Fluid mosaic model - Wikipedia

The fluid mosaic model explains various observations regarding the structure of functional cell membranes. According to this biological model, ... Fluidmosaicmodel FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Describethefluidmosaicmodelofplasmamembrane Fluidmosaicmodelofacellmembrane Thefluidmosaicmodelexplainsvariousobservationsregardingthestructureoffunctionalcellmembranes.Accordingtothisbiologicalmodel,thereisalipidbilayer(twomoleculesthicklayerconsistingprimarilyofamphipathicphospholipids)inwhichproteinmoleculesareembedded.Thephospholipidbilayergivesfluidityandelasticitytothemembrane.Smallamountsofcarbohydratesarealsofoundinthecellmembrane.Thebiologicalmodel,whichwasdevisedbySeymourJonathanSingerandGarthL.Nicolsonin1972,describesthecellmembraneasatwo-dimensionalliquidthatrestrictsthelateraldiffusionofmembranecomponents.Suchdomainsaredefinedbytheexistenceofregionswithinthemembranewithspeciallipidandproteincocoonthatpromotetheformationoflipidraftsorproteinandglycoproteincomplexes.Anotherwaytodefinemembranedomainsistheassociationofthelipidmembranewiththecytoskeletonfilamentsandtheextracellularmatrixthroughmembraneproteins.[1]Thecurrentmodeldescribesimportantfeaturesrelevanttomanycellularprocesses,including:cell-cellsignaling,apoptosis,celldivision,membranebudding,andcellfusion.Thefluidmosaicmodelisthemostacceptablemodeloftheplasmamembrane.Itsmainfunctionistoseparatethecontentsofthecellfromtheexterior. Contents 1Chemicalmakeup 2Experimentalevidence 3Subsequentdevelopments 3.1Membraneasymmetry 3.2Non-bilayermembranes 3.3Membranecurvature 3.4Lipidmovementwithinthemembrane 4Restrictionstobilayerfluidity 4.1Lipidrafts 4.2Proteincomplexes 4.3Cytoskeletalfences(corrals)andbindingtotheextracellularmatrix 5Historicaltimeline 6Notesandreferences Chemicalmakeup[edit] Components Location Functions Phospholipid Themainfabricofplasmamembrane Itprovidesselectivepermeabilitytothecellmembrane. Carbohydrates Attachedtoproteinsonoutsidemembranelayers Ithelpsincell-to-cellrecognition. Cholesterol Betweenphospholipidsandphospholipidbilayers Ithelpstheplasmamembranetoretainitsfluidity. Proteins Embeddedwithinoronthesurfaceofphospholipidlayers Theseformchannelstoallowthemovementofmolecules. Experimentalevidence[edit] Thefluidpropertyoffunctionalbiologicalmembraneshadbeendeterminedthroughlabelingexperiments,x-raydiffraction,andcalorimetry.Thesestudiesshowedthatintegralmembraneproteinsdiffuseatratesaffectedbytheviscosityofthelipidbilayerinwhichtheywereembedded,anddemonstratedthatthemoleculeswithinthecellmembranearedynamicratherthanstatic.[2] PreviousmodelsofbiologicalmembranesincludedtheRobertsonUnitMembraneModelandtheDavson-DanielliTri-Layermodel.[1]Thesemodelshadproteinspresentassheetsneighboringalipidlayer,ratherthanincorporatedintothephospholipidbilayer.Othermodelsdescribedrepeating,regularunitsofproteinandlipid.Thesemodelswerenotwellsupportedbymicroscopyandthermodynamicdata,anddidnotaccommodateevidencefordynamicmembraneproperties.[1] TheFrye-Edidinexperimentshowedthatwhentwocellsarefusedtheproteinsofbothdiffusearoundthemembraneandmingleratherthanbeinglockedtotheirareaofthemembrane. AnimportantexperimentthatprovidedevidencesupportingfluidanddynamicbiologicalwasperformedbyFryeandEdidin.TheyusedSendaivirustoforcehumanandmousecellstofuseandformaheterokaryon.Usingantibodystaining,theywereabletoshowthatthemouseandhumanproteinsremainedsegregatedtoseparatehalvesoftheheterokaryonashorttimeaftercellfusion.However,theproteinseventuallydiffusedandovertimetheborderbetweenthetwohalveswaslost.Loweringthetemperatureslowedtherateofthisdiffusionbycausingthemembranephospholipidstotransitionfromafluidtoagelphase.