Medical Microbiology

388 7 GeneralVirology

Theactualproteinsynthesisprocedureisimplemented,codedbytheviral

mRNA,withthehelpofcellularcomponentssuchastRNA,ribosomes,initia-

tionfactors,etc.Twofunctionallydifferentproteintypesoccurinviruses:

&The“noncapsidviralproteins”(NCVP)thatdonotcontributetocapsid

assembly.Theseproteinsfrequentlypossessenzymaticproperties(poly-

merases,proteases)andmustthereforebeproducedearlyoninthereplica-

tioncycle.

&Thecapsidproteins,alsoknownasviralproteins(VP)orstructuralpro-

teins,appearlaterinthereplicationprocess.

ProteinSynthesisControl

Segmentedgenomes.Aseparatenucleicacidsegmentispresentforeachprotein

(example:reoviruses).

mRNAsplicing.ThecorrectmRNAiscutoutoftheprimarytranscript(asinthecell

theexoniscutoutofthehnRNA)(examples:adenoviruses,retroviruses,etc.).

“Early”and“late”translation.ThedifferentmRNAmoleculesrequiredforassem-

blyofso-calledearlyandlateproteinsareproducedatdifferenttimesintheinfec-

tioncycle,possiblyfromdifferentstrandsofviralDNA(examples,papovaviruses,

herpesviruses).

Posttranslationalcontrol.Thisprocessinvolvesproteolyticcuttingoftheprimary

translationproductintofunctionalsubunits.Viralproteasesthatrecognizespecific

aminoacidsequencesareresponsibleforthis,e.g.,thetwopoliovirusproteasescut

betweenglutamineandglycineortyrosineandglycine.Suchproteases,someof

whichhavebeendocumentedinradiocrystallographicimages,arepotentialtargets

forantiviralchemotherapeutics(example:HIV).

Viralmaturation(morphogenesis).Inthisstep,theviralcapsidproteinsand

genomes(presentinmultiplecopiesafterthereplicationprocess)areas-

sembledintonew,infectiousvirusparticles.Insomeviralspeciesthesepar-

ticlesarealsocoveredbyanenvelope(p. 3 78f.)

Release.Thereleaseofviralprogenyinsomecasescorrelatescloselywith

viralmaturation,wherebyenvelopesorcomponentsofthemareacquired

whentheparticles“budoff”ofthecytoplasmicmembraneandareexpelled

fromthecell(Fig. 7. 6 ).Innonenvelopedviruses,releaseofviralprogenyis

realizedeitherbymeansoflysisoftheinfectedcellormoreorlesscontin-

uousexocytosisoftheviralparticles.

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Kayser, Medical Microbiology © 2005 Thieme