Human cytomegalovirus tegument proteins (pp65, pp71, pp150, pp28).

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by  Tomtishen III, John Paul

Human cytomegalovirus tegument proteins
(pp65, pp71, pp150, pp28)
Tomtishen III
TomtishenIIIVirologyJournal2012,9:22
http://www.virologyj.com/content/9/1/22(17January2012)
TomtishenIIIVirologyJournal2012,9:22
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REVIEW Open Access
Human cytomegalovirus tegument proteins
(pp65, pp71, pp150, pp28)
John Paul Tomtishen III
Abstract
Human cytomegalovirus (HCMV), a member of the Betaherpesvirinae sub-family of Herpesviridae family, is a
widespread pathogen that infects a majority of the world’s population by early adulthood. In individuals whose
immune systems are immature or weakened, HCMV is a significant pathogen causing morbidity and mortality.
There is no effective vaccine and only limited antiviral treatments against HCMV infection to date. A possible target
for novel antiviral treatments is the HCMV proteins that localize to the tegument of the virion, since they play
important roles in all stages of the viral life cycle, including, viral entry, gene expression, immune evasion, assembly,
and egress. The most likely tegument protein candidates are pp65 (immune evasion), pp71 (gene expression), and
pp150 and pp28 (assembly and egress). Although the subcellular localization of these proteins has been identified
during HCMV infections in vitro, their localization patterns have not been determined when each protein is
expressed individually in living cells. Thus, the objective of this review is elucidate the HCMV tegument as well as
present current research findings concerning the subcellular localization of the tegument proteins pp65, pp71,
pp150, and pp28 as fusions to one of several fluorescent proteins.
Keywords: Human cytomegalovirus, HCMV, Tegument, pp65, pp71, pp150, pp28, Transfection, Confocal
microscopy
Human cytomegalovirus significance hepatitis,pneumonia,gastrointestinaldiseaseand/orreti-
Human cytomegalovirus (HCMV) is a member of the nitis (inflammation of the retina) [4]. HCMV is also
Betaherpesvirinaesub-familyofHerpesviridae.Itisawide- responsible for approximately 8% of infectiousmononu-
spread pathogen that infects a majority of the world’s cleosis cases [6] and is the leading viral cause of birth
populationbyearlyadulthood[1].Infact,bytheageof40, defectsoften causing deafness and mental retardation in
between50and85%ofadultsareinfected byHCMV[2]. thefetusifawomanisinfectedduringpregnancy[7].
The virusestablishesalife-longinfection withsomecells HCMVhasbeenimplicatedinplayingaroleininflam-
beinglatentlyinfected,astatewherethevirushastheabil- matoryandproliferativediseases,includingcertaincardi-
ity to lie dormant within a cell, while others are persis- ovascular diseases and cancer [8]. Epidemiological and
tentlyinfected,wheretheinfectioncannotbeclearedfrom pathologicalstudieshaveespousedastronglinkbetween
an organism and there is intermittent shedding of infec- HCMVandatherosclerosis[8].Severalmechanismshave
tiousvirions[3].Immunocompetentindividuals,whocan been proposed in which HCMV could influence the
develop a strong immune response, typically display no developmentofatherscleroticvascularabnormalities[9].
symptomsofinfection[4].However,inindividualswhose AproposedroleofHCMVinthepathogenesisofathero-
immune systems are immature or weakened, such as sclerosis involves the reactivation of a latent HCMV
organ transplant and AIDS patients, HCMV is a signifi- infectionfollowedbyvirus-inducedenhancementofvas-
cantpathogencausingmorbidityandmortality[5].Symp- cularinflammationanddamagethroughsmoothcellpro-
tomsintheseindividualstypicallyconsistofspikingfever, liferation, uptake of low-density lipoproteins by smooth
leucopenia (decrease in white blood cells), malaise, cells, neointimal formation (thickened arterial layer via
cell migration and proliferation), and narrowing of the
vessellumen[9].
Correspondence:[email protected]
BucknellUniversity,CellBiology/BiochemistryProgram,OneDentDrive,
Lewisburg,PA17837,USA
©2012TomtishenIII;BioMedCentralLtd.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductionin
anymedium,providedtheoriginalworkisproperlycited.