[3]SingerandNicolsonrationalizedtheresultsoftheseexperimentsusingtheirfluidmosaicmodel.[2] Thefluidmosaicmodelexplainschangesinstructureandbehaviorofcellmembranesunderdifferenttemperatures,aswellastheassociationofmembraneproteinswiththemembranes.WhileSingerandNicolsonhadsubstantialevidencedrawnfrommultiplesubfieldstosupporttheirmodel,recentadvancesinfluorescencemicroscopyandstructuralbiologyhavevalidatedthefluidmosaicnatureofcellmembranes. Subsequentdevelopments[edit] Membraneasymmetry[edit] Additionally,thetwoleafletsofbiologicalmembranesareasymmetricanddividedintosubdomainscomposedofspecificproteinsorlipids,allowingspatialsegregationofbiologicalprocessesassociatedwithmembranes.Cholesterolandcholesterol-interactingproteinscanconcentrateintolipidraftsandconstraincellsignalingprocessestoonlytheserafts.[4]AnotherformofasymmetrywasshownbytheworkofMouritsenandBloomin1984,wheretheyproposedaMattressModeloflipid-proteininteractionstoaddressthebiophysicalevidencethatthemembranecanrangeinthicknessandhydrophobicityofproteins.[5] Non-bilayermembranes[edit] Theexistenceofnon-bilayerlipidformationswithimportantbiologicalfunctionswasconfirmedsubsequenttopublicationofthefluidmosaicmodel.Thesemembranestructuresmaybeusefulwhenthecellneedstopropagateanonbilayerform,whichoccursduringcelldivisionandtheformationofagapjunction.[6] Membranecurvature[edit] Themembranebilayerisnotalwaysflat.Localcurvatureofthemembranecanbecausedbytheasymmetryandnon-bilayerorganizationoflipidsasdiscussedabove.MoredramaticandfunctionalcurvatureisachievedthroughBARdomains,whichbindtophosphatidylinositolonthemembranesurface,assistinginvesicleformation,organelleformationandcelldivision.[7]Curvaturedevelopmentisinconstantfluxandcontributestothedynamicnatureofbiologicalmembranes.[8] Lipidmovementwithinthemembrane[edit] Duringthedecadeof1970,itwasacknowledgedthatindividuallipidmoleculesundergofreelateraldiffusionwithineachofthelayersofthelipidmembrane.[9]Diffusionoccursatahighspeed,withanaveragelipidmoleculediffusing~2 µm,approximatelythelengthofalargebacterialcell,inabout1second.[9]Ithasalsobeenobservedthatindividuallipidmoleculesrotaterapidlyaroundtheirownaxis.[9]Moreover,phospholipidmoleculescan,althoughtheyseldomdo,migratefromonesideofthelipidbilayertotheother(aprocessknownasflip-flop).However,flip-flopmightbeenhancedbyflippaseenzymes.Theprocessesdescribedaboveinfluencethedisorderednatureoflipidmoleculesandinteractingproteinsinthelipidmembranes,withconsequencestomembranefluidity,signaling,traffickingandfunction. Restrictionstobilayerfluidity[edit] Therearerestrictionstothelateralmobilityofthelipidandproteincomponentsinthefluidmembraneimposedbytheformationofsubdomainswithinthelipidbilayer.Thesesubdomainsarisebyseveralprocessese.g.bindingofmembranecomponentstotheextracellularmatrix,nanometricmembraneregionswithaparticularbiochemicalcompositionthatpromotetheformationoflipidraftsandproteincomplexesmediatedbyprotein-proteininteractions.[1]Furthermore,protein-cytoskeletonassociationsmediatetheformationof“cytoskeletalfences”,corralswhereinlipidandmembraneproteinscandiffusefreely,butthattheycanseldomleave.[1]Restrictiononlateraldiffusionratesofmembranecomponentsisveryimportantbecauseitallowsthefunctionalspecializationofparticularregionswithinthecellmembranes. Lipidrafts[edit] Lipidraftsaremembranenanometricplatformswithaparticularlipidandproteincompositionthatlaterallydiffuse,navigatingontheliquidbilipidlayer.Sphingolipidsandcholesterolareimportantbuildingblocksofthelipidrafts.