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A model for the photosynthetic apparatus of purple bacteria
e,
within
energy Ismnsfer pmsses and betw@@ncomplex@@ @ccuI on tim@scal@s that vary fmm tens of femtoseeonds Ifs: IO’J s) to 30400 picoseconds Ips: XFn s); the transfer d charge has been observed to last >200 f,s. Recently, energy transfer measurements have become patible at a rexdlItian of less than 50 f@. These timeresolved spectroscopic teclmiques not only enable rate cilnstants for vaIioll@ steps in energr transfer to be determined, but also make wssible the iw&igation of excited -states that seem to oscillat@‘*.” mllerent1y O”@TB period of several pieoseconds. This wealth of skuetwal and spectmsmpic information is giving crucial insight into the detailed structural features that ar@ important in photosynthesis.
The pigments that play a central ml@ in energy transport within lightharvesting complexes of plant@ and bacteria ar@ chlorophylls and bacteriacblomphylls, respectively. Carotenoid pigments also participate in energy transport, and have a semndary role in pmtecting the organism fmm photooxidation (eawd by the presence of singlet axyg@n that is created by triplet-excited chlorophylls sensitising molecular oxygen). The ability of cambnoids to perfmm a photo-protwtive role is thought ta be through kipletkiplet energy transfer from chlompbylls to camtamid pigments. In
Rhadospirillaceae, BS many as 250 to 399 bacteri@eblomphyll pigments a@ plerent for @very RC complex’? Many pigment to pigment transfers occur before chaFge separation finally takes place. It is remnrkable that, despite the large number of energy kansfer steos involved in nhomsvnthesis. the p&ss loges rlO%‘of captured @r&y. In purple bacteria, photosynthetic complexes ar@ densely packed in the membrane and awear to form extended zakes’ ofcompler;.s that are lilmtionsuy mmected’~‘“. In these bacteria, the RC sod an antenna complex called light-harvesting 1 [LHl or B875 - a&r the long wavelength absorption maximum of the bacc@ric&lomphyll a iBchl a)moleculeswithint.he compl@xlform the ‘core’ complex. The La1 complex contains 32 Bchl a molecules (Ref. 15 and A. Gall, pers. communl and has been visualized at low resolution, by eleekon micmsmw of twc&nensional CrystaIs’J~. in purple ba@t@ria therr ar@ additional antenna complex@@ called LH2 (or B800450), which in form contain 27 R&l (I molecules. These are arranged peripherally to LHl and @r@present in vaiable amount@“.‘~. The skucture of the bxterial RC was 6nt described by Deis@nh@f@ret al.’ This was the lint membrane protein to have it@ stmcture determined at bigb resolution and for this pioneering work the German groups were awarded the 1988 N&l Prize in Chemistry. Recently the crystal shucture of a peripheral ant@nna complex from the purple bacterium Rhodopseudomonas (Rp.1 acidcphila was determined (McDermott et 1~6~1.It is now possible to assemble the complete phctos]pthetic unit of the purple bacteria and discuss its skuchve and function in detail. Roth types of antenna complex ax@ asaemlded from very similar building blocks. The pigments ar@ noncwalently attached to two hydmphabic low molecular mass 6-7 kDa) apeproteins (pm+,&@ without pigments) called a and p. The intact antenna. structures ar@ oligomers of these appmtein pair@ and their associated pi,gments. Mar@ than 20 different apopmteins have been sequenced’7, and comparison cd these ha@ revealed @mu@impQrtant con@@rwd features. They all mntain a central bydmphobic span of about 20-23 amino acids that fold up to form a-helices’“‘l that span the membrane once, and both
some
theirmnamelic
