生理化学教学课件：Posttranslational Processing and Targeting

PART3:PosttranslationalProcessingandTargeting2FateofnascentpolypeptidesAssociationofnascentpolypeptidewithvariouschaperonesystemscommitsthemtofoldingpathwaysIfproteinsfailtofold(onethirdofthenascentpolypeptides),theyarerecognizedandtargetedfordegradationThemacromoleculesassistingtheformationofproteinsecondarystructureincludeMolecularChaperonProteinDisulfideIsomerase(PDI)PeptideProlylCis-transIsomerase(PPI)见单词表3.1

ProteinFolding3.2.3Peptidylprolylisomerization4Isomerizationofprolines（脯氨酸）Mostpeptidebondsaretrans(100xmorestablethancis).

Whensecondresidueisproline,transformisonly4xmorestableInnativeproteins,cis-prolinepeptidesarestabilizedbytertiarystructurebutinunfoldedstatethereisanequilibriumbetweencisandtransisomersCis-trans

isomerizationofprolinepeptides,catalyzedbyPPI,istherate-limitingstepinfoldingforsomeproteins3.3PosttranslationalProcessingandModificationsofNewlySynthesizedPolypeptidesN-andC-terminusprocessingand/ormodificationTrimmingofpeptidesthroughproteolyticcleavageCovalentmodificationofsomeaminoacids(phosphorylation,methylation,acetylation,glycosylation…)PosttranslationalModificationWhatisit?

AdditionofgroupsordeletionofpartstomakeafinishedproteinWhatgroups?Howmuch?Where?-methyl

-

acetyl

-

glyco-phosphoAndmanymore…Whatpurpose?-

targeting(eg.somelipoproteins)-

stability(eg.secretedglycoproteins)-function(eg.surfaceglycoproteins)-controlofactivity(eg.clottingfactors,caspases)Thecorrectlyfoldedproteinsneedtobetransportedtospecialcellularcompartmentstoexertdesiredbiologicalfunctions.AAssequenceontheN-terminusthatdirectsproteinstobetransportedtopropercellulartargetsitesiscalledsignalsequence.3.4

ProteinTargeting

SignalsequencesAllthesecretoryproteinshavethesignalsequences.Consistof13-36AAinthreeregionsPositivelychargedAAatN-terminusHydrophobiccoreof10-15AAinthemedialregionSmallpolarAAatC-terminus

b.MitochondrialproteinMitochondrialproteinsincytosolarepresentinprecursorforms.Signalsequenceof20-25AAatN-terminusarerichinSer,Thr,andbasicAA.

c.NuclearproteinK-K/R-X-K/RNuclearLocalizationSequenceD.NuclearExportSignalLXXXLXXLXLPART4:ProteinSynthesisinMedicine

1.MoleculardiseasesareassociatedwithdefectiveproteinsMutationsongenecodingregionsSilentmutations:codeforthesameaminoacid.Missensemutations:codeforadifferentaminoacid.Nonsensemutations:introduceastopcodonandcantruncatetheprotein.AbnormalproteinfoldingCysticfibrosiscausedbydeletionofPhe508inCFTR,resultinginimproperproteinfoldingOnlineMendelianInheritanceinMan(OMIM)isadatabasethatcataloguesalltheknowndiseaseswithageneticcomponentUniprot

()tRNAstructureandmodificationsHumandiseaseassociatedwithtRNAmodificationRibosomopathiesAtleast50%ofpatientswithDiamond-Blackfananemia(先天性再生障碍性贫血)carrymutationsinribosomalproteinsTreacher-Collinssyndrome,NorthAmericanIndianchildhoodcirrhosis,chromosome5q-syndromeisolatedcongenitalaspleniaCongenitalAsplenia(无脾症)Inisolatedcongenitalasplenia,themostrecentlydiscoveredribosomopathy,haploinsufficiencyoftheribosomal

proteinRPSApreventssplenicdevelopment.

Thesepatientsare

pronetoseverebacterialinfectionsbecausetheylackaspleen,

buttheyareotherwisehealthy

andhavenootherobservabledevelopmental

anomaliesDiamond-Blackfananemia

(戴-布二氏贫血)InthecaseofDiamond-Blackfananemia,mutationsinanyof11differentribosomalproteinsleadtobonemarrowfailure.Cirrhosis（肝硬化）InNorthAmericanIndianchildhoodcirrhosis,amutationintheribosomebiogenesisfactorhUTP4/Cirhincausesbiliarycirrhosis,forwhichtheonlytreatmentislivertransplantationrequiredbyearlyadolescence.Littleisknownaboutthemolecularmechanismsthatleadtothisdisease.Shwachman-Bodian-Diamondsyndrome(许氏症)

Shwachman-Bodian-DiamondsyndromearisesfrommutationsintheSBDSprotein,whichisinvolvedinlargeribosomalsubunitmaturation.Patientswiththisdisordersuffernotonlyfromdysfunctionofthepancreas,butalsofrombonemarrowfailure,skeletalabnormalities,andanenlargedliver.Treacher-Collinssyndrome

(颌面部骨发育不全综合征)Treacher-CollinssyndromeoftenresultsfromhaploinsufficiencyinTCOF1,thegenethatencodesTreacle,anucleolarproteininvolvedinpre-rRNAsynthesis.2.Manyvirusesusethehostcellproteinsynthesismachinery

(1)VirusmRNAismoreefficientlytranslatedthanhostcellmRNA.(2)VirusesmakeabundantmRNA.(3)SomevirusescaninhibithostcellmRNAbindingto40Ssubunit.

3.ProteinSynthesiscanbeinhibitedbyantibioticsandtoxinsTheproteinsynthesisishighlyregulated.Thisprocesscanalsobetheprimarytargetformanytoxins,antibioticsandinterferons.TheseinterferonsinteractspecificallywithproteinsandRNAstointerrupttheproteinsynthesis.

nametargetfunctionTetracycline四环素30SblocktheAsitetopreventbindingofAA-tRNAwith30SStreptomycin链霉素30SInhibitsinitiationChloromycetin氯霉素50Sblockthepeptidyl

transferase,

andinhibittheelongationCycloheximide放线菌酮60S(Eu)blockthepeptidyl

transferase,andinhibittheelongationPuromycin嘌呤霉素50S(Pro)and60S(Eu)releasetheprematuredpeptide,andterminateselongationErythromycin红霉素50SInhibitthetranslocaseAntibiotics

IthasasimilarstructuretoTyr-tRNA.Itworksforbothprokaryotesandeukaryotes.PuromycinSometoxins,suchasplantproteinRicin(篦麻毒素),isamongthemosttoxicsubstanceknown,whichactson60ssubunits.ToxinsRicinusCommuniscastor-oilplantrRNAN-glycosylaseactivityThericintargetsA4324thatiscontainedinahighlyconservedsequenceof12nucleotidesuniversallyfoundineukaryoticribosomesof60sDiphtheriatoxinDiphtheriatoxinisanexotoxinsecretedbyCorynebacteriumdiphtheriae,thepathogenb