【生物課件】細(xì)胞質(zhì)基質(zhì)與細(xì)胞內(nèi)膜系統(tǒng)

1.TheCompartmentalizationinEukaryoticCellsMembranesdividethecytoplasmofeukaryoticcellsintodistinctcompartments.

Threecategoriesineukaryoticcells:(1)theendomembranesystem:ER,Golgicomplex,Lys.,secretoryvesicles.(2)thecytosol.(3)mitochondria,chloroplasts,peroxisomes,andthenucleus.Membrane-boundstructures(organelles)arefoundinalleukaryoticcells.現(xiàn)在是1頁\一共有62頁\編輯于星期四CytoplasmicmatrixanditsfunctionsCytoplasmicMatrix:Theregionoffluidcontentofthecytoplasmoutsideofthemembranousorganelles.Aqueoussolutionoflargeandsmallmoleculesincludingfilamentsofcytoskeletonwhichactasorganizerforsomeorder.TheCytosolisthesiteofproteinsynthesisanddegradationormodification.Italsoperformsmostofthecell’sintermediarymetabolism.Cytoplasmicmatrix(Cytosol)andEndomembraneSystem現(xiàn)在是2頁\一共有62頁\編輯于星期四Functionsofcytoplasmicmatrix:Theproteinsynthesis,degradationandmodification.Cellscarefullymonitortheamountofmisfoldedproteins.Anaccumulationofmisfoldedproteinsinthecytosoltriggersaheat-shockresponse,whichstimulatesthetranscriptionofgenesencodingcytosolicchaperonesthathelptorefoldtheproteins.現(xiàn)在是3頁\一共有62頁\編輯于星期四現(xiàn)在是4頁\一共有62頁\編輯于星期四B.EndomembraneSystemEndomembraneSystem:ThestructuralandfunctionalrelationshiporganellesincludingER,Golgicomplex,lysosome,endosomes,secretoryvesicles.Membrane-boundstructures(organelles)arefoundinalleukaryoticcells.現(xiàn)在是5頁\一共有62頁\編輯于星期四Intracellularcompartment%oftotalcellvolumeCytosol54Mittchondria22RoughERcisternae9SmoothERcisternaeplusGolgicisternae6Nucleus6Peroxisome1Lysosomes1Endosomes1RelativevolumesoccupiedbythemajorintracellularcompartmentsinLiverCell現(xiàn)在是6頁\一共有62頁\編輯于星期四C.TheDynamicNatureoftheEndomembraneSystemMostorganellesarepartofadynamicsysteminwhichvesiclesmovebetweencompartments.Biosyntheticparthwaysmoveproteins,carbohydratesandlipidswithinthecell.Secretorypathwaysdischargeproteinsfromcells.Endocyticparthwaysmovematerialsintocells.Sortingsignalsarerecognizedbyreceptorsandtargetproteinstospecificsites.現(xiàn)在是7頁\一共有62頁\編輯于星期四D.Afewapproachestothestudyofcytomembranes

Insightsgainedfromautoradiography;Insightsgainedfromthebiochemicalanalysisofsubcellularfractions;Insightsgainedfromthestudyofgeneticmutants;Thedynamicactivitiesofendomembranesystemsarehighlyconserveddespitethestructuraldiversityofdifferentcelltypes.現(xiàn)在是8頁\一共有62頁\編輯于星期四DeDuve,A.ClaudeandG.Palade,1974NobelPlrize現(xiàn)在是9頁\一共有62頁\編輯于星期四2.ThestructureandfunctionsofEndoplasmicReticulum(ER)RoughendoplasmicreticulumandSmoothendoplasmicreticulum

RERhasribosomesonthecytosolicsideofcontinuous,flattenedsacs(cisternae);SERisaninterconnectingnetworkoftubularmembraneelements.現(xiàn)在是10頁\一共有62頁\編輯于星期四A.FunctionsoftherERProteinssynthesizedonribosomesofrERinclude:

secretoryproteins,integralmembraneproteins,solubleproteinsoforganelles.

