
After. binding. to. the. origin. of. assembly,. the. two-layer. disk. undergoes. a. conformational.
change. into. a. helical. “lock-washer”. motif,. whereby. additional. disks. assemble. and. the.
nucleoprotein. complex. is. elongated. (Klug. 1999),. (Figure. 7.11).. In. vitro. studies. have. also.
shown.that.tailoring.the.pH.and.ionic.strength.can.produce.several.different.intermediate.
capsid.structures,.including.small.protein.aggregates,.or.stacked.rods.(Durham.and.Klug.
1971)..The.highly.repeatable,.one-dimensional.nature.of.the.TMV.capsid.structure.have.led.
to.a.diverse.array.of.engineering.applications,.as.noted.in.the.following.sections.
7.6.2  Material Templating
Precise. nanoscale. control. of. materials. in. one. dimension. is. very. advantageous. from. an.
engineering. standpoint.. TMV. has. played. a. major. part. with. respect. to. the. engineering.
of. viruses. for. nanomaterial. biotemplating. and. biosynthesis.. The. irst. demonstration.
 
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