Medical Microbiology

MorphologyandStructure 379

OtherComponentsofViralParticles

Variousenzymes.Virusesrequireanumberofdifferentenzymesdependingon

genometypeandmodeofinfection.Inseveralvirusspeciesenzymesareacom-

ponentofthevirusparticle,forexampletheneuraminidaserequiredforinvasion

andreleaseofmyxoviruses.Otherexamplesincludenucleicacidpolymerasessuch

astheRNA-dependentRNApolymerasesinantisenseviruses,theDNApolymerases

insmallpoxvirusesandtheRNA-dependentDNApolymerase(“reversetranscrip-

tase”)inhepatitisBvirusesandretroviruses(seep.385f.).

Hemagglutinin.Someviruses(aboveallmyxovirusesandparamyxoviruses)are

capableofagglutinatingvariousdifferenthumanoranimalerythrocytes.These

virusesbearacertainsurfaceprotein(hemagglutinin)intheirenvelopethatenables

themtodothis.Thehemagglutinationphenomenoncanbemadeuseofforquan-

titativeviraltestingor—inthehemagglutinationinhibitiontest—forvirusidentifica-

tionandantibodyidentification(p.405ff.).Inbiologicalterms,hemagglutininplays

adecisiveroleinadsorptionandpenetrationofthevirusintothehostcell.

StructuralPatterns

Cubicsymmetry(rotationalsymmetry).Viruseswithrotationalsymmetryare

icosahedrons(polyhedronswith 20 equilateraltriangularfaces).Thenumberof

capsomerspervirionvariesfrom 32 to 252 anddependsonthenumberofcap-

somers(twotosix)makinguponesideoftheequilateraltriangle.Thecapsomers

inavirionneednotallbethesame,eitherintheirmorphology,antigenmake-upor

biologicalproperties.Purifiedicosahedralvirusescanbecrystallized,sothatimages

ofthemcanbeobtainedusingthemethodsofradiocrystallography.Anumberof

virusimageshavebeenobtainedwiththismethodataresolutionof 2 A.

Helicalsymmetry.Helicalsymmetryispresentwhenoneaxisofacapsidislonger

thantheother.Thenucleicacidandcapsidproteinarecloselyassociatedinthe

ribonucleoprotein(RNP),inwhichtheproteinistightlyarrayedaroundthenucleic

acidstrand.ThisRNA-proteincomplexisknownasthenucleocapsid,whichtakes

theformofahelixinsidetheviralenvelope.Fig. 7 .3bshowsthisininfluenzavirus

theenvelopeofwhichhasbeenpartlyremovedandFig. 7 .3aillustratesthesesym-

metriesschematically.

Complexsymmetry.Complexstructuralpatternsarefoundinbacteriophagesand

thesmallpoxvirus(seeFig. 7. 1 ,right).Tbacteriophages,forexample,haveanico-

sahedralheadcontainingtheDNAandatubeliketailthroughwhichtheDNAis

injectedintothehostcell.

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Kayser, Medical Microbiology © 2005 Thieme