現(xiàn)在是11頁\一共有62頁\編輯于星期四SynthesisofmembranelipidsMostmembranelipidsaresynthesizedenterlywithintheER.Therearetwoexceptions:sphingomyelinandglycolipids,(beginsinER;completedinGolgi);(2)someoftheuniquelipidsoftheMitandChlmembranes(themself).Themembranesofdifferent0rganelleshavemarkedlydifferentlipidscomposition.Transportbybudding：ER→GC、Ly、PMTransportbyphospholipidexchangeproteins(PEP)：ER→otherorganelles（includingMitandChl）現(xiàn)在是12頁\一共有62頁\編輯于星期四B.FunctionsofthesERSynthesisofsteroidsinendocrinecells.Detoxificationoforganiccompoundsinlivercells.Systemofoxygenases---cytochromep450familyReleaseofglucose6-phosphateinlivercells.SequestrationofCa2+.Ca2+-ATPase現(xiàn)在是13頁\一共有62頁\編輯于星期四3.ThestructureandfunctionsofGolgicomplexA.ThepolarityofGolgicomplex現(xiàn)在是14頁\一共有62頁\編輯于星期四a)CiscisternaeofGolgicomplex:reducedosmiumtetroxide(OsO4);b)ReactionforenzymemannosidaseII,localizedinthemedial;c)Reactionforenzymenucleosidediphosphatase,localizedinthetranscisternae.RegionaldifferencesinmembranecompositionacrosstheGolgistack現(xiàn)在是15頁\一共有62頁\編輯于星期四B.TheFunctionsofGolgicomplexGlycosylationintheGolgicomplexGolgicomplexplaysakeyroleintheassemblyofthecarbohydratecomponentofglycoproteinsandglycolipids.現(xiàn)在是16頁\一共有62頁\編輯于星期四ThecorecarbohydrateofN-linkedoligosaccharidesisassembledintherER.ModificationstoN-linkedoligosaccharidesarecompletedintheGolgicomplex.O-linkedoligosaccharidestakesplaceinGolgicomplex.現(xiàn)在是17頁\一共有62頁\編輯于星期四StructureoftypicalO-andN-linkedoligosaccharidesCoreRegionAfterR.KornfeldandS.Kornfeld,1985,Annu.Rev.Biochem.

45:631現(xiàn)在是18頁\一共有62頁\編輯于星期四Whatisthepurposeofglycosylation?N-linkedglycosylationisprevalentinalleucaryotes,butisabsentfromprocaryotes.Itdon’trequireatemplate.ThereisanimportantdifferencebetweentheconstructionofanoligosaccharideandthesynthesisofDNA,RNA,andprotein.Importantfunctions:

(1)Onemightsuspectthattheyfunctiontoaidfoldingandthetransportprocess;forexample,carbohydrateasamarkerduringproteinfoldinginERandtheuseofcarbohydrate-bindinglectinsinguidingER-to-Golgitransport.(2)Limittheapproachofothermacromoleculestotheproteinsurface,moreresistanttodigestionbyproteases.(3)Regulatoryrolesinsignalingthroughthecell-surfacereceptorNotch,toallowsthesecellstorespondselectivelytoactivatingstimuli.現(xiàn)在是19頁\一共有62頁\編輯于星期四TheGolginetworksareprocessingandsortingstationswhereproteinsaremodified,segregatedandthenshippedindifferentdirections.現(xiàn)在是20頁\一共有62頁\編輯于星期四

VesivulartransportwithintheGolgiapparatus:

Twoviews:cisternalmaturationmodelandvesiculartransportmodelTwopossiblemodelsexplainingtheorganizationoftheGolgicomplexandthetransportfromonecisternatothenext.現(xiàn)在是21頁\一共有62頁\編輯于星期四十

