Pleasenotethesewebpagesarepartofanassignmentfora

graduatecourseinAdvancedBiochemistryandMolecularBiology

BCMB8010attheUniversityofGeorgia.Questionsshouldbe

directedtoJasonMock1atmockj@rx.uga.edu

Transthyretin Abstract

Transthyretinisaserumprotein,chieflysynthesizedintheliver,whosefunctionisofvitalimportancetometabolism,cellulardifferentiation,anddevelopment.Transthyretinprimarilyservesasatransportproteinforthethyroidhormonethyroxine,themostabundantofallofthethyroidhormonesproducedinthebody.However,transthyretincanalsoformacomplexwithretinolbindingproteintoassistinthetransportofretinol.

Thyroxineisaprohormone,meaningthat,althoughitisactivetosomeextent,itisaprecursortoamoreactivehormonecalledtriiodothyronine.Bothhormonesareactiveintheirfreeforms,whichispreciselywhytransthyretinissoimportant.Lessthan1%ofallofthethyroxinesecretedintothebloodstreamisfree,whiletherestisboundbyoneofthreetransportproteins:transthyretin,thyroxinebindingglobulin,orserumalbumin.Thistightregulationofthethyroidhormoneskeepsmetabolismandcellulardevelopmentfunctioningproperly.

Structurally,transthyretinissimple,yetbeautiful.Itisahomotetramerthatismadeupoffourmonomersthatare127residueseach.Eachmonomerconsistsprimarilyofsheets,buteachonealsocontainsasinglehelix.Hydrophobicinteractionsbetweenthesheetslendonemonomertoformastructuralmotifwithanothermonomer1ContactJasonMockviaemail:mockj@rx.uga.eduortraditionalmail:DepartmentofPharmaceuticalandBiomedicalSciencesUniversityofGeorgia250WestGreenStreetAthens,Georgia306022352

knownasabetasandwich.Twoofthesedimerscanthenassociateinopposingorientationstoformthetetramericcomplex,whichhastwoactivebindingsitesbetweenthetwohomodimers.

Whilenormalfunctioningofthisproteinhelpsthebodytooperateproperly,abnormalactivitycanhavedireconsequences.Transthyretinisoneoftwentyknownsolubleproteinsthatcanforminsolublefibrilscalledamyloids,whichcanthendepositinanorgan,causingdysfunctionandaconditionknownasamyloidosis.

Transthyretinamyloidosiscanresultfromthenormalsequenceoftheproteinorasaresultofamutationthatcausesavariantformoftheprotein.Wildtypetransthyretinamyloidosishasalateonsetandgenerallyaffectstheheart,althoughsomeotherorgansmaybeinvolved.Whiletheexactstimulithatresultinfibrilformationarenotknown,thereisaclearcorrelationwithonsetandaging.

Therearemanydifferentvariantsequenceformsoftransthyretinamyloidosis.Nearly80variantformsoftheprotein,resultingfrompointmutationsinthegene,havebeenassociatedwithsomeformofamyloidosis.Variantsequencetransthyretinamyloidosiscanaffectorgansotherthantheheart,liketheGItractorthecarpalligaments,buttheheartandthenervesarecommonlyinvolved.Lastly,inspiteofitsseeminglysimplestructure,transthyretinisactuallyveryunique.Notonlydoitstwobindingsitesexhibitnegativecooperativity,arareoccurrenceasfarasproteinsgo,theyarealsoextremelyversatileintermsofthetypesofligandstheycanbindasidefromthyroxine.Furthermore,theproteincomplexformedbytransthyretinandretinolbindingproteinisanimmensestructure,withmanynoteworthyaspects. BackgroundandHistory

Transthyretinisaserumtransportproteinthatplaysaroleinthetransportofthyroxine,athyroidprohormone,andretinol,viaacomplexwithretinolbindingprotein[1].Thoughtheprimarypurposeofthispaperistoaddressthestructuralintricaciesofthisprotein,aproperunderstandingofthehistory,function,andclinicalrelevanceoftransthyretinisvitalinordertoappreciatethesignificanceitsstructure.TheevolutionofanameThecurrentunderstandingoftheproperties,structure,andfunctionoftransthyretin,shownbelowinFigure1,istheproductofmorethanfiftyyearsofresearch.Initiallycalledprealbuminwhenitwasdiscoveredbecauseitseparated

aheadofalbuminonserumproteinelectrophoresisgels,thenametransthyretinwassettleduponasinsightintoitsrolesinthebodywereelucidated.