[10] Proteincomplexes[edit] Cellmembraneproteinsandglycoproteinsdonotexistassingleelementsofthelipidmembrane,asfirstproposedbySingerandNicolsonin1972.Rather,theyoccurasdiffusingcomplexeswithinthemembrane.[1]Theassemblyofsinglemoleculesintothesemacromolecularcomplexeshasimportantfunctionalconsequencesforthecell;suchasionandmetabolitetransport,signaling,celladhesion,andmigration.[1] Cytoskeletalfences(corrals)andbindingtotheextracellularmatrix[edit] Someproteinsembeddedinthebilipidlayerinteractwiththeextracellularmatrixoutsidethecell,cytoskeletonfilamentsinsidethecell,andseptinring-likestructures.Theseinteractionshaveastronginfluenceonshapeandstructure,aswellasoncompartmentalization.Moreover,theyimposephysicalconstraintsthatrestrictthefreelateraldiffusionofproteinsandatleastsomelipidswithinthebilipidlayer.[1] Whenintegralproteinsofthelipidbilayeraretetheredtotheextracellularmatrix,theyareunabletodiffusefreely.Proteinswithalongintracellulardomainmaycollidewithafenceformedbycytoskeletonfilaments.[11]Bothprocessesrestrictthediffusionofproteinsandlipidsdirectlyinvolved,aswellasofotherinteractingcomponentsofthecellmembranes. S.cerevisiaeseptinsSeptinring-likestructures(ingreen)canpinchcellmembranesandsplitthemintosubdomains. SeptinsareafamilyofGTP-bindingproteinshighlyconservedamongeukaryotes.Prokaryoteshavesimilarproteinscalledparaseptins.Theyformcompartmentalizingring-likestructuresstronglyassociatedwiththecellmembranes.Septinsareinvolvedintheformationofstructuressuchas,ciliaandflagella,dendriticspines,andyeastbuds.[12] Historicaltimeline[edit] 1895–ErnestOvertonhypothesizedthatcellmembranesaremadeoutoflipids.[13] 1925–EvertGorterandFrançoisGrendelfoundthatredbloodcellmembranesareformedbyafattylayertwomoleculesthick,i.e.theydescribedthebilipidnatureofthecellmembrane.[14] 1935–HughDavsonandJamesDanielliproposedthatlipidmembranesarelayerscomposedbyproteinsandlipidswithpore-likestructuresthatallowspecificpermeabilityforcertainmolecules.Then,theysuggestedamodelforthecellmembrane,consistingofalipidlayersurroundedbyproteinlayersatbothsidesofit.[15] 1957–J.DavidRobertson,basedonelectronmicroscopystudies,establishesthe"UnitMembraneHypothesis".This,statesthatallmembranesinthecell,i.e.plasmaandorganellemembranes,havethesamestructure:abilayerofphospholipidswithmonolayersofproteinsatbothsidesofit.[16] 1972–SJSingerandGLNicolsonproposedthefluidmosaicmodelasanexplanationforthedataandlatestevidenceregardingthestructureandthermodynamicsofcellmembranes.[2] Notesandreferences[edit] ^abcdefghNicolsonGL(2014)."TheFluid—MosaicModelofMembraneStructure:Stillrelevanttounderstandingthestructure,functionanddynamicsofbiologicalmembranesaftermorethan40years".BiochimicaetBiophysicaActa(BBA)-Biomembranes.1838(6):1451–146.doi:10.1016/j.bbamem.2013.10.019.PMID 24189436. ^abcSingerSJ,NicolsonGL(Feb1972)."Thefluidmosaicmodelofthestructureofcellmembranes".Science.175(4023):720–31.doi:10.1126/science.175.4023.720.PMID 4333397.S2CID 83851531. ^FryeLD,EdidinM(1970)."Therapidintermixingofcellsurfaceantigensafterformationofmouse-humanheterokaryons".JCellSci.7(2):319–35.doi:10.1242/jcs.7.2.319.PMID 4098863. ^SilviusJR(2005)."Partitioningofmembranemoleculesbetweenraftandnon-raftdomains:Insightsfrommodel-membranestudies".BiochimicaetBiophysicaActa(BBA)-MolecularCellResearch.1746(3):193–202.doi:10.1016/j.bbamcr.2005.09.003.PMID 16271405. ^MouritsenOG,BloomM(1984)."Mattressmodeloflipid-proteininteractionsinmembranes".BiophysJ.46(2):141–153.doi:10.1016/S0006-3495(84)84007-2.PMC 1435039.PMID 6478029. ^vandenBrink-vanderLaanE;et al.