十

十

C.GolgiBiogenesisStagesofGolgigrowthanddivision.ShownarethinsectionelectronmicrographsofT.gondiiRHtachyzoitesreplicatingbyendodyogenyinHFFcells.CellswereplacedinoneoffourcategoriesaccordingtothenumberandsizeoftheGolgi:a,singleGolgi;b,single,elongatedGolgi;c,twoGolgi;d,Golgi,oftenmorevesiculated,ineachnascentdaughtercell,delineatedbythegrowinginnermembranecomplex(IMC).a,apicoplast;dg,densegranules;er,ER;es,ERexitsitesontheouterflattenedpartofthenuclearenvelope;G,Golgi;m,micronemes;mit,mitochondria;r,rhoptries.Scalebar,0.5mm.現(xiàn)在是22頁\一共有62頁\編輯于星期四StableexpressionofmammalianGolgiproteins.a,b,OverlaidimmunofluorescenceandphaseimagesofGRASP–YFP(a)andNAGTI–YFP(b)instable,transgeniccelllinesofToxoplasmagondii.c–h,ImmunofluorescenceimagesofatransgeniccelllineexpressingbothGRASP–CFP(green)andNAGTI–YFP(red)before(c–e)orafter(f–h)treatmentwith5mg/mlBFAfor10minat37oC.Mergedimagesareshownontheright.AsterisksindicateasecretedformofNAGTI–YFPthataccumulatesintheparasitophorousvacuole.Scalebars,5mm.現(xiàn)在是23頁\一共有62頁\編輯于星期四Immunoelectronmicroscopyoftransgenicparasites.a–c,CryosectionsofGRASP–YFP(a,c)orNAGTI–YFP(b)transgenicparasites,pretreatedfor2hwith50mg/mlcycloheximide,beforebeingfixedandimmunolabelledforYFPusingpolyclonalantibodiesagainstGFPfollowedbyproteinAcoupledto5-nmgoldparticles.NotethehighdensityoflabellingrestrictedtoGolgimembranes.Inc,GRASP–YFPtransgenicparasitesweretreatedwithBFA(5mg/ml)for30minbeforeimmunolabelling.Notethetubulo-vesicularappearanceoftheGolgicausedbylossofGolgienzymestotheER.d,Quantificationofimagesinaandb.Resultsarepresentedasmean±s.d.goldparticles/um2.現(xiàn)在是24頁\一共有62頁\編輯于星期四BiogenesisoftheGolgiapparatusinlivingparasites.a–h,TransgenicparasitesstablyexpressingIMC1–CFP(blue)weretransfectedwithplasmidDNAencodingGRASP–YFP(green).After20hofinfectioninHFFs,fourparasiteswereimagedbytime-lapsevideofluorescencemicroscopy.Imagesweretakenevery10minfor7hat37°C.Representativeimagesattheindicatedtimesareshown.NotethatT.gondii.Replicatessynchronouslyinagivenvacuole,whichpermitssimultaneousimagingofseveralcellsatthesamecell-cyclestage.i,j,TransgenicparasitesexpressingNAGTI–YFP(green)wereimagedovertimeandsampleimageslateincelldivisionareshown.ForbothGolgimarkersnotetheinheritanceoftwostructuresbyeachnascentdaughter(f,i,j)andtheireventualcoalescence(arrowingandh).k,Threedimensionalreconstructionoftwoparasitesduringmitosis.TheGolgiwasselectivelyoutlinedinredandotherelectron-densestructureswerecolouredingreenordarkbluetodifferentiatethetwoformingdaughtercells.Golgiareinheritedbybothcells,andinthecompletereconstructionofonedaughter(right)twoGolgistructuresarevisible(arrows).NotethattheotherdaughterwasonlyreconstructedpartiallyandcontainsasingleGolgistructure.現(xiàn)在是25頁\一共有62頁\編輯于星期四4.ThestructureandfunctionsofLysosomesA.CharacteristicsofLysosomes①Lysosomeisaheterogenousorganelle:PrimarylysosomesSecondlysosomesheterophagicautophagicResidualbodyPrimaryLys.SecondLys現(xiàn)在是26頁\一共有62頁\編輯于星期四

Figure

6-19

Histochemicalvisualizationoflysosomes.

Electronmicro-graphsoftwosectionsofacellstainedtorevealthelocationofacidphosphatase,amarkerenzymeforlysosomes.Thelargermembrane-boundedorganelles,containingdenseprecipitatesofleadphosphate,arelysosomes,whosediversemorphologyreflectsvariationsintheamountandnatureofthematerialtheyaredigesting.Theprecipitatesareproducedwhentissuefixedwithglutaraldehydeisincubatedwithaphosphatasesubstrateinthepresenceofleadions.TwosmallvesiclesthoughttobecarryingacidhydrolasesfromtheGolgiapparatusareindicatedbyredarrowsinthetoppanel.(CourtesyofDanielS.Friend.)

現(xiàn)在是27頁\一共有62頁\編輯于星期四②Lysosomescontainplentyacidhydrolasesthatcandigesteverykindofbiologicalmolecule.---theprincipalsitesofintracellulardigestion.Markerenzyme:acidphosphatase③Lysosomemembrane:H+-pumps:internalprotonconcentrationiskepthighbyH+-ATPaseGlycosylatedproteins:mayprotectthelysosomefromself-digestion.

Transportproteins:transportingdigestedmaterials.現(xiàn)在是28頁\一共有62頁\編輯于星期四Figure

13-18

ThelowpHinlysosomesandendosomes.

ProteinslabeledwithapH-sensitivefluorescentprobe(fluorescein)andthenendocytosedbycellscanbeusedtomeasurethepHinendosomesandlysosomes.ThedifferentcolorsreflectthepHthatthefluorescentprobeencountersintheseorganelles.ThepHinlysosomes(red)isabout5,whilethepHinvarioustypesofendosomes(blueandgreen)rangesfrom5.5to6.5.(CourtesyofFredMaxfieldandKennethDunn.)現(xiàn)在是29頁\一共有62頁\編輯于星期四Figure

13-20

Theplantcellvacuole.