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ThereisnoeffectivevaccineagainstHCMV,anddrugs double-stranded DNA. The capsid is surrounded by a
thatinhibitviralreplicationexistbutareineffectivedueto proteinaceous tegument and an outer lipid envelope
hightoxicity,low bioavailability, andthe development of [1]. Virions gain entry into a cell through a membrane
drug-resistant virus strains [8,10]. The primary antiviral fusion event involving the outer membrane of the cell
agentsusedtotreatHCMVinfectionsinpatientswithan and glycoproteins on the lipid envelope of virions.
impaired immune system are ganciclovir, foscarnet, and Once the fusion of these two membranes occurs, the
cidofovir[9].Thesedrugshaveimprovedthesurvivaland DNA-containing protein capsid and the tegument pro-
qualityoflifeofimmunocompromisedindividualssuffer- teins are released into the cell [11].
ingfromHCMV,buttheyarefarfromidealduetomajor Thegeneexpressionpatternfollowsasimilarcascadeas
hematologic, renal, and neutropenia toxicity [9]. There- usedbyotherherpesviruses asreviewedinKalejta(2008)
fore, the lack of an effective treatment for HCMV infec- [12].Duringthelyticinfection,viralimmediate-earlygenes
tions,especiallyinimmunocompromisedindividuals,has areexpressed[8].Theexpressionofthesegenesresultsin
resultedinintensestudyfortheidentificationofproteins theproductionofviralimmediate-earlyproteinsthatmod-
and processes that could be targeted by novel antiviral ulatethehostcellenvironmentandstimulatetheexpres-
treatments[8]. sion of viral early genes [1]. The viral-immediate early
genesproduceproteinsthatareresponsibleforreplicating
Human cytomegalovirus structure and life cycle thedouble-strandedviralgenomicDNA;afterDNArepli-
HCMV has the prototypical herpesvirus virion struc- cation,theseimmediate-earlygenesturnontheexpression
ture (Figure 1) and the replication cycle has a well con- of viral late genes [1]. The viral late proteins are mainly
trolled cascade of gene expression [9]. The virion has structural components of the virion that assist in the
an icosahedral protein capsid that contains the 235-kb assembly and egress of newly formed viral particles [1].
Figure1AcartoondepictingthestructureoftheHCMVvirion.(Imageobtainedfrom(http://www.virology.net/big_virology/bvdnaherpes.
html)courtesyofDr.MarkoReschkeinMarburg,Germany).
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Immediate-earlygenesinHCMVcanbesilencedincertain lifecycle,including,viralentry,geneexpression,immune
celltypesuponinfectionthough,whichresultsinalatent evasion, assembly, and egress [1,11]. There are several
infection [13]. A latent infection is characterized by the tegumentproteinsthatareofparticularinterestduetothe
minimizationofviralgeneexpressionandtheinhibitionof role(elucidatedbelow)thattheyplayintheHCMVrepli-
theassemblyandegressofnewviralprogeny[11].Latent cationcycle,includingpp65,pp71,pp150,andpp28.
infectionscanreactivateintoalyticinfectionuponcertain
environmentalcues,whichcausesdiseaseandallowsviral Tegument protein pp65
spread[1,12]. Pp65 is the most abundant tegument protein and the
HCMVinfectedcellsalsoproducenoninfectiousenvel- major constituent of extracellular virus particles [18].
oped particlesand dense bodies inadditionto infectious However,pp65isnotessentialfortheproductionofnew
virions as reviewed in Kalejta (2008) [12]. Noninfectious infectiousvirusparticlesas evidenced in strains that lack
envelopedparticlesaresimilartoinfectiousvirionsinthat the pp65 gene [18] which can still replicate in culture.
theycontainanearlyanidenticalassortmentofenvelope, Pp65isthemajortegumentproteinresponsibleformodu-
tegument,and capsid proteins, but they lackthe double- lating/evading the host cell immune response during
strandedviralgenomewithintheicosahedralcapsid[14]. HCMV infections [19]. As reviewed in Kalejta (2008),
Dense bodies onthe otherhandare envelopedtegument pp65isimplicatedincounteractingbothinnateandadap-
proteins that lackcapsidsandareprimarilycomposedof tive immune responses during HCMV infections [12].