AfterDewittGoodmanandcolleagues,whowereresearchingtheinteractionbetweentransthyretinandretinolbindingprotein,sequencedtransthyretinin1974,theyattemptedtohavethenameoftheproteinchangedfromprealbumintotransretin[2].However,othersbelievedthatitsroleinthetransportofthyroxinewasmoreimportant,andtherefore,shouldprovideabasisforthename.Thus,inordertoappeasebothfactions,thenomenclaturecommitteeoftheInternationalUnionofBiochemistryandMolecularBiologyeventuallydecidedonthenametransthyretinin1981[1].BodilydistributionWhilemosttransthyretinisproducedintheliveritisalsosynthesizedinseveralothertissuesincludingthekidney[3]andthechoroidplexus[4].Ofcourse,transthyretincanbefoundcirculatinginserum,whereitwasfirstisolated,butsubsequentstudieshavealsofounditpresentintheretinalpigmentepithelium,thepinealgland,andpancreaticisletcells[5,6].Butperhapsmoreimportantly,transthyretinmakesupasmuchas25%oftheproteincontentfoundinthecerebrospinalfluid[4].Withoutthehelpoftransport,transthyretinwouldhavedifficultycrossingthebloodcerebrospinalfluidbarrier;thus,theproductionofthis

proteinbythechoroidplexus,whichmainlyfunctionsintheproductionofthecerebrospinalfluid,isextremelyimportant.Experimentationhasvalidatedthistheory,byshowingthatmostofthetransthyretinsynthesizedinthechoroidplexusissecreteddirectlyintothecerebrospinalfluidratherthantheblood,establishingthechoroidplexusasthepredominatesourceofthisproteininthecentralnervoussystem[7].FunctionoftransthyretinNotsurprisingly,thedistributionoftransthyretininthebodyisassociatedwithitsfunction.Transthyretinhastwoprimaryduties:totransportthyroxine,themostabundanthormoneproducedinthethyroid,andtohelptransportretinolbycomplexingwithretinolbindingprotein[1].Transthyretinisoneofthreethyroxinetransportproteinsintheserum,theothersbeingalbuminandthyroxinebingingglobulin.Intheserum,transthyretinisresponsibleforbindingandtransportinganywherefrom1520%ofthethyroxinepresent,butintheCNS,itbindsandtransportsupto80%ofthyroxine,makingittheprimarythyroidhormonetransportproteinfoundinthecerebrospinalfluid[8].Thebindingofthyroxineiscrucialtomaintainingappropriatelevelsofthishormoneintheplasmaandwithincells.Unboundthyroxineismetabolicallyactive,andinitsactiveform,thyroxinecanhaveprofoundimpactsonacell,helpingtofacilitatecellulardifferentiation,development,andmetabolism.Thus,itisimportanttonotethattransthyretinandtheotherthyroidtransportproteinskeepnearly99%ofthyroxineinitsbound,inactivestate.Thissurplusofthyroxineinthebodyallowsforsensitivesignaling,astransthyretincantransportthyroxinetotissueswhereitisneededandquicklybindthehormonewhenitisnot[9].Furtherexemplifyingthecorrelationbetweenlocationandfunction,transthyretinisalsoresponsibleforassistinginthetransportofretinol.Asitwaspreviouslynoted,mostserumtransthyretinissynthesizedintheliver,whichalsohappenstobetheprimarystoragesiteofretinol.Retinolistransportedtothemembraneoftargetcellsbyassociatingwithretinolbindingprotein.However,retinolbindingproteinisarelativelysmall,withamolecularweightof21,000Da,allowingittobereadilyfilteredoutofthebloodbythekidneysandpreventingpropertransport[10].Complexingwithtransthyretin,whichhasamolecularweightof~55kDa[11],preventsglomerularfiltrationandincreasestheaffinityofretinolbindingproteinforretinol,allowingtransporttotargetcellsmorespecifically[10].BiologicalandGeneticAspectsofInterest