Thiselectronmicrographofcellsinayoungtobaccoleafshowsthatthecytosolisconfinedbytheenormousvacuoletoathinlayer,containingchloroplasts,pressedagainstthecellwall.Themembraneofthevacuoleiscalledthetonoplast.(CourtesyofJ.Burgess.)現(xiàn)在是30頁\一共有62頁\編輯于星期四B.TheFunctionsofLysosomesLysosomesareinvolvedinthreemajorcellfunctions:①phagocytosis;②autophagy;③endocytosis.Primarylysfusewitheitherphagocyticorautophagicvesicles,formingresidualbodiesthateitherundergoexocytosisorareretainedinthecellaslipofuscingranules.現(xiàn)在是31頁\一共有62頁\編輯于星期四C.LysosomesandDiseasesDisordersresultingfromdefectsinlysosomalfunction:①Autolysis:Abreakorleakinthemembraneoflysreleasesdigestiveenzymesintothecellwhichdamagesthesurroundingtissues(Silicosis).②

Lysosomalstoragediseasesareduetotheabsenceofoneormorelysosomalenzymes,andresultinginaccumulationofmaterialinlysosomesaslargeinclusions.OneseveretypeofthediseaseisI-celldisease(inclusion–celldisease,GlcNAc-Phosphotransferasegenemutant).

Tay-Sachsdiseaseresultsfromadeficiencyoftheenzyme(-N-hexosaminidaseA)whosefunctionistodegradegangliosides,amajorcomponentofbraincellmembranes.現(xiàn)在是32頁\一共有62頁\編輯于星期四表1.神經(jīng)鞘脂貯積病疾病缺失酶類主要貯積底物后果GM1神經(jīng)節(jié)苷脂貯積癥GM1-半乳糖苷酶神經(jīng)節(jié)苷脂GM1智力遲鈍，肝臟肥大，骨骼受累，2歲前死亡泰－薩二氏病己糖胺酶A神經(jīng)節(jié)苷脂GM2智力遲鈍，失明，3歲前死亡法布萊氏病-半乳糖苷酶A三己糖神經(jīng)酰胺皮疹，腎功能喪失，下肢疼痛山霍夫氏病己糖胺酶A和B神經(jīng)節(jié)苷脂GM2和紅細(xì)胞糖苷酯與泰－薩氏疾病癥狀相似，但發(fā)展更快高歇氏病葡糖腦苷酯酶葡糖腦苷脂肝臟和脾臟腫大，長骨腐蝕，只在嬰兒期發(fā)生智力遲鈍尼-皮二氏病鞘磷脂水解酶鞘磷脂肝臟和脾臟腫大，智力遲鈍Farber’s脂肪肉芽腫病神經(jīng)酰胺水解酶神經(jīng)酰胺疼痛性與退行性的關(guān)節(jié)變形，皮膚瘤，幾年內(nèi)死亡Krabbe’s病半乳糖腦苷酯酶半乳糖腦苷脂髓磷脂缺失，智力遲鈍，2歲前死亡腦硫脂沉積芳基硫酸酯酶腦硫脂智力遲鈍，前十年死亡現(xiàn)在是33頁\一共有62頁\編輯于星期四D.BiogenesisofLysosomesFigure

6-23

Thetransportofnewlysynthesizedlysosomalhydrolasestolysosomes.

Theprecursorsoflysosomalhydrolasesarecovalentlymodifiedbytheadditionofmannose6-phosphateintheCGN.TheythenbecomesegregatedfromallothertypesofproteinsintheTGNbecauseaspecificclassoftransportvesiclesbuddingfromtheTGNconcentratesmannose6-phosphate-specificreceptors,whichbindthemodifiedlysosomalhydrolases.Thesevesiclessubsequentlyfusewithlateendosomes.AtthelowpHofthelateendosomethehydrolasesdissociatefromthereceptors,whicharerecycledtotheGolgiapparatusforfurtherroundsoftransport.Inlateendosomesthephosphateisremovedfromthemannoseonthehydrolases,furtherensuringthatthehydrolasesdonotreturntotheGolgiapparatuswiththereceptor.現(xiàn)在是34頁\一共有62頁\編輯于星期四Mannose6-phosphateresiduestargetproteinstolysosomesTargetingofsolublelysosomalenzymestoendosomesandlysosomesbyM-6-Ptag現(xiàn)在是35頁\一共有62頁\編輯于星期四

PhosphorylationofmannoseresiduesonlysosomalenzymescatalyzedbytwoenzymesRecognitionsitebindstoSignalpatchGlcNAcphosphotransferasephosphodiesterase現(xiàn)在是36頁\一共有62頁\編輯于星期四Figure6-40.Themannose6-phosphate(M6P)pathway,themajorroutefortargetinglysosomalenzymestolysosomes.

