(4.1)-拓展知识（1）蛋白质定位与信号肽1999

TheNobelPrizeinPhysiologyorMedicine1999wasawardedtoGünterBlobel"forthediscoverythatproteinshaveintrinsicsignalsthatgoverntheirtransportandlocalizationinthecell."SummaryAlargenumberofproteinscarryingoutessentialfunctionsareconstantlybeingmadewithinourcells.Theseproteinshavetobetransportedeitheroutofthecell,ortothedifferentcompartments–theorganelles–withinthecell.Howarenewlymadeproteinstransportedacrossthemembranesurroundingtheorganelles,andhowaretheydirectedtotheircorrectlocation?Thesequestionshavebeenansweredthroughtheworkofthisyear’sNobelLaureateinPhysiologyorMedicine,DrGünterBlobel,acellandmolecularbiologistattheRockefellerUniversityinNewYork.Alreadyatthebeginningofthe1970shediscoveredthatnewlysynthesizedproteinshaveanintrinsicsignalthatisessentialforgoverningthemtoandacrossthemembraneoftheendoplasmicreticulum,oneofthecell’sorganelles.DuringthenexttwentyyearsBlobelcharacterizedindetailthemolecularmechanismsunderlyingtheseprocesses.Healsoshowedthatsimilar“addresstags”,or“zipcodes”,directproteinstootherintracellularorganelles.TheprinciplesdiscoveredanddescribedbyGünterBlobelturnedouttobeuniversal,operatingsimilarlyinyeast,plant,andanimalcells.Anumberofhumanhereditarydiseasesarecausedbyerrorsinthesesignalsandtransportmechanisms.Blobel’sresearchhasalsocontributedtothedevelopmentofamoreeffectiveuseofcellsas“proteinfactories”fortheproductionofimportantdrugs.SeveralimportantfunctionsAnadulthumanbeingismadeupofapproximately100,000billioncells.Acellcontainsmanydifferentcompartments,organelles,eachsurroundedbyamembrane.Theorganellesarespecializedtocarryoutdifferenttasks.Thecellnucleus,forinstance,containsthegeneticmaterial(DNA)andthusgovernsallfunctionsofthecell.Themitochondriaarethe“powerplants”producingenergyneededbythecell,andtheendoplasmicreticulumis,togetherwiththeribosomes,responsibleforsynthesizingproteins.Everycellcontainsapproximatelyonebillionproteinmolecules.Thedifferentproteinshavealargenumberofimportantfunctions.Someconstitutethebuildingblocksforconstructingthecellwhileothersfunctionasenzymescatalyzingthousandsofspecificchemicalreactions.Theproteinswithinacellareconstantlydegradedandresynthesized.Thenumberofaminoacids–thebuildingblocksmakingupallproteins–mayinasingleproteinrangefromabout50toseveralthousands,forminglong,foldedchains.Howdoproteinscrossthebarriers?Thus,itwasforalongtimeapuzzlehowlargeproteinscouldtraversethetightlysealed,lipid-containing,membranessurroundingtheorganelles.Somedecadesago,itwasalsounknownhownewlymadeproteinsweredirectedtotheircorrectlocationsinthecell.GünterBlobelwasgoingtosolvebothofthesepuzzles.Attheendofthe1960shejoinedthefamouscellbiologylaboratoryofGeorgePaladeattheRockefellerInstituteinNewYork.Here,duringtwodecades,scientistshadstudiedthestructureofthecellandtheprinciplesforthetransportofnewlysynthesizedproteinsoutofthecell.ThisworkearnedGeorgePaladetheNobelPrizeinPhysiologyorMedicinein1974(whichhesharedwiththeBelgianscientistsAlbertClaudeandChristiandeDuve).“Thesignalhypothesis”GünterBlobel’sresearchwasbuiltonthetraditionsofPalade´slaboratory.Inparticular,Blobelstudiedhowanewlymadeprotein,destinedtobecometransportedoutofthecell,istargetedtoaspecializedintracellularmembranesystem,theendoplasmicreticulum.In1971heformulatedafirstversionofthe“signalhypothesis”.Hepostulatedthatproteinssecretedoutofthecellcontainanintrinsicsignalthatgovernsthemtoandacrossmembranes.Basedonelegantbiochemicalexperiments,Blobeldescribedin1975thevariousstepsintheseprocesses.Thesignalconsistsofapeptide,i.e.asequenceofaminoacidsinaparticularorderthatformanintegralpartoftheprotein.Healsosuggestedthattheproteintraversesthemembraneoftheendoplasmicreticulumthroughachannel(Fig.1).Duringthenexttwentyyears,Blobelandcoworkersstepbystepcharacterizedthemoleculardetailsoftheseprocesses.Eventuallyitwasshownthatthesignalhypothesiswasbothcorrectanduniversal,sincetheprocessesoperateinthesamewayinyeast,plant,andanimalcells.Fig.1.