Whiletransthyretindoesnotfunctionenzymaticallyorundergoanyexcitingconformationalchanges,itsfunctionwithinthebodyiscrucial.Asthenextsectionwilldescribe,transthyretinisinvolvedinthedevelopmentofseveraldiseases,andthebiologicalandgeneticvariabilityoftransthyretinthathasbeenimplicatedintheformationofthesediseaseswillbecoveredindetail.

DiseasestatesrelatedtotransthyretinAmyloidosisisadiseasestatecharacterizedbydepositsofinsolubleproteinfibrilsthatcanaccumulateinvarioustissuesthroughoutthebodyandinterferewiththenormalfunction[1].Therearearoundtwentyproteinsandpolypeptides,ofwhichtransthyretinisone,thatareknowntobeassociatedwiththistypeoffibrilformation,andwhiletheexactmechanismoffibrilformationremainsamystery,allfibrils,regardlessoftheirparentprotein,sharethreetraits:fibrilsstronglybindtothedyeCongored,theyarelong,unbranched,andgenerallyhaveadiameterofapproximately100,andtheyconsistofsheetsthatcrossparalleltotheaxisofthefibril[12].Ofthevariousamyloiddiseases,includingprionandAlzheimersdisease,transthyretinisspecificallylinkedtofamilialamyloidpolyneuropathy,familialamyloidcardiomyopathy,andsenilesystematicamyloidosis.Infamilialamyloidpolyneuropathy,mostamyloiddepositsformintheheartandperipheralnerves[13],andalthoughthisdiseaseisusuallyaresultoftransthyretinfibrilaccumulation,geneticvariantsofapolipoproteinA1havealsobeenknowntoresultinthiscondition[1].Bothfamilialamyloidcardiomyopathyandsenilesystematicamyloidosisusuallycauseamyloidbuildupintheheart,andmaybeoneofthecausesofunexplainedcardiomyopathyandarrhythmia[14]Thediscoveryoftransthyretinsroleinamyloiddiseasewasmadein1978whenCostaetal.showedthatamyloidmaterialtakenfromcertainpatientsreactedwithantiserumofplasmatransthyretin[15].Thisfindingacceleratedresearchassociatedwithtransthyretinandhowamyloiddiseasesdevelopfromendogenousproteins.Recentresearchhasexpandedourunderstandingofhowthesefibrilsform.Inthecaseoftransthyretin,thehomotetramermustdissociateintomonomers,whichcanthenassembleintoamyloidfibrils[14].Itislikelythatthesefibrilsassembleheirchially,withindividualpolypeptidesformingprotofilaments,whicharethoughttobecomposedofsheetsthattwistaroundoneanotheroracore,whichmayormaynotbehollow.Jaroniecetal.usedmagicanglespinningNMRtodeterminehowanamyloidfibrilmightformfromapolypeptideregionoftransthyretin.MagicanglespinningNMRisuniqueinthatitallowedtheresearcherstocircumventtheproblemofalackofroutinestructurethathadpreventedelucidationviaxraycrystallographyandnormal,liquidstateNMR.Theyshowedthatan11residuefragmentoftransthyretin,whichisshowninFigure2,wascapableofselfassemblingintoanamyloidfibril[12].Theseresultsprovidevaluableinsightintohowthisdiseasemayprogress.