PrecursorsoflysosomalenzymesmigratefromtherERtothecis-Golgiwheremannoseresiduesarephosphorylated.IntheTGN,thephosphorylatedenzymesbindtoM6Preceptors,whichdirecttheenzymesintovesiclescoatedwiththeclathrin.Theclathrinlatticesurroundingthesevesiclesisrapidlydepolymerizedtoitssubunits,andtheuncoatedtransportvesiclesfusewithlateendosomes.Withinthislow-pHcompartment,thephosphorylatedenzymesdissociatefromtheM6Preceptorsandthenaredephosphorylated.ThereceptorsrecyclebacktotheGolgi,andtheenzymesareincorporatedintoadifferenttransportvesiclethatbudsfromthelateendosomeandsoonfuseswithalysosome.ThesortingoflysosomalenzymesfromsecretoryproteinsthusoccursintheTGN,andthesetwoclassesofproteinsareincorporatedintodifferentvesicles,whichtakedifferentroutesaftertheybudfromtheGolgi.[G.Griffithsetal.,Cell

52:329;S.Kornfeld,Annu.Rev.Biochem.

61:307;andG.GriffithsandJ.Gruenberg,TrendsCellBiol.

1:5]現(xiàn)在是37頁\一共有62頁\編輯于星期四5.ProteinSorting

Overviewofsortingofnuclear-encodedproteinsineukaryoticcellsProteinsareimportedintoorganellesbythreemechanisms:GatedTransport:TransportthroughnuclearporesTransmembranetransport:ER,Mit,Chl,PerVesiculartransport:ER-Golgi-PM-Lys,Endosome現(xiàn)在是38頁\一共有62頁\編輯于星期四

Roadmapofproteinsorting

現(xiàn)在是39頁\一共有62頁\編輯于星期四Proteinsorting:Proteinmoleculesmovefromthecytosoltotheirtargetorganellesorcellsurfacedirectedbythesortingsignalsintheproteins.現(xiàn)在是40頁\一共有62頁\編輯于星期四SignalpeptidesandSignalpatchesFigure

6-8

Twowaysthatasortingsignalcanbebuiltintoaprotein.

(A)Thesignalresidesinasinglediscretestretchofaminoacidsequence,calledasignalpeptide,thatisexposedinthefoldedprotein.Signalpeptidesoftenoccurattheendofthepolypeptidechain,buttheycanalsobelocatedelsewhere.(B)Asignalpatchcanbeformedbythejuxtapositionofaminoacidsfromregionsthatarephysicallyseparatedbeforetheproteinfolds;alternatively,separatepatchesonthesurfaceofthefoldedproteinthatarespacedafixeddistanceapartcouldformthesignal.現(xiàn)在是41頁\一共有62頁\編輯于星期四Gatedtransport:

Throughgatedpores—Nuclearpores;Nuclearlocalizationsignal(NLS);Foldedandassemblyformtotransport.TransmembranetransportERsignalsequence,Mit,Chl,Per:Leadersequence;Throughtranslocononthemembrane;SingleandUnfoldform;Helpedbymolecularchaperons現(xiàn)在是42頁\一共有62頁\編輯于星期四VesiculartransportBudding,transporting,dockingandatlastfusionwithtargetmembrane;Assemblycoatedproteinsonthevesicles(Clathrin,COPIIandCOPI);OnlyProperlyfoldedandassembledproteins;Theorientationoftransportedproteinsandlipidsisnotchangedduringtransporting.現(xiàn)在是43頁\一共有62頁\編輯于星期四B.SignalHypothesis

--G.Blobel&D.Sabatini,1971.AmodelfortheSignalMechanismofCotranslationalImportEvidenceThatProteinSynthesizedonRibosomesAttachedtoERMembranesPassDirectlyintotheERLumen(D.Sabatini)Milstein:IgG現(xiàn)在是44頁\一共有62頁\編輯于星期四Milsteinetal:StudyingthesynthesisoflightchainofIgG(incell-freesystems,20AalongeratN-terminalendthantheauthenticlightchain)AddingERmembranestothissystemleadstotheproductionofanIgGlightchainofthecorrectsize.現(xiàn)在是45頁\一共有62頁\編輯于星期四

ASchematicmodelforthesynthesisofasecretoryproteinonamembrane-boundribosomeoftheroughER現(xiàn)在是46頁\一共有62頁\編輯于星期四Signal-recognitionparticle,SRP:Sixdifferentpolypeptidescomplexedwitha300-nucleotide(7S)moleculeofRNA.ERsignalsequence:Typically15-30aminoacids:Consistofthreedomains:apositivelychargedN-terminalregion,acentralhydrophobicregion,andapolarregionadjoiningthesitewherecleavagefromthematureproteinwilltakeplace.AsignalsequenceonnascentseretoryproteinstargetsthemtotheERandisthencleavedoffSRPreceptor(GTPbindingprotein)SRPhavethreemainactivesites:OnethatrecognizesandbindstoERsignalsequence;Onethatinteractswiththeribosometoblockfurthertranslation;OnethatbindstotheERmembrane(dockingprotein))現(xiàn)在是47頁\一共有62頁\編輯于星期四ThesortingsignalofVSVglycoproteins:Asp-X-Gln或DXEFigure6-43.Thesortingofproteinsdestinedfortheapicalandbasolateralplasmamembranesofepithelialcells.