“Thesignalhypothesis”.Proteinswhicharetobeexportedoutofthecellaresynthesizedbyribosomes,associatedwiththeendoplasmicreticulum.ThegeneticinformationfromDNAistransferredviamessengerRNA(mRNA).Thisinformationdetermineshowtheaminoacidsbuilduptheproteins.First,asignalpeptideisformedasapartoftheprotein.Withthehelpofbindingproteins,thesignalpeptidedirectstheribosometoachannelintheendoplasmicreticulum.Thegrowingproteinchainpenetratesthechannel,thesignalpeptideiscleaved,andthecompletedproteinisreleasedintothelumenoftheendoplasmicreticulum.Theproteinissubsequentlytransportedoutofthecell.“Addresstags”fororganellelocalizationIncollaborationwithotherresearchgroups,GünterBlobelwassoonabletoshowthatsimilarintrinsicsignalstargetthetransportofproteinsalsotootherintracellularorganelles.Onthebasisofhisresults,GünterBlobelformulatedin1980generalprinciplesforthesortingandtargetingofproteinstoparticularcellcompartments.Eachproteincarriesinitsstructuretheinformationneededtospecifyitsproperlocationinthecell.Specificaminoacidsequences(topogenicsignals)determinewhetheraproteinwillpassthroughamembraneintoaparticularorganelle,becomeintegratedintothemembrane,orbeexportedoutofthecell.Arangeofsignalsthatgovernproteinstothedifferentpartsofthecellhavenowbeenidentified(Fig.2),showingthattheprinciplesformulatedbyBlobelarecorrect.Thesesignalscanbecomparedtoaddresstagsorzipcodeswhichensurethatatraveler’sluggagearrivesatthecorrectdestination,oraletterreachesitscorrectaddressee.Thesesignalsequencesareinfactachainofdifferentaminoacidspresenteitherasashort“tail”atoneendoftheprotein,orsometimeslocatedwithintheprotein.Fig.2.

Examplesofdirectedtransportmediatedbytopogenicsignals.Thefigureshowsaschematiccellwithsomeofitscompartments,theorganelles.(Achloroplastisanorganellethatispresentinplantcellsbutnotinanimalcells).Theorganelleshavespecialfunctionsandtheyaresurroundedbymembranes.Newlysynthesizedproteinsareprovidedwithspecial“addresstags”,signalsequencesortopogenicsignals,whichdirecttheproteinstoacorrectplacewithinthecellandallowthemtocrossthemembranesoftheorganelles.Thesignalitselfconsistsofachainofaminoacids.Itisanintegralpartoftheprotein,anditisoftenlocatedatoneendoftheprotein.SignificanceofBlobel’sdiscoveryGünterBlobel’sdiscoveryhashadanimmenseimpactonmoderncellbiologicalresearch.Whenacelldivides,largeamountsofproteinsarebeingmadeandneworganellesareformed.Ifthecellistofunctioncorrectly,theproteinshavetobetargetedtotheirproperlocations.Blobel´sresearchhassubstantiallyincreasedourunderstandingofthemolecularmechanismsgoverningtheseprocesses.Furthermore,knowledgeaboutthetopogenicsignalshasincreasedourunderstandingofmanymedicallyimportantmechanisms.Forexample,ourimmunesystemusestopogenicsignals,e.g.intheproductionofantibodies.Blobel’sresearchhashelpedexplainthemolecularmechanismsbehindseveralgeneticdiseases.Ifasortingsignalinaproteinischanged,theproteincouldendupinawronglocationinthecell.Oneexampleisthehereditarydiseaseprimaryhyperoxaluria,whichcauseskidneystonesalreadyatanearlyage.Insomeformsoffamilialhypercholesterolemia,averyhighlevelofcholesterolinthebloodisduetodeficienttransportsignals.Otherhereditarydiseases,e.g.cysticfibrosis,arecausedbythefactthatproteinsdonotreachtheirproperdestination.FutureapplicationsInthenearfuturetheentirehumangenomewillbemapped.Asaresultonecanalsodeducethestructureandtopogenicsignalsoftheproteins.Thisknowledgewillincreaseourunderstandingofprocessesleadingtodiseaseandcanbeusedtodevelopnewtherapeuticstrategies.Alreadytodaydrugsareproducedintheformofproteins,e.g.insulin,growthhormone,erythropoetinandinterferon.Usuallybacteriaareusedfortheproductionofthedrug,butinordertobefunctionalcertainhumanproteinsneedtobesynthesizedinmorecomplexcells,suchasyeastcells.Withthehelpofgenetechnologythegenesofthedesiredproteinsareprovidedwithsequencescodingfortransportsignals.Thecellswiththemodifiedgenescanthenbeefficientlyusedasproteinfactories.Increasedknowledgeabouttheprocessbywhichproteinsarebeingdirectedtodifferentpartsofthecellalsomakesitpossibletoconstructnewdrugsthataretargetedtoaparticularorganelletocorrectaspecificdefect.Theabilitytoreprogramcellsinaspecificwaywillalsobeimportantforfuturecellandgenetherapy.