NaturallyoccurringvariantsoftransthyretinandamyloidosisWhilemostamyloiddiseasesareconsideredtobeproductsofregularaging,inwhichthewildtypeproteinbeginstoformfibrilsthatwillthenaccumulateasanindividualgetsolder,thereareapproximately80knownpointmutationsoftransthyretinthatareassociatedwithinheritedamyloidosis[1].Ofthesevariants,roughly70areknowntocontributetothedevelopmentoffamilialamyloidpolyneuropathy,whileonlyafewareassociatedwithfamilialamyloidcardiomyopathyandsenilesystematicamyloidosis[14].Thediscoverythatvariantsoftransthyretinarecloselyrelatedtoamyloidosisinitiatednewdirectionsinresearch,bothmolecularlyandgenetically.Someresearchersbeganlookingatthyroxinebindinganditsinvolvementindiseaseprogression,whileothershuntedforthesequenceofthegenebehindtheproblematicprotein.Ingeneral,thesevariantmutationsareautosomaldominant,meaningthatanindividualcanhaveonewildtypeandonemutantalleleandstillexhibitaphenotype.Mostcarriersareheterozygous,andinterestingly,theyusuallyexhibitmostlynormalproteinintheplasma,eventhoughitisassumedthatbothallelesaretranscribedequally[16].Attemptsweremadetodefine3Dstructuresofthesevariantsinordertoextrapolatewhytheymaybemorepredisposedtoformingamyloidfibrils;however,thisundertakingwascomplicatedbythefactthatmostindividualswithamutationareheterozygous,andtransthyretinhasauniqueability

toformhybridtetramers,inwhichanyofthemonomersubunitscanbeavariantorwildtypeformoftheprotein[1].Thisinterestingaspectoftheproteinmadetheelucidationofitsgenesequencethatmuchmorecrucial,asrecombinantDNAtechnologiescouldprovideamorefeasibleapproachtostudyingtheoveralleffectsofthesemutationsontheproteinstructure.SequencingofthetransthyretingeneanditsuseinresearchAlthoughothergroupshadreportedthesequenceofthecDNAofhumantransthyretin[1],thecompletegenesequencewasnotelucidateduntil1985byTsuzukietal.Therearethreeintronsseparatingthefourexonsofits6,945bpsequence,andoddly,therearetwoopenreadingframeswithinthefirstandthirdintrons(oneineach,respectively),butitisnotknownwhetherornottheseORFs2areactuallyexpressed[17].Thegeneislocatedonthelongarmofchromosome18[18],anditsaccessionnumberisNC_000018.Ofcourse,aftersequencingthehumantransthyretingene,theputativenextstepindeterminingtheprocessoffibrillogenesisandamyloidosiswastocreatearecombinantanimalmodel.Thegenewasalsosequencedinmice,anditwasextremelysimilartothehumangene:containingthesameconservedsequences,hepatocytespecificbindingsites,andchangesinonly25outofthe127residues.Yet,fibrilscontainingtransthyretinhaveneverbeenobservedinmurineamyloidosis[19].Researchershopedtoovercomethisbygeneratingrecombinantmicethatwouldexpressvariantformsofhumantransthyretin,andwhilemanyhavetried,therehavebeenvaryingamountsofsuccess.Althoughresearcherswereabletocauseamyloidformationsintheirmousemodels,therewasneveranyaccumulationintheperipheralnervoustissue,whichisoneofthehallmarksoftransthyretinrelatedamyloidosis.So,althoughthereasonwhyaccumulationneveroccurredintheperipheralnervoustissueisnotcompletelyunderstood,itisclearthatamousemodelwillnotsufficeasamodelsystemforstudyinghumantransthyretinamyloidosis[1].StructuralBiology

Asmentionedabove,transthyretinisanextremelyimportantproteinintermsofregularbodilyfunctionanddiseasestates.Althoughtransthyretiniscomposedoffouridenticalmonomers,unlessanindividualisexpressingoneoftheaforementionedvariantformsoftheprotein,itsstructuralfunctionsaresurprisinglydiverse.Inthenextsections,thestructuralbiologyoftransthyretinandhowitbindsthyroxine,exogenoussubstrates,andretinolbindingproteinwillbecovered.2ORF=OpenReadingFrame