WhenculturedMDCKcellsareinfectedsimultaneouslywithVSVandinfluenzavirus,theVSVglycoproteinisfoundonlyonthebasolateralmembrane,whereastheHAglycoproteinoftheinfluenzavirusisfoundonlyontheapicalmembrane.Liketheseviralproteins,somecellularproteinsaresorteddirectlytotheapicalmembraneandotherstothebasolateralmembraneviaspecifictransportvesiclesthatbudfromthetrans-Golginetwork.Incertainotherpolarizedcells,someapicalandbasolateralproteinsaretransportedtogethertothebasolateralsurface;theapicalproteinsthenmoveselectively,byendocytosisandtranscytosis,totheapicalmembrane.[K.Simonsetal.,Cell

62:207;K.Mostovetal.,JCB.

116:577]現(xiàn)在是48頁\一共有62頁\編輯于星期四Start-transferSequence&Stop-transferSequence現(xiàn)在是49頁\一共有62頁\編輯于星期四Figure6-24.SynthesisandinsertionintotheERmembraneoftheGLUT1glucosetransporterandotherproteinswithmultipletransmembranea-helicalsegments.

TheN-terminalahelixfunctionsasaninternal,uncleavedsignal-anchorsequence(red),directingbindingofthenascentpolypeptidechaintotherERmembraneandinitiatingcotranslationalinsertion.BothSRPandtheSRPreceptorareinvolvedinthisstep.Followingsynthesisofhelix2,whichfunctionsasastop-transfermembrane-anchorsequence,extrusionofthechainthroughthetransloconintotheERlumenceases.ThefirsttwoahelicesthenmoveoutofthetransloconintotheERbilayer,anchoringthenascentchainasana-helicalhairpin.TheC-terminusofthenascentchaincontinuestogrowinthecytosol.Subsequenta-helicalhairpinscouldinsertsimilarly,althoughSRPandtheSRPreceptorarerequiredonlyforinsertionofthefirstsignalanchorsequence.Althoughonlysixtransmembraneahelicesaredepictedhere,GLUT1andproteinsofsimilarstructurehavetwelveormore.[H.P.Wesselsetal.,Cell

55:61.]現(xiàn)在是50頁\一共有62頁\編輯于星期四TheOrientationofNascentPolypeptide

TheNascentpolypeptideisorientedwithintransloconsothatthepositivelychargedflankingsequencefacesthecytosol現(xiàn)在是51頁\一共有62頁\編輯于星期四C.

AModelforthePostranslationalImportofPolypeptidesintotheMit.Post-translationalmodificationandqualitycontrolintherERDisulfidebondsareformedandrearrangedintheERlumenOnlyinERlumenistherearedoxenvironmentforoxidationof–SHgroups.PDI:proteindisulfideisomerase,foundinabundanceintheERlumen現(xiàn)在是52頁\一共有62頁\編輯于星期四6.

TypesofVesicleTransportandTheirFunctionsA.Thethreedifferenttypesofcoatedvesicles.Differentcoatproteinsselectdifferentcargoandshapethetransportvesiclesthatmediatethevariousstepsinthebiosynthetic-secretoryandendocyticpathways.現(xiàn)在是53頁\一共有62頁\編輯于星期四COPII-coatedvesiclesmovematerialsfromtheERtotheGolgi.TheassemblyofaCOPII-coatedvesicles.Sar—GTPbindingprotein:Sar-GTPbindstotheER;Sar-GDPdissociatesfromtheERAntibodiesisabletoblockthebuddingofvesiclefromERbuthavenoeffectonvesicletransportfromoneGolgicompartmenttoanotherinmammaliancell.現(xiàn)在是54頁\一共有62頁\編輯于星期四COPI-coatedvasiclestransportingEscapedERresidentProteinsBacktotheER.TheassemblyofaCOPI-coatismediatedbyADP-ribosylationfactor(ARF),GTPbindingprotein,whichisrequiredforvesicletransferbetweencisternae.COPIcoatedvesiclesmayselectspecificcargo.

ERisanopenprison.SolubleERproteinbearRetrievingsignal—KDEL(Lys-Asp-Glu-Leu)inmammalandHDELinyeast,whereasERmembraneproteinsbearthesignalKKXX.TheKDELreceptorpresentinvesiculartubularclustersandtheGolgiapparatus..(3)COPI-coatedvesiclewerefirstidentifiedbytreatmentofGTPanalogues---COPI-coatedvesicleaccumulatedwithinthecellandcouldbeisolatedbycentrifugation.現(xiàn)在是55頁\一共有62頁\編輯于星期四Clathrin-coatedvesicle:TransportingCargofromtheTGNtoendosomes,Lysosomes,andplantvacuolesandalsomovematerialsfromthePMtocytoplasmiccompartmentsalongtheendocyticpathway.TheTGNofGolgiistheSourseofClathrin-coatedvesicle.(2)Clathrin-coatscontain:

proteinclathrin-----whichformsastructuralscaffold,adaptors----multisubunit,whichformsaninnershell.

