班主任的翻译.docx

班主任的翻译.docx

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班主任的翻译

Theoutersurfaceofthehyphalorsporewallsinthesecasesisusuallyfoundtobecomposedofalayerof10-nm-widerodletscomposedofproteinswhichmodifythebiophysicalpropertiesofthewallsurface;themostcommonlyencounteredfamilyofsuchproteinsarecalled hydrophobins （Wessels,1996;Wösten&deVocht,2000;Sunde etal.,2008;Cox&Hooley,2009）.Hydrophobinsbelongtoalarge,diversegroupofrelatedproteinsfoundwidelyinthefungi;whenexpressedtothemaximumtheymayconstituteupto10%oftotalwallprotein.Eachmoleculeconsistsofahydrophobicdomainandahydrophilicdomain;thatis,theyare amphipathic （atermyoumayhavemetinrelationtothephospholipidsthatmakeupbiologicalmembranes,whichalsohaveahydrophilicgroupatoneendandahydrophobicgroupattheother）.Theiramino-terminalpartdeterminesthehydrophilicsideoftheassemblage.Theamphipathicstructureofhydrophobinsprovidesthemoleculeswithanextraordinarypotentialarrayoffunctions:

∙enablehyphaetobreakthroughthewater/airinterfaceoffluidhabitats;

∙providethehydrophobicityrequiredbyhyphaeandsporesincontactwithair;

∙participateinmorphogeneticsignalling,initiatingconidiationandfruitbodyformation;

∙haveimportantrolesintissueformation,particularlyincontrollingfluidandairspaces;

∙promoteadhesionbetweenthecellwallofthefungusandthehydrophobicsurfacesofplantsandinsects,andsopotentiateinfectionandaidpenetrationofthehostsurface;

∙failtoactivatetheimmunesystem;e.g.aerialconidiaof Aspergillus, Penicillium and Cladosporium thathavesurfacelayersofhydrophobin.Thehydrophobinrodletsaresaidto‘immunologicallysilenceairbornemoulds’,whichmeansthatalthoughfungalsporesareubiquitousintheairwebreathetheyneithercontinuouslyactivatehostimmunitynorinduceinflammatoryresponsesafterinhalation（Aimanianda etal.,2009）;

∙mediatesymbioticinteractionswithplantroots（mycorrhizas）andalgae（lichens）.

Themoleculesthatdoallthisarerelativelysmallproteins,usuallyaround100aminoacids,thathaveextensivehomologiesandcharacteristicallycontainsignalsequencesforsecretion,andeightcysteineresiduesconservedinthesameposition.Theseeightconservedcysteineresiduesformfourdisulphidebridgesandtheypreventself-assemblyofthehydrophobinintheabsenceofahydrophilic/hydrophobicinterface.HydrophobinsareuniquetomycelialfungibutareexpressedbybothAscomycotaandBasidiomycota.Eachfungushasgenesformorethanone,oftenmorethantendifferenthydrophobins,andthegenesareusuallyexpressedatdifferenttimes.In Schizophyllumcommune thehydrophobinfoundinthevegetativehyphalwalldiffersfromthatexpressedinthehyphalwallsofthefruitbody（Wessels,1996）.

Hydrophobinsareexcretedfromthehyphaltip;ifthehyphaisinanaqueousenvironment,thehydrophobinspassintosolution.Buttheproteinmoleculesareabletoself-assembleintocoveringfilmsatthewater/air（i.e.hydrophilic/hydrophobic）interfaceandwhenahyphaemergesfromthesolution,thepolypeptidepolymerisesonthesurfaceofthehyphalwall,forminganarrayofparallelrodlets.Eachhydrophobinmoleculeisboundtothefungalwallbyitshydrophilicend,thehydrophobicdomainbeingexposedtotheoutsideworld（Linder etal.,2005;Cox&Hooley,2009）.Adifferenceinsolubilityoftheseassemblagesdivideshydrophobinsintotwogroups:

classIhydrophobinsformhighlyinsolublemembranesdissolvedonlybytrifluoroaceticacidandformicacid,whileassembliesofclassIIhydrophobinsdissolvereadilyinethanolorsodiumdodecylsulfate（SDS）.

Thehydrophobinassemblyonthehyphareduceswatermovementthroughthewall,givingprotectionfromdesiccation,buttheexposedhydrophobicsurfaceenablesbondingtootherhydrophobicsurfaces.Thishappensbecausehydrophobesarenotelectricallypolarisedandthelowestenergystatefortwohydrophobesisforthemtobondtogethertoexcludeelectricallypolarisedwatermolecules.Afungalwallcoatedwithhydrophobinswillbeabletousethishydrophobicinteractiontobondtootheraerialhyphae,leadingtotheformationofmulticellularhyphalstructures.Ahydrophobin-coatedsporecouldalsoattachimmediatelyandfirmlytothehydrophobic（forexample,waxy）surfaceofitsplantorinsecthost,givingtimeforformationofappressoriaorotherpenetrationstructures.

Withinfungaltissues,andthatincludesthetissuesoflichenthalli,hydrophobinsprovidecontroloverthemovementofwaterandgaseswithinthetissuebecausetheexposedhydrophobicdomainspreventwaterlogginginairspaces,andallowthefungustocontrolwhichchannelsthroughthetissueareusedformovementofwaterandaqueousnutrients,andwhicharekeptfreeoffluidandusedformovementofgases（Wösten&deVocht,2000;Sunde etal.,2008）.Theremarkableabilityofhydrophobinstochangethenatureofasurface（theyturnhydrophobicsurfaceshydrophilicandhydrophilicsurfaceshydrophobic）makehydrophobinsinterestingcandidatesforuseincommercialandmedicalapplications（Cox&Hooley,2009;Cox etal.,2009）.