StructureAspreviouslystated,transthyretinisahomotetramer;however,itsstructureisanythingbutlackluster.Transthyretinmonomersarecomposedprimarilyofsheetsinasandwichorientations,withoneshorthelix,consistingofonlynineresidues.Around45%oftheremainingaminoacidsmakeupthemonomerseightstrands.Asaresultofsevenaromaticresiduesonthesideoftheproteinoppositethehelix,thesheetsaremorespreadoutatthatend,asopposedtothesectionsofthesheetsthatareclosertothehelix,whicharetightlypacked[1].Themonomersformdimersviaextensivehydrogenbondingatapseudotwofoldaxisofsymmetry.Thehydrogenbondsoccuralongtwopairsofstrandsinwhichtheresiduesarecloselyjuxtaposed[1].ThesestandsandtheiradjacentresiduescanbeseeninFigure3,coloredmagenta.Thisbroadareaofhydrogenbondingallowstransthyretintoreadilyformdimersthatareextremelystable,whichaccountsforthefactthatthedimeristhebasicunitofthisproteininvivo,ratherthanthemonomer[9].

Unlikethetightlyassociateddimers,thetetramericformoftransthyretinisnotquiteasstrong.Thereisonlyasmallcontactregionbetweentheloopsofonedimerwiththesheetsoftheother.Thisbondresultsprimarilyfromhydrophilicandhydrophobicinteractions.TheseimportantloopregionscanbeseenhighlightedaboveinFigure4.Impressively,inspiteofthesmallareaofcontact,transthyretintetramershavebeenshowntobestableinapHrangebetween3.5and12[1].However,themostimportantaspectoftheformationofthistetramericstructurearethebindingsitesthatitcreatesforthyroxine.ThyroxineBindingThyroxineisthenaturalligandfortransthyretin.Therearetwobindingsitesforthishormone,bothofwhicharelocatedinthechannelthatisformedbythetetramericcomplex.Eachsiteisapproximately50longand8wide,andiscapableofbindingonethyroxinemoleculeatatime[1].Ataglance,theaboveinformationwouldmaketransthyretinseemcommonplace,asfarasproteinsgo;however,itisactuallyexceptionallyuniqueinseveralways.First,althoughtherearetwobindingsitespertetramer,transthyretinexhibitsnegativecooperativity,meaningitwillonlybindonemoleculeofthyroxineatatime,andtheotherbindingsitewillremainvacant.Negativecooperativityisrare