theviralpp65protein[14].Thesignificanceofthenonin- pp65’sroleinimmuneevasionislargelyattributabletoits
fectious enveloped particles and dense bodies is not targetingofbothhumoralandcellularimmunityaswellas
knowninwild-typestrainsofHCMV[14]. servingasthedominanttargetantigenofcytotoxicTlym-
phocytes [19]. It has been demonstrated that pp65 not
Tegument structure and function onlypreventsimmediate-earlyproteinsfrombeingrecog-
A possible target for novel antiviral treatments is the nized by components of the immune system, but it also
HCMV proteins that localize to the tegument. As men- inhibitsthesynthesisofthevariouscomponentsinvolved
tioned, the tegument in HCMV is located between the inthe host cell’simmuneresponse[20].Oneofthe ways
outerlipidmembraneandtheicosahedralproteincapsid, inwhichpp65counteractsadaptive immunityisthrough
which contains the viral genomic double-stranded DNA itsenzymatickinaseactivity[21].Itwasshownthatpp65
[11]. The tegument is generally thought to be unstruc- mediates the phosphorylation of viral immediate-early
turedandamorphousinnaturealthoughsomestructuring proteins,whichblockstheirpresentationtothemajorhis-
isseenwiththebindingoftegumentproteinstothepro- tocompatibilitycomplexclassImolecules[22].Thekinase
tein capsid [15]. The tegument proteins comprise more activity of pp65 has also been implicated in causing the
thanhalfofthetotalproteinsfoundwithininfectiousvir- degradationofthealphachaininthemajorhistocompat-
ions[16].Tegumentproteinsarephosphorylated,butthe ibilityclassIIcellsurfacereceptor,HLA-DR,viaanaccu-
significance of this and otherposttranslational modifica- mulationofHLAclassIImoleculesinthelysosome[20].
tionstotheseproteinsremainslargelyunexplored[11].A Furthermore,severalstudieshavepresentedevidencethat
common sequence to direct proteins into the tegument pp65isinvolvedinmediatingadecreaseintheexpression
has not been identified through either experimental or of major histocompatibility complex class II molecules
bioinformaticapproaches[17].Theprocessofassembling [20]. This is significant in that major histocompatibility
theviraltegumentuponviralegressanddisassemblyupon complexclassIandIImoleculesareresponsibleforlym-
viralentryintocellsislargelyunknown[17].Howeveras phocyterecognitionandantigenpresentationwithclassI
reviewed in Kalejta (2008), phosphorylation, subcellular molecules presenting to cytotoxic T lymphocytes CD8+
localizationtotheassemblysite,andinteractionwithcap- andclassIImoleculestohelperTlymphocytesCD4+[20].
sids or the cytoplasmic tails of envelope proteins, likely Another pivotal role that pp65 has in immune evasion
facilitate the incorporation of proteins into the HCMV duringHCMVinfectionsisthroughtheinhibitionofnat-
tegument[12]. uralkillercellcytotoxicity[23].Specifically,itwasshown
As mentioned above, virions gain entry into a cell thatpp65canact as an antagonisticligandthat canbind
through a membrane fusion event involving the outer to theNKp30activatingreceptortoprotectthekillingof
membrane of the celland thelipidmembrane ofvirions. infectedcellsaswellasinterferewiththeabilityofNKp30
The entry of the tegument proteins as well as the DNA- tocross-talkbetweenothernaturalkillercellsanddendri-
containingproteincapsiduponviralentryoccursafterthe ticcells[23,24].
fusionofthesetwomembranes[11].Uponreleaseintothe Finally,pp65hasbeenshowntoattenuatetheinterferon
cytoplasm,tegumentproteinsbecomefunctionallyactive, response[25].Itisthoughtthatpp65isinvolvedindown
where they play important roles in all stages of the viral modulatingbetainterferonandanumberofchemokines,
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whichisbasedontheobservationofanelevatedexpression capsidsthenbudintoGolgiapparatus-derivedvesiclesand
of interferongenesininfections with a strainlacking the obtaintheirfinalenvelope.Whenthesevesiclesfusewith
pp65protein[25]. thecellmembrane,theenvelopedvirionisreleased.Pp28
Thus, the role that pp65 has in immune evasion in islargelyresponsibleforthecytoplasmic envelopmentof
HCMV infections is to prevent infected cells from being tegument proteins and capsids in HCMV during the
destroyed by the immune system. Furthermore, it has assemblyandegressprocess[12,32].