現(xiàn)在是56頁\一共有62頁\編輯于星期四B.TheSNAREHypothesisforTransportVesicleTargetingandFusionSpecificityinvesicledockingandfusionisthoughttobeattainedthroughspecificinteractionsbetweenspecificv-SNAREproteinsonthevesiclemembranesandt-SNAREproteinsonthemembranesofthetargetcompartment.SNAREsareaproteinfamily.Thereareatleast20differentSNAREsinananimalcell.TheroleofSNAREsinguidingvesiculartransport.現(xiàn)在是57頁\一共有62頁\編輯于星期四我勸一個草率結(jié)婚的朋友離婚。她平靜的告訴我，如果說當(dāng)初魯莽結(jié)婚是個錯誤。那么，現(xiàn)在草率離婚是一錯再錯。這位朋友后來還是離婚了，大家一致認(rèn)為她的行為很理性。

同樣的故事正在互聯(lián)網(wǎng)搜索巨頭谷歌身上發(fā)生，但是谷歌選擇了草率“離婚”。

飲鴆止渴

由于急于抑制蘋果iphone手機(jī)翻天覆地的產(chǎn)業(yè)沖擊，谷歌采取急功近利的粗糙型開放策略。飲鴆止渴的策略一時取得了成果。市場研究公司尼爾森最近公布的數(shù)據(jù)顯示，在通過VerizonWireless、AT&T和SprintNextel三大運營商經(jīng)銷后，谷歌Android手機(jī)在美國市場上的銷售量已經(jīng)超過iPhone。另一家市場研究公司iSuppli甚至認(rèn)為，全球范圍內(nèi)使用Android操作系統(tǒng)的手機(jī)數(shù)量將在2012年超過蘋果iPhone。

表面繁華的背后，是Android生態(tài)系統(tǒng)的一團(tuán)糟，谷歌正在為自己的粗放型開放策略買單。

用戶對谷歌手機(jī)的態(tài)度從開始的好奇、后來的猶豫，變成強(qiáng)烈的批評?！按蠖郃ndroid手機(jī)程序都是垃圾，亂七八糟的”，一位手機(jī)發(fā)燒友迅速投奔了iphone的陣營：“同樣的植物人大戰(zhàn)僵尸游戲，在谷歌手機(jī)和iphone手機(jī)上的體驗簡直沒法比”

混亂，還是混亂。一切一切的亂象，折射出谷歌已經(jīng)失去對Android生態(tài)系統(tǒng)的控制。這一切的根源，我的判斷是開放策略初期過于寬松，導(dǎo)致失去控制權(quán)?；靵y的生態(tài)系統(tǒng)表現(xiàn)在用戶手機(jī)上，就是應(yīng)用程序的混亂和粗燥。

一錯再錯

為此，谷歌開始采取對策。最近，有國內(nèi)廠商稱新的Android3.0開始關(guān)閉應(yīng)用程序的API(應(yīng)用程序編程接口)，統(tǒng)一Android界面。這意味著，谷歌將放棄其初始開放策略，開始封閉管理。

粗看之下，谷歌認(rèn)識到自己的錯誤。既然是過度開放導(dǎo)致的錯誤，那么收緊開放尺度是很自然的邏輯，無懈可擊。但我認(rèn)為，谷歌倉促收緊開放策略仍然是個錯誤。打個比喻。如果過度開放的政策是草率結(jié)婚，那么草率的封閉就是草率離婚。這么判斷的原因很簡單，谷歌把Android開放出去的那一天，Android已經(jīng)不屬于谷歌。谷歌沒有認(rèn)識到這一點，還以為Android只是自己的。

合作伙伴對谷歌封閉政策的反應(yīng)加強(qiáng)了我的判斷結(jié)論。經(jīng)濟(jì)觀察報報道，國內(nèi)第一家生產(chǎn)基于Android平臺手機(jī)的設(shè)計公司創(chuàng)楊通信，近日已經(jīng)被迫出售。創(chuàng)楊通信負(fù)責(zé)人給出的出售理由是，“因為不愿意甘當(dāng)炮灰而選擇放棄。”

按照目前Android3.0將統(tǒng)一界面的想法，未來的手機(jī)市場將出現(xiàn)毫無差異化的產(chǎn)品。這對于企業(yè)來說，幾乎意味著不可避免的價格戰(zhàn)。利潤空間的微薄，導(dǎo)致合作伙伴生存環(huán)境惡劣。于是大量退出幾乎是一種必然。