amongstproteins,andinthecaseoftransthyretin,itoccursbecausethediametersofthetwobindingsitesareslightlydifferent[20].Thebindingsiteisdividedintothreepartscalledhalogenbindingpockets3,orHBPs[14].TheprimarybindingsiteiswiderattheouterandinnerHBPsandmorerestrictiveinthemiddlepocket,whiletheoppositeistrueofthesecondarysite.Thus,thyroxinecanmoreeasilyenterandbindintheprimarysite.Oncebound,thyroxinecausesanincreaseinthediameteroftheprimarysite.Thisslightchangeinstructureresultsinachangeinconformationatthesecondarysite.Despitethefactthatthesesitesarenotdirectlyconnected,thechangeinonesitecanbefeltintheotherduetothestronghydrogenbondsconnectingthemonomers[20].Theconformationalchangeinthesecondarysiteresultsintheopeningofthesitesothatthedimensionsaresimilartothatoftheinitialprimarysite.Thus,athyroxinemoleculemayenterthesecondarysite,butifthisoccurs,itwillforceaconformationalshiftintheoppositedirectionastheonethattookplaceafterthefirstligandwasbound.Sincethefirstthyroxinehasalreadyestablishedbindinginteractionswiththeprimarysite,thisbackwardsshiftcannotoccurasreadily,andtheconformationalchangesthatwouldbenecessarytotightlybindthethyroxineinthesecondarysitecannottakeplace.Asaresult,thesecondarysitewillnotbindthyroxineafteramoleculeofthehormoneisboundintheprimarysite,thusexplainingthenegativelycooperativenatureoftheprotein[20].Furthermore,transthyretinisuniqueinitsmodeofbindingthyroxine.Thebindingsitesforthyroxinearelocatedwithinatwofoldaxisofsymmetrybetweenthetwodimersofthetetramericcomplex.Asaresult,thebindingpocketitselfhasatwofoldaxisofsymmetry,whichallowsthethyroxinemoleculetobindintwodifferentmodesororientations.AsshowninFigure5,athyroxinemoleculecanfaceeitherwaywithinthebindingsiteandstillhavethenecessaryinteractionstoremainbound.Ineithermode,thehydrophilictailofthyroxineispositionedattheopeningofthebindingsitetoexposeittotheaqueousenvironmentandthesameactiveresidues,detailedinFigure5,interactwiththerestofmolecule,withtheconformationalchangesdescribedabovemakinghydrogenbondingpossiblewitheitheralignmentofthemolecule[14].TheseactiveresiduesarearrangedintothreeunitscalledHalogenBindingPockets,orHBPs,and,asthenamesuggests,theyinteractwiththeiodineatomsonthyroxine.HBP3istheinnermostpocket,coloredblueinFigure5,anditconsistsofSer117,Thr118and119,Ala108and109,andLeu110.HBP2,coloredcyaninFigure5,sharesLeu110andAla109withHBP3,butalsocontainsLys15,andLeu17.Lastly,HBP1,theoutermostpocket,ismadeupofAla108,Thr106,Met13,andLys15,anditiscoloredmagentainFigure5[14]. 3HBP=HalogenBindingPocket

Therelativeversatilityofthebindingsitesoftransthyretinisnotonlysignificantwithrespecttoitsbindingofthyroxine,butalsotonaturalandexogenousanaloguesofthehormonethatmaybeofclinicalimportance.TheimportanceofothersubstrateTodate,theonlyeffectivetreatmentforfamilialamyloidpolyneuropathyislivertransplantation[Original],asmosttransthyretinisproducedwithintheliver,butsincestudieshavesuggestedthatvariantproteinproductioninthechoroidplexusismorelikelytobetheprimarycauseofthiscondition,itisanincompletesolutionatbest[21].However,thereisstillhopeinthefighttoslowtheprogressionofthisdiseaseandtheothertransthyretinassociatedamyloiddiseasesintheformofthyroxineanalogues.Thesemoleculesactasinhibitorsoftransthyretinamyloidosisbybindingintheactivesite,whichstabilizesthetetramericstructureoftheprotein.Sincetransthyretinhastodissociateintomonomersinorderforfibrillogenesistooccur,thestabilizationofthenativetetramericformoftheproteincouldprovideatherapeuticapproachtoslowingtheprogressionofamyloidosiswithouttheneedforinvasivesurgery.Figure6showsthestructureofthyroxine,flufenamicacid,resveratrol,andoFLU,fourmoleculesthatbindtransthyretinandarerepresentativeofdifferentclassesofamyloidosisinhibitors:thenaturalligand,nonsteroidalantiinflammatorydrugs,naturalcompounds,anddesignedamyloidosis

inhibitors,respectively[14].Thankstoitsflexiblebindingsites,therearemanyotherexamplesofmoleculesthatcanbindtransthyretinaswell,butthoseshowninFigure6shouldprovidesomeideaofthetypesofclassesandgeneralstructuresofthesepotentialamyloidosisinhibitors.