apopmteins contain conserved histidine residues, which have been shown by reso-ce Raman spectroscopy tn be emrdinated to magnesium iona O&+) attbeceIltIE&be.beIct&cblolinacteriochloFinrings of the 875.nm- and 85Oawmvelengthabsorbing Bchl a molecules’? Models for the ligbt~harvesting tomplexesz~ and their arrangement around the RC (Ref. 17) have been proposed in the past These models were based on the primsrg structures of antenna mmplexes and the general principles of helix-helix nackine It is now clear
predominant forces in light-barvestingcomplex formation. The determination of the three dimensional structure of the LHZ cnm plex, together with the primary sequence homology between LHI and LHZ, provides all the neceasan information required to build an atomic model of Ml. The stoicbiometric LHl:RC ratiois 16a@:1. although the ratio of the peripheral LEZ mmpleres to this LHl-RC core cumplex is vtiable and depends on the gmwth conditions of the bacteria. CeuS grown in low light wnditioas synthesise mm. plexes with the hi&& R&l a RC ratio, apprcmimately 250-34 B&l a molecules pm RC (Ref 12). With 32 B&l a in IHl and 27 in each LHZ mmplex, tbis impties &IO Lx2 complexes associated with each LHI-RC complex. In this article, we present a model of the RC-LHl-LHZ mmplex, which has been assembled l?om the moat detailed shyetural information now ava&¶ble for these StN~. ibis infommtian has been used to impose omstminta on model building based mainly on gometric arguml?nts. Manual manipulations on a molecular gmpbics system have been kept to a minimum so as not to bias the madelliug away from this experimental data. The stmctums included am: LHZ fmm Rp. widop.Wa strain UIO~O, Aresolution~; RC from Rhodobncler (Rh.) sphoemides straia R-26, at a resolutian of 2.8 A (Ref. 7); and an electmn density map of LHl from Rhodospidum (Rs.) rubrum’s at 8.5 A resolution. The structures fmm the Rbodcspirillaceae family used in this work are closely related, and it has been sasumed that
:
the
determined at2.5
raay associated 0 and pmteim.The bacteriocbloti
the
(3 apa.
planes witbb2 pairs are appmxbnately parallel, whereas the interplanar angb?sare greatm between neighbouring R&l a oiements of adiacent nairs. Adiacent a ringof16 apoprotein pairs. It
ia pas% ibletoaeearineofwaksbetweenthe
helice3,wkdch&&umedto~
sentaringofBchlomolecule8.~l
molerules and phytol chains of Mb I%50 and B800 molecules bkstwiw in the space behveen. Retinement of this structure bldicate~ that there is &a& exact ninefold aymmehy be. tween pmtamers of the complex.The nmameric skuchxe found in tlte crystal appears to be identical 0 the ba* terial structure as jxdged by pigment abmrpticmqectm. which are identical but for tbe loss of the strong Bhoubler at 870475 nm cawed by the removal ofLHlandtbeRC.
antaimonly one kind of E&l (1 mole&e, B875. which can be structurally equatedwithR&wofLHz.TbeLH1 complex has an outer diameter of 116Aa@daniMerdiameterof68k Tbereisah&intbecentmofbothtbe LH1rmdLRZstmctwes;inLH2,ti hole is 6lled with lipid, but in IX1 it is ocolpied hy a single RC complex(Ref. 21 and A Gall, pem. commun.).The majvraslssvedfeatmesintheprimary stNctures of the apoproteinsfrom purple bklcterial antenna cumpleses are tbesameforbotbLH1endLEI2Gtef. 17). Espeeiauy important are the coneei-ved Hi6 residne5 that nnrrdinate
perspectives These twocriteria am achieved by placingthespecialpattbeoriginc# JJIl and mtatingtbe laeal twofoldaxis onto the normal of the membrane plane. A final trawlatbm in applied to the RC along the membrane normal enable a model tar the smlchue of EmtitbeMgatomsofthespecialpeir LH1tobebuiufmmLH2e. areattheeameIevefintmembmne The atnmic StlWtWBOfLHl~k astbe?dgatom80fLH1.TbereismbuiItfromLB2andinttEsamecrystatkmal ambiguity of the RC eomptex tanogmpbic mordinate eaystwaa LIB, ahoot the membrane wrmal of about by the f&wieg prwdnre. 