除了為合作伙伴找到新的利益空間，谷歌還將面臨開放陣營精神層面的聲討，這對谷歌的挑戰(zhàn)會更大。如果說谷歌為了自己競爭的私利利用了開放，贏得了名聲。那么，谷歌不能一腳把開放踢開，他現(xiàn)在還需要為這種名聲買單。

如果只顧自己收網(wǎng)，谷歌會被面臨鋪天蓋地的道德譴責(zé)。谷歌，希望你準(zhǔn)備好了，三思而行。

木桶效應(yīng)就是指一個水桶無論有多高，它盛水的高度取決于其中最低的那塊木板。這在選購手機(jī)的時候也同樣適用。尤其是很多用戶在購買手機(jī)的時候，都會專注于某一個參數(shù)，比如要求處理器主頻要高達(dá)1GHz，但一部手機(jī)的整機(jī)表現(xiàn)是由多個因素組成，所以在購買手機(jī)的時候一定要從整體的角度上來看一部手機(jī)的性除了處理器主頻以外，其實還有很多影響整機(jī)表現(xiàn)的元素，比如運行內(nèi)存(RAM)、機(jī)身內(nèi)存(ROM)、操作系統(tǒng)、廠商對系統(tǒng)的優(yōu)化都會有所影響。不過在很多用戶眼中，這幾項卻遠(yuǎn)沒有處理器主頻重要。而如果忽略這幾項的話，可能買到一個主頻很高，但整機(jī)性能卻仍然不令人滿意的機(jī)型。

用戶在選機(jī)過程中切勿只關(guān)注一個硬件參數(shù)，這樣很可能并不能買到一款理想的機(jī)型，就算買的手機(jī)主頻再高，運行內(nèi)存低的話仍然無法同時運行數(shù)個程序、機(jī)身內(nèi)存小的話則無法把太多的軟件安裝到機(jī)器上，這對整機(jī)耗電和運行速度方面都有所影響、而廠商優(yōu)化更是決定著一部手機(jī)的穩(wěn)定性和部分運行速度。綜上所述，大家在選購手機(jī)的時候一定要綜合考慮一款手機(jī)的硬件規(guī)格。除此之外，也不要把硬件看的太過重要，就比如蘋果iPhone3GS在硬件配置上并不出眾，但卻在操控手感以及軟件資源上目前難有機(jī)型企及。更高分辨率能獲得更為逼真細(xì)膩的顯示效果，所以對于屏幕的分辨率絕大多數(shù)人都會偏向于分辨率更高的機(jī)型。但對于筆者所說的高分辨率未必是好事會有所懷疑。其實這里說的高分辨率“不好”更多是指采用非主流的高分辨率機(jī)型。在此前，就有幾款“悲情”機(jī)型在分辨率上吃了不小的虧。

大名鼎鼎的HTCDiamond就是一款頗具代表意義的機(jī)型，Diamond上市的手機(jī)市場還處于QVGA時代、只有少數(shù)旗艦機(jī)皇采用WVGA這樣級別的屏幕，由于HTCDiamond卻采用了VGA這一過渡型的分辨率，而也正是因為這一點，Diamond很多軟件都未有支持或無法完美運行，可謂是一個不小的遺憾。除此之外，曾經(jīng)非常經(jīng)典的

現(xiàn)在是58頁\一共有62頁\編輯于星期四附贈人生心語人生太短，聰明太晚人生太短，聰明太晚(1)

我們都老得太快

卻聰明得太遲把錢省下來，等待退休后再去享受結(jié)果退休后，因為年紀(jì)大，身體差，行動不方便，哪里也去不成。錢存下來等養(yǎng)老，結(jié)果孩子長大了，要出國留學(xué)，要創(chuàng)業(yè)做生意，要花錢娶老婆，自己的退休金都被拗走了。人生太短，聰明太晚(2)

當(dāng)自己有足夠的能力善待自己時，就立刻去做，老年人有時候是無法做中年人或是青少年人可以做的事，年紀(jì)和健康就是一大因素。小孩子從小就告訴他，養(yǎng)你到高中，大學(xué)以后就要自立更生，要留學(xué)，創(chuàng)業(yè)，娶老婆，自己想辦法，自己要留多一點錢，不要為了小孩子而活我們都老得太快卻聰明得太遲，我的學(xué)長去年喪妻。這突如其來的事故，實在叫人難以接受，但是死亡的到來不總是如此。學(xué)長說他太太最希望他能送鮮花給他，但是他覺得太浪費，總推說等到下次再買，結(jié)果卻是在她死后，用鮮花布置她的靈堂。這不