RetinolBindingProteinComplexAsidefromtransportingthyroxine,transthyretinisalsophysiologicallyimportantforitsroleintransportingretinol,orvitaminA.Retinolisstoredintheliver,andinordertobesecretedtoothertissuesitmustbindretinolbindingprotein4,orRBP,inahandinglovemanner,inwhichtheretinolmoleculefitssnuglyintotheeightstrandbarrelthatmakesupthemajorityoftheRBPstructure[11].However,asinglemoleculeofRBPisonly21kDa,anditcanreadilybefilteredoutofthebloodstreambytheglomerulusinthekidneys,preventingRBPfromdeliveringretinoltothetissuesthatneedit.Luckily,thisproblemcanbeovercomewhenRBPformsacomplexwithtransthyretin[1]ThistransportcomplexconsistsofonetetramerictransthyretinmoleculeandtwoRBPs.EachRBPbindstransthyretinataninterfacebetweentwomonomersinatwofoldaxisofsymmetryonoppositedimers.Eachtransthyretinmonomer4RBP=RetinolBindingProtein

contributesfouraminoacidresiduestothebindingsite:Arg21,Val20,Leu82,andIle84.TheseeightresiduesfromtransthyretinprovideaninterfaceforinteractionwithfourresiduesfromRBP:Trp67,Phe96,andLeu97and63.Furthermore,extensivehydrophobicinteractionbetweentheCterminusofRBP,whichhasanhelicalsecondarystructure,andtransthyretinensurethattheproteinsstaytightlyboundwhileincirculation[11].ThesecontactpointsarehighlightedbelowinFigure7.

ThetransthyretinRBPcomplexisextremelyimportantforretinoltransportnotonlybecauseitwouldscarcelytakeplacewhereitnotforcomplexion,butalsobecausethebindingofRBPtotransthyretinincreasestheproteinsaffinityforretinol,whichhelpspreventnonspecificdeliveryofretinoltounintendedtargettissues[10].Additionally,itmaybeofinteresttonotethatpointmutationscausinggeneticvariantsoftransthyretincanhaveaprofoundimpactonretinoltransportanddistribution.Asingleaminoacidsubstitutionatposition84canproducedramaticeffects,emphasizingthisresiduesimportancetothebindingprocess.Specifically,individualsthathaveapointmutationthatsubstitutesthewildtypeisoleucineatthispositionwithserineorasparaginehadsignificantlydecreasedamountsofRBPincirculation,asitwasunabletoformthetransportcomplexwithtransthyretin[22].

ConclusionInsummary,transthyretinisanextremelyuniqueserumtransportprotein.Itfacilitatesthetransportofthethyroidhormonethyroxineandretinolviaaversatile,negativelycooperativebindingsitesandamultiproteintransportcomplex.Furthermore,asidefromitsnormalrolesinthebody,itcanalsoplayaroleindiseaseformationthroughaprocessknownasfibrillogenensisthatresultsinamyloidformationsinvarioustissues.Thus,athoroughunderstandingofthisproteinanditsstructureisnotonlyrelevantinaphysiologicalsense,butalsointermsofclinicalapplication.

References

1. HamiltonJ.A.andBensonM.D.(2001)Transthyretin:areviewfromastructuralperspective.Cell.Mol.LifeSci.58:149115212. KandaY.,GoodmanD.S.,CanfieldR.E.andMorganF.J.(1974)Theaminoacidsequenceofhumanplasmaprealbumin.J.Biol.Chem.249:679668053. WakasugiS.,MaedaS.andShimadaK.(1986)Structureandexpressionofthemouseprealbumingene.J.Biochem.(Tokyo)100:49584. HerbertJ.,Wilcox,J.N.,PhamK.T.C.,FremeauR.T.,ZevianaM.,DworkA.,SopranoD.R.etal.(1986)Transthyretin:achoroidsplexusspecifictransportproteininhumanbrain.Neurology36:9009115. MartoneR.L.,MizunoR.andHerbertJ.(1992)Themammalianpinealisasyntheticsitefortransthyretinandretinalbindingprotein.2ndInternationalSymposiumonFamilialAmyloidoticPolyneuropathyandOtherTransthyretinRelatedDisorders,Skelleftea,Sweden.ProgramandAbstracts,0.3,p.156. KamekoM.,IchikawaM.,KatsuyamaT.,KanaiM.,KatoM.andAkamatsuT.(1986)Immunohisochemicallocalizationofplasmaretinolbindingproteinandprealbumininhumanpancreaticislets.Histochem.J.18:1641687. SchreiberG.,AldredA.R.,JaworowskiA.,NilssonC.,AchenM.G.andSegalM.B.(1990)Thyroxinetransportfrombloodtobrainviatransthyretinsynthesisinchoroidplexus.Am.J.Physiol.258:R338R3458. HagenG.A.andEllliotW.J.(1973)Transportofthyroidhormonesinserumandcerebrospinalfluid.J.Clin.Endocrinol.37:4154229. RobbinsJ.,ChengS.Y.,GershengornM.C.,GlinoerD.,CahnmannH.J.,EdelnochH.(1978)Thyroxinetransportproteinsofplasma:molecularpropertiesandbiosynthesis.RecentProg.Horm.Res.34:47751910. GoodmanD.S.andRazA.(1972)Extractionandrecombinationstudiesoftheinteractionofretinolwithhumanplasmaretinolbindingprotein.J.LipidRes.13:33834711. NaylorH.M.andNewcomerM.E.(1999)Thestructureofhumanretinol