22.5”.which ia the angular separation (1)Tkecoor~tesafan.gapopmtein ofonepairofL.Hlapopmteinstium paaivmL.H2are~witbRithassodated wtber.TBisermrcouldber&ced~ Rchl a and ealutenoid molecules. car&d d&g of the M subunit oftbe (2)TbeR3cQRcblamolecnleis~t RCtoLHI.Tbejusti6cationoftlm iaLH1,andisremoved.Tbisnow arbitrarya6swnptioninoisbomeout cmTespondstotbeR32Osubnnitthat by the &servaIicmthat the tmnsmemcanbeblo1atedastbeminimumbuildbrswportiarmdRC&f.7~Ul&htbW ing black of LHl G?& 22 and 2% of LB2 quite cloaelyl, and hence, by (3) The 8820 unit in movedradiauy, in definition, those of LHl. For exampIe, the plane of the membrane,away tiom in L&l2the tlansmembrnne a-t&x of tile (L&in stops t H&37,while the t!Jeoriginbythewor1a9(theratio of apoprotein pairs in Ix1 and LH2). m a-b&?, l&ned A, on (4) The Ii320 struetnral motifis mpied theMeubunitoftbeRC.~atGln77. InmumcdeItbeCaatnmsofthssetwo 16tiDESthlUUgh~~&iIOt&iOU of 23.5” about the memblnne normal residuesarefoundtabeattbeaame tbai rwming tbmugh the origin of the ce lewl in rhe membnme to witbin 0.3 k &Idition (2). It tbe&re ordinate&eIu Figure 2 Show8the prediaea stlucbue thesiwandsymmetwofLHlmuldin ThestrnctueisnowbuiRwitbita oftbeRGLHlcumpIexnexttoar1LH2 CelltIe of ma@Sover tile origin of the complex.It is impartant to empbaaise that this model ha8 been prcduced stating aH2) alordinate system. flvmtbeknowrl~ ofLE2za.d RC.andtheringsizeofLH1. BLwiwnmrwciimcklfoln~ TkestnwhueoftbeRCfromRb. IJI2t0LHl,tb~hVC-&hlOringSyP sphakdes’ isa mmplex oftbreaproterns must be brought into cIme pmrtein subunits and 14 cofactors.It lacks the tightly bound c&&mme of Rp. uiridiss and is compxd of protsin sublmitadesignat.edH,MandLThe total width of the RC, in the di+km normaltotbemembraoe.is75AThe membrane-spanning part of the cornplex has an elliptic@cmss section with aXeSOf7Oand4OA%?‘Sp&dpair’ of Rcbl a molecule.?.which eaptwe energy from LHl, had &wxt exact twofold symmetry; it is known that thislccala?..isofsymmetryispalaRel tLl the membrane rmrmale. Two asmmlption8 axe made here about the p&ion of the RC relative to MI. (1) The spedal pair is equidistant from alItheBchlomolecuIesinLEI1;the specidpairdpigmen~isne~~the centreoftbeRC.whicbiturnisconStrainedbyth0LRlC0Zllpl~Xt4lbeing quite claw to its own centre. (2) TIE q!cial pair h 8%the same level inthemembraneastheringingDfBehlo condition (1) is very restrictive, as molecldesofLHIandLH2.TbiaentbediameteraofLIilandLXi26xtbe .suratheclosestp~a-rkhiQ distala ef their &seat approach.TIE? intereompIex distances to fadlit& requirement that LH2 eompl@xes &icient energy transfer.
the E&50and B375 &hl a m&c&s. These His residues are found at the same place in a port&n ofhydr&&ie !ransmembrane peptide. which puts them at the same depth in the mem-
cmdd have been achieved with neuM amino acids. There are 110 equivalent amino acids in the LHl sequence and it would bepredi*.sd that no speeik interactions are made with LH2. Rotation of of LH2 Eomplexes about the LHl centre does not seem to be Ritical, a5 the requirement that LEE complexes only just touch forces a radial distance for La2 that resuks in ne&ibls interacttons with LHI. Helixhelix interactions am achieved by mtatig LH2 about its own membrane normal until el0.S contact8 are made a helix on the neighbaring LIE This plvxedure .P&omaticalIy Seems to achieve close interaction between the gmups of-0 and Asp17 amino acids
thering
with
tit
&tempted; It is therefore estimated that the likely error of the rot&km of L&I2about the membrane normal, running through its centre, is about 5”.
eight LHZ: Fig. 3) is very striking in ap pearanca. An RC complex is surrounded by an LHI complex, which is in turn surrmmded by eight JA-E complexes. The view ewmp&ng a narrow layer kbaut 5 A) of the membrane around the ringa of R&l a pigments reveals the overall geometry of tbe eIwgy transfer partofttephot~~yntb&icunit (Fig. 3a).
