Seeonlineversionforlegendandreferences.808Cell138,August21,2009©2009ElsevierInc.DOI10.1016/j.cell.2009.08.004SnapShot:ImportandSortingofMitochondrialProteinsNilsWiedemann,MartinvanderLaan,andNikolausPfannerInstitutfürBiochemieundMolekularbiologie,ZBMZ,UniversityofFreiburg,79104Freiburg,Germany808.e1Cell138,August21,2009©2009ElsevierInc.DOI10.1016/j.cell.2009.08.004SnapShot:ImportandSortingofMitochondrialProteinsNilsWiedemann,MartinvanderLaan,andNikolausPfannerInstitutfürBiochemieundMolekularbiologie,ZBMZ,UniversityofFreiburg,79104Freiburg,GermanyMitochondriaareorganellessurroundedbytwomembranes.Theycontain~1000differentproteins,ofwhich99%areencodedbyDNAinthenucleus.Thesemitochon-drialproteinsaresynthesizedoncytosolicribosomesasprecursorproteins.Theycontainspecifictargetingsignalsthatguidethemtooneofthefourmitochondrialcompartments:outermembrane(OM),intermembranespace(IMS),innermembrane(IM),andmatrix.Transportofmitochondrialprecursorproteinsandtheirassemblyintomatureproteincomplexesrequiresseveralintricatemolecularmachinesinthefourmitochondrialcompartmentsthatarededicatedtotransportingdistinctclassesofsubstrates.(A)Thetranslocaseoftheoutermembrane(TOMcomplex)isthegeneralentrygateforvirtuallyallnuclear-encodedmitochondrialprecursorproteins.ThecoreoftheTOMcomplexconsistsofaprotein-conductingchannel,whichisformedbyTom40,thecentralreceptorTom22,andthreesmallproteins,Tom5,Tom6,andTom7.Associatedwiththecorecomplexaretheprimaryreceptors,Tom20andTom70.Mim1isrequiredforbiogenesisoftheseα-helicaloutermembraneproteins,whichareanchoredattheirNterminus.(B)Thesortingandassemblymachinery(SAMcomplex)oftheoutermembrane,whichconsistsofSam35,Sam37,Sam50,andMdm10,isdedicatedtothebiogen-esisofβ-barrelmembraneproteins,suchasTom40andporin.TheseproteinsaretransferredfromtheTOMcomplextotheSAMcomplexwiththehelpofsmallTIMchaperonesintheintermembranespace.Sam35recognizesthesortingsignalattheCterminusofβ-barrelproteins(β-signal)andtransferstheseprecursorstoSam50,whichintegratesthemint...