been shown to protect infected cells from the immune
response by binding to components of the immune sys- Tegument protein subcellular localization
tem, thereby inhibiting their activation [24]. Asillustrated,thetegumentproteinsplayimportantroles
in all stages of the viral life cycle, including, viral entry,
Tegument protein pp71 gene expression, immune evasion, assembly, and egress
Pp71, by comparison, plays an important role in the [11].However,thesubcellularlocalizationofthetegument
activation of immediate-early gene expression at the proteinsaftertheirreleaseintothecytoplasmatthebegin-
start of the lytic replication cycle [26]. Although this ningstageofinfectionhasnotbeenfullyelucidated.This
protein is not absolutely essential, it is necessary for effi- isespeciallytrueforthelocalizationofthetegumentpro-
cient viral replication as reviewed in Kalejta (2008) [12]. teinswhentheyareexpressedindividuallyorincombina-
A proposed mechanism for how pp71 activates viral tionswithouttherestoftheHCMVvirion.Furthermore,
gene expression is by neutralizing the effects of the cel- thestructureofthetegumentitselfisnotknown,andthe
lular Daxx protein, which is recruited to promoters by processofassembling thetegument upon viral egress, as
DNA-binding transcription factors, resulting in the well asthe disassembly ofthe tegumentuponviral entry
repression of transcription [27]. Pp71 can bind to two intocells,ispoorlyunderstood[17].Therefore,theobjec-
inherent domains on Daxx and induce its proteasomal tiveofthisresearchwastodeterminethesubcellularloca-
degradation [28]. Additionally, it was demonstrated that lization of the primary tegument proteins pp65, pp71,
pp71 increases the infectivity of viral genomic DNA pp150, and pp28, after transfection of plasmid DNA
when transfected into cultured cells [29]. expressing each protein as a fluorescent protein fusion.
Recently though, pp71 has also been implicated in Sincethesetegumentproteinsplaypivotalrolesinseveral
immuneevasion,similartopp65,bydisruptingthemajor stages of the viral life cycle, knowledge of where and the
cell surface expression of components of the immune mechanism of how these proteins localize upon release
response[30].Specifically,pp71appearstotargetthecell could be fundamental in the development of effective,
surface receptors of major histocompatibility complex novel antiviral treatments for this widespread human
class I proteins by slowing their intracellular transport pathogen, which would have great therapeutic value for
[30].Thislimitstheabilityofinfectedcellstodisplayviral immunocompromisedindividuals.
antigens totheimmunesystemand preventsrecognition When HCMV virions fuse with the membrane of host
bycytotoxicTlymphocytes. cells, some tegument proteins remain in the cytoplasm,
whileothersmigratetothenucleusofthecell[33].Other
Tegument proteins pp150 and pp28 tegument proteins will remain tightly associated to the
Pp150andpp28arehighlyimmunogenicandplayrolesin nucleocapsids, and mediate their delivery to the nuclear
the assembly and egress of virus particles. Both of these porecomplexviathemicrotubuleassemblyasreviewedin
tegumentproteins play verysimilarroles,but have some Kalejta(2008)[12].Severaltegumentproteinsthoughwill
distinctfunctions.pp150,thesecondmostabundanttegu- have a specific localization within the cell depending on
ment protein behind pp65, is necessary to incorporate thestageofthelyticcycle.
nucleocapsidsintovirus particles[16]. Pp150isessential In the early stage of infection, pp65 tends to indepen-
for maintaining the stability of the cytoplasmic capsids dentlymigratetothenucleoliofthecell[34].Thelocaliza-
anddirectingtheirmovement[1,31]. tion of pp65 to the nucleoli suggests a functional
Pp150alsoplaysaroleinthereorganizationofthecyto- relationship between the localization of pp65 and the
plasmic assembly compartment during virion assembly development of the lytic cycle of HCMV [34]. However,
[31]. The process of virion assembly in HCMV has been pp65beginstomigratetothecytoplasm48hintothelytic
reviewedinKalejta(2008)[12].Aftertheviralgenomeand cycle withnuclearpore complex becoming devoidofthe
late genes are expressed, capsid formation and DNA protein [35]. This migration appears to be mediated by
packaging into the preformed capsids begins to occur in cyclin-dependentkinaseactivityandaCrm1exporter[36].
the nucleus. Capsids acquire a primary envelope when Pp71,by comparison, hasa similar localization pattern
they bud through the inner nuclear membrane into the as pp65 during the lytic cycle of HCMV infections. The
perinuclearspace,whichtheyloseuponbuddingthrough subcellular localization of pp71upon viralentry istothe
the outer nuclear membrane into the cytoplasm. The nucleusofthehostcell [33].Thislocalizationappearsto
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be essential for the initiation of either a lytic or latent unpublished data). After the plasmid and tegument pro-
infection [37]. During the later stages of infection, some teinsundergorestrictiondigests,theresultantstickyends
pp71appearstolocalizesubcellularlytoboththenucleus fuse to form the fluorescently labeled tegument protein
andthecytoplasm[38]. plasmid. The fluorescent tegument protein fusions are
Unlike the subcellular localization of pp65 and pp71, identified by transforming them into bacterial cells on a
which appears to be dependent on the stage of the lytic plate containing a specific antibiotic. Since the plasmid
life cycle of HCMV, the localization of pp150 has not contains the antibiotic resistance gene, the colonies that
been well-defined. Some studies have suggested that the aretransformedwiththefusionplasmidareabletosurvive
subcellular localization of pp150 initially when it associ- on the plate. The fusion plasmids are then isolated from
ates with the viral nucleocapsids is to the nucleus, while thebacterialcoloniesthatweretransformed.