content
Tbe total B&l a cd the pIlotssynthetic unit is 239, quite close. to the value of 259.399 stained fium purple
kact&,&asRb..~des,gmwn in low lightmnditioDE’? There iea
superiicial resemblance
betmen tbis mcdelandmeoftbemcdebprqmed by Tab&~. This latter model wan mmpcm.zd of B hemmer of m& motifs, forming qr& rings within which an RC complex could be situated. In tII m&I described in this article, the number of a@mits invohred is very different, and sn important role ia played by pigmenepigment and pigmen& protein interactions in determinin3 the immt shwhlmI interactions that
d#3&inatetheilight~~ complex structures. The &se+.t Mg-Mg atom distances between Bchl a pigme& in differ+ mmpIexea are: Ia? t+H2, about 19A: uI2toIHl~abcut24A;aLHl~oQe of gle SPxmI pair of pi@Ents, about 43 A. There is a sienifcant diEeremx b&weenringsofliehlapigmentsin LIZlandL?I2.InLIB,tbean&s bekwn bacteric&Ioti planes of the BcbIapaimk.tbwEicbIanioIecuIes assoeiatedwitbthe~ameapopmteiopair) are about 140; the angks between pairs are about 26”. In contrast, the an3llIar dispIamments bekveeacbIa p+enta within LIIl are 8” and 14’. The angle titbintbetruestnlctureofLHIi8 pmbabIy around ll”, which is half the angle between a@ apopmtein pairs. Tbis more reg&r arraugement leads support to the idea that LII2 mmpkxes were later and more speck&d additions to pbotosyntbetic mmpkxes, and may also explain the lower ahwption energg of LHl, &semed at 875 nn5 cnmpaIEdtatbatofLH2atS59rlm. Contacts between LHl and LH2 mmplexes are sparse - tb=eOnIy pIace whemtbeyammadeisattbeperipIa5 mie side, near the B850 and B875 pigments. Tbia is where the p &in &&c?s lean ouward to acmdaetherin@ofp&ments.TbeLH2 amino acid residues Phe31 and Phe34 make van du WaaIs contacts with VaI31 and Trp34 of LEIl. In park&r, Trp34 of Ix1 is bvgE enan3b fcr cIw.e rIw overlap with Pbe34 of LIE?. TIE LH2mmpIexbaBmanymolwbydm phobic intertions aIon the len&h of its h&es that rue partiaIIy wrapped around one another. The only chai%ed tranaaembmne amim acids a&found at Asp17 and A&O. Specitic irksactions between Asp17 and AC@0 cm p cbaio apopmteins of different IB?
Bcbl 2t-m~~ RC-LHl mmplexes sbmvs hexagonally packed units of
&UtlZOAdi@i?&~~~~.ThiS~~ interpreted as an RC surrounded by sii or 12 c@ pairs. The six- m 12fold symmetry may have been impos?d on theimagesbythepzn2efaofimage enharmement. In any case the msolution is not sacient to resolve the true building blwks of the complex. There has not been any published work, on tbe intact membrane. to mnl&m or disprove our model of the photcwnthetic unit. The observed ratio of 25@-300 R&l a Ic&cul~ per RC, for bacteria gmwn in low light canditions, requires photosynthetic units to be packed closely to&her. A hexagonal lattice ia the mast likely regular packing of photosynthetic units of equal size. On lint wnsideration it would seem to be a problem for a cornplex with eightfold symmetry in pack in anays of sixfold symmetry. Careful mtideration suggest4 that this may not be so. The LIE! cQmplexes arrqed anmad I&I1 have near&neiebiwnn anenlar intervals of 135”
Witi;tkP~~liUkSkhV~~
and A@?, on each side of an LIT2 compla spaced hy an angle of 120”. A hexa& network of pbo~synthetic tits can be built (Fig. 4a), with dis tarted sixfold symmetry, that is cornlaib1.e with local cctagonal symmetry, while at the same tie maserving tbe Arg-Asp Ii&.. This is possible because these amino acids repeat witbin LII3
must aeeom&d& the oct&dral symme!~ oftbe pmposed photasystem ifclosepackiugistohemaintajned. Tbiscanbeacbievedthmugllhmughdistorted hexagonaI packing amongst the Bulk’ MI3 mmplema. In reality, the lattice can only be appmximately regular 68 it must incorporate other membmne pmt&u, such as the protondriven ATP syntbetase and cytwbmme b/c, corn$.2X Even in this more dis‘xdered model Al!-Asp links can &ill be made between Buflicient numbem of LIB complexes (Fig. 4b) to link the pbtp synthetic unita cl&y together. There is sufficient mnformational tlexlbiliti intbesidechtiofArgandAsp~ accommodate the.92 distortions. The recent determination of a crystal stmctute of LR.2 from Rs. moltihianum revealed it is an wtamer (a&i.). It isalsopxslbletbatwithinagiven species there may be d&r& LID rine sizes oresent in the native mem&cH~l,~awmulLlItmay well be, therefore, that tbe baeiful, regularUlOd&presentedinRg.4aand 4b are nahuallv dintatted bv the additionofafew Ii& of dilTei-& diameter. This could then easily allow the intact systeminthephc&+heticmembmne to aeemnmodate other important manbrane pm&ins. The packing arrange ment is consistent with the lake m&l of energy tmnster~~.~*, for which erperimental evidelxe aists to indicate that at least ten photcaynthetic units are energetically linked together. The zg--&i lb&s pm@ here give a mechanism by which these l&scanformandbestabili&.Aring of large ammatic residues has been ob. semiI the membrane hydrophobic hvdi-wbilic interfacP in Dxin. the RC abd s&e eukaryatic III;& pm. teins.Itisposhdatedtbattheeraidws help to stabilie and p&ion proteins in the lipid bilayer. The absence of suchlwgetidueswithinLH3mayindieate that pmtein-prG&in ilIt.?& am more important than pmteb+lipid interactions, and ia con&tent with our model of the photosyntb&ic unit.