(2004)."Nonbilayerlipidsaffectperipheralandintegralmembraneproteinsviachangesinthelateralpressureprofile".BiochimBiophysActa.1666(1–2):275–88.doi:10.1016/j.bbamem.2004.06.010.PMID 15519321. ^FrostA;et al.(2009)."TheBARdomainsuperfamily:membrane-moldingmacromolecules".Cell.137(2):191–6.doi:10.1016/j.cell.2009.04.010.PMC 4832598.PMID 19379681. ^Rodríguez-GarcíaR;et al.(2009)."Bimodalspectrumforthecurvaturefluctuationsofbilayervesicles:purebendingplushybridcurvature-dilationmodes".PhysRevLett.102(12):128101.doi:10.1103/PhysRevLett.102.128101.PMID 19392326. ^abcAlbertsB,JohnsonA,LewisJ,et al.(2008).MolecularBiologyoftheCell(5th ed.).NewYork:GarlandScience.pp. 621–622.ISBN 978-0-8153-4105-5. ^LingwoodD,SimonsK(2010)."Lipidraftsasamembrane-organizingprinciple".Science.327(5961):46–50.doi:10.1126/science.1174621.PMID 20044567.S2CID 35095032. ^G.Vereb;et al.(2003)."Dynamic,yetstructured:ThecellmembranethreedecadesaftertheSinger–Nicolsonmodel".PNAS.100(14):8053–8058.doi:10.1073/pnas.1332550100.PMC 166180.PMID 12832616. ^JuhaSaarikangas;YvesBarral(2011)."Theemergingfunctionsofseptinsinmetazoans".EMBOReports.12(11):1118–1126.doi:10.1038/embor.2011.193.PMC 3207108.PMID 21997296. ^Overton,E(1895)."UberdieosmotischenEigenshafterderLebendenPflanzenundtierzelle".VJSCHRNaturfGesZurich.40:159–201. ^E.Gorter;F.Grendel(1925)."OnBiomolecularLayersofLipoidsontheChromocytesoftheBlood".JournalofExperimentalMedicine.41(4):439–443.doi:10.1084/jem.41.4.439.PMC 2130960.PMID 19868999. ^JamesDanielli;HughDavson(1935)."Acontributiontothetheoryofpermeabilityofthinfilms".JournalofCellularandComparativePhysiology.5(4):495–508.doi:10.1002/jcp.1030050409. ^JohnE.Heuser(1995)."InMemoryofJ.DavidRobertson"(PDF).NewsletteroftheAmericanSocietyofCellBiology. vteStructuresofthecell/organellesEndomembranesystem Cellmembrane Nucleus Endoplasmicreticulum Golgiapparatus Parenthesome Autophagosome Vesicle Exosome Lysosome Endosome Phagosome Vacuole Acrosome Cytoplasmicgranule Melanosome Microbody Glyoxysome Peroxisome Weibel–Paladebody Cytoskeleton Microfilament Intermediatefilament Microtubule Prokaryoticcytoskeleton Microtubuleorganizingcenter Centrosome Centriole Basalbody Spindlepolebody Myofibril Undulipodium Cilium Flagellum Axoneme Radialspoke Pseudopodium Lamellipodium Filopodium Endosymbionts Mitochondrion Plastid Chloroplast Chromoplast Gerontoplast Leucoplast Amyloplast Elaioplast Proteinoplast Tannosome Otherinternal Nucleolus RNA Ribosome Spliceosome Vault Cytoplasm Cytosol Inclusions Proteasome Magnetosome External Cellwall Extracellularmatrix vteStructuresofthecellmembraneMembranelipids Lipidbilayer Phospholipids Lipoproteins Sphingolipids Sterols Membraneproteins Membraneglycoproteins Integralmembraneproteins/transmembraneprotein Peripheralmembraneprotein/Lipid-anchoredprotein Other Caveolae/Coatedpits Celljunctions Glycocalyx Lipidraft/microdomains Membranecontactsites Membranenanotubes Myelinsheath NodesofRanvier Nuclearenvelope Phycobilisomes Porosomes Retrievedfrom"https://en.wikipedia.org/w/index.php?title=Fluid_mosaic_model&oldid=1098008683" Categories:MembranebiologyOrganellesCellanatomyHiddencategories:ArticleswithshortdescriptionShortdescriptionisdifferentfromWikidata Navigationmenu Personaltools NotloggedinTalkContributionsCreateaccountLogin Namespaces 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