other studies suggest that it is to the cytoplasm [39,40]. In total, three variant fluorescent proteins (Cyan-blue,
The subcellular localization of pp28 on the other hand RFP(DsRed2)-red,GFP-green)wereusedtotagandlabel
appears to be to specific cytoplasmic compartments thetegumentproteins. Pp65wastaggedwithCyan,pp71
[41]. This localization appears to be essential for the withRFP,andpp150andpp28withGFP.Thecharacteri-
production of viral progeny, since it is localized at the zationofeachfluorescentproteinmoleculecanbeseenin
site of final envelopment [41]. Table1.Additionally,thelocationofthefluorescenttags
oneachproteincanbeseeninFigure2.
Fluorescence proteins as localization tags
Thedevelopment of fluorescentproteinmoleculestoact Experimental outline and objectives [45]
aslocalizationtagsforsubcellularcomponentshasrevolu- Asmentioned,theobjectivesofthisstudyweretoidentify
tionized the biomedical sciences [42]. The first instance the subcellular localization of HCMVtegument proteins
occurredwhenAequoreavictoriajellyfishwild-typegreen pp65, pp71, pp150, and pp28 to understand how these
fluorescent protein (GFP) was used to highlight sensory proteinsinteractwiththecellandfunctionduringthelytic
neurons in the nematode [43]. Fluorescence is such a cycle ofHCMVinfections. Additionally, identificationof
powerfultoolinthatitallowsonetodistinguishandiden- the HCMVtegument protein localizationpatterns could
tify cellular componentsthat are either toosmall or lack assist in the development of a better targeted effective,
littlecontrastwiththebackgroundwithtraditionalmicro- novel antiviral treatment ofHCMV infections, especially
scopytechniques.Thewild-typegreenfluorescentprotein in immunocompromised individuals. This was accom-
moleculewasmodifiedtoyielddifferentvariantsduetoits plished through the use of cell culture and microscopy
complexemissionspectrum [44]. Nonetheless, extensive techniques. The tegument proteins of interest were first
researchhasoccurredinrecentyearstoproducenewand fusedtoafluorescenceprotein(Cyan-blue,RFP-red,GFP-
improvedfluorescenttagsthatarebrighter,coverabroad green), as described to in the previous section, that will
spectral range, and also exhibit enhanced photostability, emit/fluoresce when exposed to a certain wavelength of
reduced oligomerization, pH insensitivity, and faster light. The DNA that encodes these modified tegument
maturationrates[42]. proteinswerethentransfectedintoanestablishedhuman
Duetothesuccessofvariousfluorescentproteinmole- cell lines (HeLa cells). Confocal microscopy, which is an
culesbeingusedastagstoassessthesubcellularlocaliza- imagingtechniquethatisusedtoincreasetheopticalreso-
tion and of many cellular and viral proteins, several lution and contrast of photographs obtained through a
distinctfluorescentproteinmoleculeswereutilizedinthis microscope,wasthenutilizedtodeterminethesubcellular
experiment to identify the subcellular localization of localization of each tegument protein within the trans-
HCMV tegumentproteins pp65, pp71, pp150, and pp28, fectedcellsbynotingwheretheCyan,RFP,orGFPmole-
respectively[44].Thefluorescenttegumentproteinfusions culefluorescestoreconstructthree-dimensionalstructures
wereconstructedpreviouslybyinsertingtheopenreading oftheobtainedimages.Additionally,theimagesobtained
frameofeachtegumentproteinintoaplasmidcontaining from the fluorescence tags were compared and overlaid
afluorescencegenesothatthetwoproteincodingregions withimagesobtainedbyconcurrentlystainingwithDAPI
arein-frameandproduceafusionprotein(Pizzornoetal., orHoechst33342,whicharefluorescentstainsthathavea
Table 1Characterization ofthe fluorescent molecules used to tag HCMVtegument proteins pp65,pp71, pp150, and
pp28
FluorescentMolecule TegumentProtein(s)Labeled ExcitationPeak(nm) EmissionPeak(nm) Brightness Photostability pKa
Cyan pp65 433/445 475/503 13 64 4.7
RFP pp71 584 607 12.5 8.7 4.5
GFP pp150andpp28 488 507 34 174 6.0
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Figure2FluorescentlylabeledHCMVtegumentproteinconstructs.