+&at
at
p1exandthepmpoaedmoftbebacterial photasyntbetic unit explain the kinetinr oc energy tansfer deb?rmined by time-resolved spectroscopy? A
transfer-%mc from LIIltotbeRCthatisaboutanwderof magnitude slower than transfer frw LI-IZ to LHI. The eme~ntauv ob
distance, tranaition&pole-~~~ pbemsneaon-Fm_nuggtraoafaR. l%isbasallR”dependeme,whereRis thediaancebetweendomandaseptor molecules. It baa been shown that it t&~s.sprortbeeneqytobe+Jalw fen-al fmm R&i0 to R875 (i.e. LHZ to LHlP? Since the ring8 of Rcbl a pigments am 105&d at the mme depth in the membrane. the diatawe between tbemcaIlbeminimwand,tberefare,
within
energy Ismnsfer pmsses and betw@@ncomplex@@ @ccuI on tim@scal@s that vary fmm tens of femtoseeonds Ifs: IO’J s) to 30400 picoseconds Ips: XFn s); the transfer d charge has been observed to last >200 f,s. Recently, energy transfer measurements have become patible at a rexdlItian of less than 50 f@. These timeresolved spectroscopic teclmiques not only enable rate cilnstants for vaIioll@ steps in energr transfer to be determined, but also make wssible the iw&igation of excited -states that seem to oscillat@‘*.” mllerent1y O”@TB period of several pieoseconds. This wealth of skuetwal and spectmsmpic information is giving crucial insight into the detailed structural features that ar@ important in photosynthesis.
The pigments that play a central ml@ in energy transport within lightharvesting complexes of plant@ and bacteria ar@ chlorophylls and bacteriacblomphylls, respectively. Carotenoid pigments also participate in energy transport, and have a semndary role in pmtecting the organism fmm photooxidation (eawd by the presence of singlet axyg@n that is created by triplet-excited chlorophylls sensitising molecular oxygen). The ability of cambnoids to perfmm a photo-protwtive role is thought ta be through kipletkiplet energy transfer from chlompbylls to camtamid pigments. In
Rhadospirillaceae, BS many as 250 to 399 bacteri@eblomphyll pigments a@ plerent for @very RC complex’? Many pigment to pigment transfers occur before chaFge separation finally takes place. It is remnrkable that, despite the large number of energy kansfer steos involved in nhomsvnthesis. the p&ss loges rlO%‘of captured @r&y. In purple bacteria, photosynthetic complexes ar@ densely packed in the membrane and awear to form extended zakes’ ofcompler;.s that are lilmtionsuy mmected’~‘“. In these bacteria, the RC sod an antenna complex called light-harvesting 1 [LHl or B875 - a&r the long wavelength absorption maximum of the bacc@ric&lomphyll a iBchl a)moleculeswithint.he compl@xlform the ‘core’ complex. The La1 complex contains 32 Bchl a molecules (Ref. 15 and A. Gall, pers. communl and has been visualized at low resolution, by eleekon micmsmw of twc&nensional CrystaIs’J~. in purple ba@t@ria therr ar@ additional antenna complex@@ called LH2 (or B800450), which in form contain 27 R&l (I molecules. These are arranged peripherally to LHl and @r@present in vaiable amount@“.‘~. The skucture of the bxterial RC was 6nt described by Deis@nh@f@ret al.’ This was the lint membrane protein to have it@ stmcture determined at bigb resolution and for this pioneering work the German groups were awarded the 1988 N&l Prize in Chemistry. Recently the crystal shucture of a peripheral ant@nna complex from the purple bacterium Rhodopseudomonas (Rp.1 acidcphila was determined (McDermott et 1~6~1.It is now possible to assemble the complete phctos]pthetic unit of the purple bacteria and discuss its skuchve and function in detail. Roth types of antenna complex ax@ asaemlded from very similar building blocks. The pigments ar@ noncwalently attached to two hydmphabic low molecular mass 6-7 kDa) apeproteins (pm+,&@ without pigments) called a and p. The intact antenna. structures ar@ oligomers of these appmtein pair@ and their associated pi,gments. Mar@ than 20 different apopmteins have been sequenced’7, and comparison cd these ha@ revealed @mu@impQrtant con@@rwd features. They all mntain a central bydmphobic span of about 20-23 amino acids that fold up to form a-helices’“‘l that span the membrane once, and both
some
theirmnamelic
apopmteins contain conserved histidine residues, which have been shown by reso-ce Raman spectroscopy tn be emrdinated to magnesium iona O&+) attbeceIltIE&be.beIct&cblolinacteriochloFinrings of the 875.nm- and 85Oawmvelengthabsorbing Bchl a molecules’? Models for the ligbt~harvesting tomplexesz~ and their arrangement around the RC (Ref. 17) have been proposed in the past These models were based on the primsrg structures of antenna mmplexes and the general principles of helix-helix nackine It is now clear
predominant forces in light-barvestingcomplex formation. The determination of the three dimensional structure of the LHZ cnm plex, together with the primary sequence homology between LHI and LHZ, provides all the neceasan information required to build an atomic model of Ml. The stoicbiometric LHl:RC ratiois 16a@:1. although the ratio of the peripheral LEZ mmpleres to this LHl-RC core cumplex is vtiable and depends on the gmwth conditions of the bacteria. CeuS grown in low light wnditioas synthesise mm. plexes with the hi&& R&l a RC ratio, apprcmimately 250-34 B&l a molecules pm RC (Ref 12). With 32 B&l a in IHl and 27 in each LHZ mmplex, tbis impties &IO Lx2 complexes associated with each LHI-RC complex. In this article, we present a model of the RC-LHl-LHZ mmplex, which has been assembled l?om the moat detailed shyetural information now ava&¶ble for these StN~. ibis infommtian has been used to impose omstminta on model building based mainly on gometric arguml?nts. Manual manipulations on a molecular gmpbics system have been kept to a minimum so as not to bias the madelliug away from this experimental data. The stmctums included am: LHZ fmm Rp. widop.Wa strain UIO~O, Aresolution~; RC from Rhodobncler (Rh.) sphoemides straia R-26, at a resolutian of 2.8 A (Ref. 7); and an electmn density map of LHl from Rhodospidum (Rs.) rubrum’s at 8.5 A resolution. The structures fmm the Rbodcspirillaceae family used in this work are closely related, and it has been sasumed that
:
the
determined at2.5
raay associated 0 and pmteim.The bacteriocbloti
the
(3 apa.
planes witbb2 pairs are appmxbnately parallel, whereas the interplanar angb?sare greatm between neighbouring R&l a oiements of adiacent nairs. Adiacent a ringof16 apoprotein pairs. It
ia pas% ibletoaeearineofwaksbetweenthe
helice3,wkdch&&umedto~
sentaringofBchlomolecule8.~l
molerules and phytol chains of Mb I%50 and B800 molecules bkstwiw in the space behveen. Retinement of this structure bldicate~ that there is &a& exact ninefold aymmehy be. tween pmtamers of the complex.The nmameric skuchxe found in tlte crystal appears to be identical 0 the ba* terial structure as jxdged by pigment abmrpticmqectm. which are identical but for tbe loss of the strong Bhoubler at 870475 nm cawed by the removal ofLHlandtbeRC.