1999年诺贝尔生理学或医学奖授予了GünterBlobel，“是因为发现蛋白质具有内在的信号来控制其在细胞中的运输和定位。”摘要在我们的细胞内不断产生大量执行基本功能的蛋白质。这些蛋白质必须转运到细胞外，或转运到细胞内的不同区室-细胞器。新产生的蛋白质如何穿过细胞器周围的膜运输，如何引导至正确的位置？这些问题已经通过今年的诺贝尔生理学或医学奖得主，纽约洛克菲勒大学细胞与分子生物学家GünterBlobel博士的回答得到了回答。早在1970年代初，他就发现新合成的蛋白质具有内在信号，这对于控制它们进入细胞内细胞器之一的内质网膜至关重要。在接下来的20年中，Blobel详细描述了这些过程的分子机制。他还表明，类似的“地址标签”或“邮政编码”会将蛋白质引导至其他细胞内细胞器。昆特·布洛贝尔（GünterBlobel）发现和描述的原理普遍存在，在酵母，植物和动物细胞中的运作方式相似。这些信号和转运机制中的错误导致许多人类遗传性疾病。布洛贝尔（Blobel）的研究还有助于开发更有效地利用细胞作为生产重要药物的“蛋白质工厂”。几个重要功能一个成年人大约由1万亿个细胞组成。一个细胞包含许多不同的隔室，细胞器，每个隔室都被膜包围。细胞器专门负责执行不同的任务。例如，细胞核包含遗传物质（DNA），因此支配着细胞的所有功能。线粒体是产生细胞所需能量的“发电厂”，内质网与核糖体一起负责蛋白质的合成。每个细胞包含大约十亿个蛋白质分子。不同的蛋白质具有大量重要功能。有些构成了构建细胞的基础，而另一些则充当了催化数千种特定化学反应的酶。细胞内的蛋白质不断降解并重新合成。氨基酸的数量（构成所有蛋白质的组成部分）在单个蛋白质中的范围可能在50到数千之间，形成长的折叠链。蛋白质如何穿越障碍？因此，长期以来，一个难题是，大型蛋白质如何穿越细胞器周围的紧密密封，含脂质的膜。几十年前，还不清楚如何将新制备的蛋白质定向到细胞中的正确位置。GünterBlobel将解决这两个难题。1960年代末，他加入了纽约洛克菲勒学院著名的乔治帕拉德细胞生物学实验室。在这里，二十年来，科学家们研究了细胞的结构以及新合成的蛋白质从细胞中转运出来的原理。这项工作使乔治·帕拉德（GeorgePalade）于1974年获得了诺贝尔生理学或医学奖（他与比利时科学家阿尔伯特·克劳德（AlbertClaude）和克里斯蒂安·德·杜夫（ChristiandeDuve）分享了这一奖项）。“信号假说”GünterBlobel的研究建立在Palade实验室的传统基础上。特别是，Blobel研究了一种注定要被转运出细胞的新蛋白质如何靶向专门的细胞内膜系统，即内质网。1971年，他制定了“信号假说”的第一版。他推测，分泌到细胞外的蛋白质包含一个内在信号，该信号控制它们进入和穿过膜。基于出色的生化实验，Blobel在1975年描述了这些过程的各个步骤。信号由肽组成，即形成蛋白质组成部分的特定顺序的氨基酸序列。他还建议该蛋白通过一个通道穿过内质网的膜（图1）。在接下来的20年中，Blobel及其同事逐步描述了这些过程的分子细节。最终表明，信号假说既正确又通用，因为在酵母，植物和动物细胞中这些过程以相同的方式运行。图1.“信号假设”。核糖体与内质网结合，将要输出到细胞外的蛋白质合成。来自DNA的遗传信息通过信使RNA（mRNA）传递。该信息确定氨基酸如何构成蛋白质。首先，形成信号肽作为蛋白质的一部分。在结合蛋白的帮助下，信号肽将核糖体引导至内质网中的通道。增长的蛋白质链穿透通道，信号肽被切割，完成的蛋白质被释放到内质网腔中。随后将蛋白质转运出细胞。“地址标签”用