Anotherfungalwallproteinthatdeservesspecificmentionistheglycoproteinknownas glomalin, whichisproduced abundantly inthewallsofhyphaeandsporesofarbuscularmycorrhizalfungiinsoilandinroots.Whatmakesthisproteindeservingofmentionistheamountofitthatisproduced.Itpermeatessoilorganicmatter,andcanaccountsforaround 30%ofthecarboninsoil.Inthesoil,glomalinformsclumpsofsoilgranulescalledaggregatesthataddstructuretosoil.ItissuchamajorcomponentofsoilorganicmatterthattheAgriculturalResearchServiceoftheUnitedStatesDepartmentofAgriculture,whichisnotnormallysensationalist,publishedanarticleinthe September2002issueofits AgriculturalResearch magazineentitled'Glomalin:

hidingplaceforathirdoftheworld’sstoredsoilcarbon',whichexplainsthat:

‘Glomalingivessoilitstilth-thatsubtletexturethatenablesexperiencedfarmersandgardenerstojudgegreatsoilbyfeelingthesmoothgranulesastheyflowthroughtheirfingers’（CLICKHERE toviewthepage[requiresInternetconnection]）.

GlomalinisproducedonlybymembersoftheGlomeromycota,fungithatformarbuscularmycorrhizas.Whenfirstdiscovereditwasclearlypresentinsuchquantitythatitmustmakeamassivecontributiontotheaggregationofsoilparticles.Soitwasfirstthoughtthatitmustbesecretedorotherwisereleasedintothesoilbyarbuscularmycorrhizalfungispecificallytocontrolsoilstructure,amechanismthathasbeencalledhabitatengineering.Theviewbeingthatincreasedsoilaggregation（i.e.improvedtilth）wouldbenefitthehostplant,andtherebytheassociatedmycorrhizalfungus,andsojustifytheenergetic‘cost’ofproducingtheglomalin.Thereissomeexperimentalsupportforthisideathoughthereisalsoevidencethatglomalinisnotsecreted,butiscovalentlyboundintothehyphalwallmatrixwhereitprotectsthehypha.Itmaybethatthecharacteristicsthatenableglomalintoprotectthehyphalwallalsoallowtheproteintopromotesoilaggregation（Driver etal.,2005;Purin&Rillig,2007）.

Updated February25,2011

6.9Thefungalwallasaclinicaltarget

Wewilldealwiththerapeuticantifungalagentsandtheirtargetsindetailin Chapter18,butwecan’tendadiscussionofhyphalcellwallswithoutmentioningtheobviouspointthatsynthesisandassemblyofthefungalcellwallisanattractivetargetforantifungalchemotherapy.Antifungalsthattargetchitinsynthesisareoflimiteduseatthemomenteventhoughtheimportanceofchitininthestructureofthefungalwallwouldseemtomakeitanexcellenttarget.Currently,thechitinsynthesisinhibitorsavailablearethenaturallyoccurring nikkomycins and polyoxins,andtheirsyntheticderivatives.Theseareanaloguesofthechitinsynthasesubstrate,UDP-N-acetylglucosamine,andactascompetitiveinhibitorsofchitinsynthase.Theycanbeeffectivewhenusedinconjunctionwithotherantifungalagents,buttendtobeineffectivealonebecauseoflimiteduptakeoftheinhibitors（Bowman&Free,2006）.

Theonlyaspectofwallsynthesisthatiseffectivelytargetedbycommerciallyavailableantifungalagentscurrentlyistheβ1,3-glucansynthase,whichisinhibitedbythe echinocandins.Thesenon-competitiveinhibitorsoftheglucansynthaseareknowntobindtotheglucansynthasecatalyticsubunit.Echinocandinscausefungalcellstoswellandthewallslyseatplacesofactivecellwallsynthesis.

Mostpharmaceuticalscurrentlyinusefortreatmentofmycosestargetaspectsoffungalbiologyotherthanthecellwalls.Themostcommonlyusedantifungalsareazolesandpolyeneantibioticswhichtargetergosterolinthefungalplasmamembrane.However,thestructureofthecellwallisuniquetothefungiandtherearemanystepscontrolledbyenzymesthatlackhomologuesinthehumangenomeandarethereforegoodcandidatesastherapeutics:

asidefromchitinandglucansynthasesandenzymesinvolvedincross-linkingwallcomponents,therearethemannosyltransferasesandglycosyltransferasesintheGolgiapparatus,andthestepsinvolvedinattachingGPIanchorstocellwallproteins（Bowman&Free,2006）.Thesearethetargetsforfutureresearch.

Updated February25,2011

Chapter6.10Referencesandfurtherreading

Adams,D.J.（2004）.Fungalcellwallchitinasesandglucanases. Microbiology, 150:

2029-2035.DOI:

 http:

//dx.doi.org/10.1099/mic.0.26980-0.

Aimanianda,V.,Bayry,J.,Bozza,S.,Kniemeyer,O.,Perruccio,K.,Elluru,S.R.,Clavaud,C.,Paris,S.,Brakhage,A.A.,Kaveri,S.V.,Romani,L.&Latgé,J.-P.（2009）.Surfacehydro