highaffinityforDNA.SincetheDNAislocatedwithinthe subcellularlylocalizetothenucleusofthecell.However,it
nucleusofcells,thisallowedforadefinitiveidentification wasseenthatpp65localizestothecytoplasmofthetrans-
ofwherethesetegumentproteinslocalizesubcellularlyin fectedHeLacells.
transfectedcells. Theobservedlocalizationpatternofpp65suggeststhat
Therewerethreedistinctseriesofstepsindetermining something else in the HCMV virion, most likely another
thelocalizationofthetegumentproteins.Thelocalization tegument protein, is crucial for pp65 to localize to the
ofthetegumentproteinswerefirstidentifiedinfixedcells nucleus of a host cell in the early stages of infection.
thatwere transfectedwithplasmids containing the tegu- Moreover, the localization of pp65 to the nucleoli in the
ment proteins of interest. Since the localization of each earlystagesofinfectionsuggestsafunctionalrelationship
protein may be affected by the presence or absence of betweenthelocalizationofpp65andthedevelopmentof
othertegumentproteins,aseriesoftransfectionsoccurred, thelyticcycleofHCMV[24].Thisimpliesthatifanovel,
includingthetegumentproteinsbythemselvesandacom- antiviraltreatmentcouldtargettheothermoleculeinthe
bination of the different tegument proteins of interest. HCMV virion that is required to assist pp65 to get into
Since we have three different fluorescence tags, we were the nucleus of the host cell, HCMV infections and its
abletoobservethelocalizationofthreetegumentproteins devastating effects, especially in immunocompromised
inasinglecell.Thelocalizationpatternsinthefixedcells individuals, could be alleviated. This new data suggests
werethencomparedtothesameseriesoftransfectionsin that a potential effective, novel, antiviral treatment for
live cells to acquire more accurate observations of the HCMV infections could be synthesized from inhibiting
localization patterns as well as to rule out the possibility the localization of pp65 to the nucleus of the cell in the
thatartifactsofthefixationprocessmayhaveaffectedthe earlystagesofinfection.
subcellularlocalizationpatternsofthetegumentproteins. The results that pp65 does not localize to the nucleus
areinteresting,sinceitisknownthatpp65hasabipartite
Summary of results [45] nuclear localization signal [46]. This implies that the
Itwasshownviafluorescentconfocalmicroscopythatthe mechanism by which the other molecule within the
localizationofeachtegumentproteinwasindependentof HCMVvirionthatisnecessaryforpp65nuclearlocaliza-
each other, and that live-cell imaging experiments gave tion during the initial stages of the lytic infection is to
betterresultsthanfixedcellimagingexperiments.Further- expose the pp65 nuclear localization signal. It is known
more, the subcellular localization of pp71 and pp150 thatproteinsorothermoleculeswithnuclearlocalization
(nucleus), and pp28 (cytoplasm) were identical to what signals do not localize to the nucleus until the signal is
occurs in a typical HCMV lytic infection, suggesting a exposed and recognized by an importin protein. Thus,
strongcorrelationbetweenlocalizationandfunction.The theothermoleculewithintheHVMCvirionmaybindto
most significant piece of information that can be drawn pp65, which yields a conformational change in pp65,
from this experiment, though, concerns that subcellular thereby,exposingitsnuclearlocalizationsignaltoimpor-
localization pattern that was observed for pp65. In the tin proteins during the early stages of the lytic cycle.
early stage of a normal HCMV infection, pp65 tends to Thus, a novel antiviral treatment could target the other
independentlymigratetothenucleoliofthecell[34].This molecule in the HCMV virion, and prevent pp65 from
localizationtothenucleolisuggestsafunctionalrelation- getting into the nucleus; subsequently inhibiting the
shipbetweenthelocalizationofpp65andthedevelopment HCMV lytic infection. This research has increased our
ofthelyticcycleof HCMV [34].Inaccordance withpre- understandingofHCMVlyticinfectionsaswellasidenti-
vious experiments, it was hypothesized that pp65 would fiesapotentialtargetforanovel,antiviraltreatment.
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doi:10.1186/1743-422X-9-22
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