SnapShot:FlypiRNAs,PIWIProteins,andthePing-PongCycleJogenderS.Tushir,PhillipD.Zamore,andZhaoZhangHHMIandDepartmentofBiochemistryandMolecularPharmacology,UniversityofMassachusettsMedicalSchool,Worcester,MA01605,USASeeonlineversionforlegendandreferences.634Cell139,October30,2009©2009ElsevierInc.DOI10.1016/j.cell.2009.10.021Seeonlineversionforlegendandreferences.,October30,2009©2009ElsevierInc.DOI10.1016/j.cell.2009.10.021634.e1Cell139,October30,2009©2009ElsevierInc.DOI10.1016/j.cell.2009.10.021SnapShot:FlypiRNAs,PIWIProteins,andthePing-PongCycleJogenderS.Tushir,PhillipD.Zamore,andZhaoZhangHHMIandDepartmentofBiochemistryandMolecularPharmacology,UniversityofMassachusettsMedicalSchool,Worcester,MA01605,USAInanimals,PIWI-interactingRNAs(piRNAs)repressexpressionoftransposonsandotherrepetitivegenomicsequences,ensuringfaithfultransmissionofthegeneticmaterialfromonegenerationtothenext.These23–30nucleotide-longsmallRNAsaregenerallyfoundinthegermline.Theybindto“PIWI”proteins,aspecializedsubclassoftheArgo-nauteproteinfamily,whosemembersusesmallRNAguidestosilencegeneexpression.InzebrafishandDrosophila,piRNAsarerequiredforfertilityinbothmalesandfemales,andlossofpiRNAsresultsinmalesterilityinmice.piRNAsinFlyOogenesisThethreeflyPIWIproteins—Piwi,Argonaute3(Ago3),andAubergine(Aub)—areexpressedinbothovariesandtestes.Inovaries,Piwi,whichlocalizestothenucleus,isfoundbothinthegermlineandintwotypesofsomaticcells:thosethatformthegermlinestemcellnicheandthesomaticfolliclecellsthatsurroundthedevelopingoocyte.UnlikePiwi,Ago3andAubresideinthenuage,astructurethatringsthenucleus;Ago3andAubarerestrictedtotheoocyteanditsnursecellsisters.DrosophilapiRNAsareproducedinthenursecellsbyacycleofreciprocal,PIWI-proteincatalyzedcleavageeventsthatbothamplifyasmallinitialpoolofprimarypiRNAsandbiasthepiRNApopulationtowardantisense.This“ping-pong”cyclecreatessensesecondarypiRNAsboundtoAgo3.Inturn,thesesensepiRNAsgeneratenewantisense,Aub-boundpiRNAsthatcansilencetransposonsandalsocreateyetmoresense...