bindingprotein(RBP)withitscarrierproteintransthyretinrevealsaninteractionwiththecarboxyterminusofRBP.Biochem.38:2647265312. JaroniecC.P.,MacPheeC.E.,BajajV.S.,McMahonM.T.,DobsonC.M.,GriffinR.G.(2004)HighresolutionmolecularstructureofapeptideinanamyloidfibrildeterminedbymagicanglespinningNMRspectroscopy.ProcNatlAcadSci.USA.101(3):71171613. SackG.H.,DumarsK.W.,GummersonK.S.,LawA.andMcKusickV.A.(1981)Threeformsofdominantamyloidneuropathy.JohnsHopkinsMed.J.149:23924414. KlabundeT.,PetrassiH.M.,OzaV.B.,RamanP.,KellyJ.W.,SacchettiniJ.C.(2000)Rationaldesignofpotenthumantransthyretinamyloiddiseaseinhibitors.Nat.Struct.Biol.7:31232115. CostaP.P.,FigueraA.S.andBravoF.R.(1978)Amyloidfibrilproteinrelatedtoprealbumininfamilialamyloidoticpolyneuropathy.Proc.Natl.Acad.Sci.USA.75:4499450316. DwuletF.E.andBensonM.D.(1986)CharacterizationofaprealbuminvariantassociatedwithfamilialamyloidoticpolyneuropathytypeII(Indiana/Swiss).J.Clin.Invest.78:88088617. TsuzukiT.,MitaS.,MaedaS.,ArakiS.andShimadaK.(1985)Structureofthehumanprealbumingene.J.Biol.Chem.260:122231222718. WallaceM.R.,DwuletF.E.,ConneallyP.M.andBensonM.D.(1986)Biochemicalandmoleculargeneticcharacteristicofanewvariantprealbuminassociatedwithhereditaryamyloidosis.J.Clin.Invest.78:61219. WakasugiS.,MaedaS.,ShimadaK.,NakashimaH.andMigitaS.(1985)Structuralcomparisonsbetweenmouseandhumanprealbumin.J.Biochem.(Tokyo)98:1707171420. NeumannP.,Cody,V.,andWojtczak,A.(2001)Structuralbasisofnegativecooperativityintransthyretin.Acta.Biochim.Pol.48(4):86787521. TakahashiK.,YiS.,KimuraY.andArakiS.(1991)Familialamyloidoticpolyneuropathytype1inKumamoto,Japanaclinicopathologic,histochemical,immunohistochemical,andultrastructuralstudy.Hum.Pathol.22:51952722. WaitsR.P.,YamadaT.,UemichiT.andBensonM.D.(1995)Lowplasmaconcentrationsofretinolbindingproteininindividualswithmutationsaffectingposition84ofthetransthyretinmolecule.Clin.Chem.41:12881291