antaimonly one kind of E&l (1 mole&e, B875. which can be structurally equatedwithR&wofLHz.TbeLH1 complex has an outer diameter of 116Aa@daniMerdiameterof68k Tbereisah&intbecentmofbothtbe LH1rmdLRZstmctwes;inLH2,ti hole is 6lled with lipid, but in IX1 it is ocolpied hy a single RC complex(Ref. 21 and A Gall, pem. commun.).The majvraslssvedfeatmesintheprimary stNctures of the apoproteinsfrom purple bklcterial antenna cumpleses are tbesameforbotbLH1endLEI2Gtef. 17). Espeeiauy important are the coneei-ved Hi6 residne5 that nnrrdinate
perspectives These twocriteria am achieved by placingthespecialpattbeoriginc# JJIl and mtatingtbe laeal twofoldaxis onto the normal of the membrane plane. A final trawlatbm in applied to the RC along the membrane normal enable a model tar the smlchue of EmtitbeMgatomsofthespecialpeir LH1tobebuiufmmLH2e. areattheeameIevefintmembmne The atnmic StlWtWBOfLHl~k astbe?dgatom80fLH1.TbereismbuiItfromLB2andinttEsamecrystatkmal ambiguity of the RC eomptex tanogmpbic mordinate eaystwaa LIB, ahoot the membrane wrmal of about by the f&wieg prwdnre. 22.5”.which ia the angular separation (1)Tkecoor~tesafan.gapopmtein ofonepairofL.Hlapopmteinstium paaivmL.H2are~witbRithassodated wtber.TBisermrcouldber&ced~ Rchl a and ealutenoid molecules. car&d d&g of the M subunit oftbe (2)TbeR3cQRcblamolecnleis~t RCtoLHI.Tbejusti6cationoftlm iaLH1,andisremoved.Tbisnow arbitrarya6swnptioninoisbomeout cmTespondstotbeR32Osubnnitthat by the &servaIicmthat the tmnsmemcanbeblo1atedastbeminimumbuildbrswportiarmdRC&f.7~Ul&htbW ing black of LHl G?& 22 and 2% of LB2 quite cloaelyl, and hence, by (3) The 8820 unit in movedradiauy, in definition, those of LHl. For exampIe, the plane of the membrane,away tiom in L&l2the tlansmembrnne a-t&x of tile (L&in stops t H&37,while the t!Jeoriginbythewor1a9(theratio of apoprotein pairs in Ix1 and LH2). m a-b&?, l&ned A, on (4) The Ii320 struetnral motifis mpied theMeubunitoftbeRC.~atGln77. InmumcdeItbeCaatnmsofthssetwo 16tiDESthlUUgh~~&iIOt&iOU of 23.5” about the memblnne normal residuesarefoundtabeattbeaame tbai rwming tbmugh the origin of the ce lewl in rhe membnme to witbin 0.3 k &Idition (2). It tbe&re ordinate&eIu Figure 2 Show8the prediaea stlucbue thesiwandsymmetwofLHlmuldin ThestrnctueisnowbuiRwitbita oftbeRGLHlcumpIexnexttoar1LH2 CelltIe of ma@Sover tile origin of the complex.It is impartant to empbaaise that this model ha8 been prcduced stating aH2) alordinate system. flvmtbeknowrl~ ofLE2za.d RC.andtheringsizeofLH1. BLwiwnmrwciimcklfoln~ TkestnwhueoftbeRCfromRb. IJI2t0LHl,tb~hVC-&hlOringSyP sphakdes’ isa mmplex oftbreaproterns must be brought into cIme pmrtein subunits and 14 cofactors.It lacks the tightly bound c&&mme of Rp. uiridiss and is compxd of protsin sublmitadesignat.edH,MandLThe total width of the RC, in the di+km normaltotbemembraoe.is75AThe membrane-spanning part of the cornplex has an elliptic@cmss section with aXeSOf7Oand4OA%?‘Sp&dpair’ of Rcbl a molecule.?.which eaptwe energy from LHl, had &wxt exact twofold symmetry; it is known that thislccala?..isofsymmetryispalaRel tLl the membrane rmrmale. Two asmmlption8 axe made here about the p&ion of the RC relative to MI. (1) The spedal pair is equidistant from alItheBchlomolecuIesinLEI1;the specidpairdpigmen~isne~~the centreoftbeRC.whicbiturnisconStrainedbyth0LRlC0Zllpl~Xt4lbeing quite claw to its own centre. (2) TIE q!cial pair h 8%the same level inthemembraneastheringingDfBehlo condition (1) is very restrictive, as molecldesofLHIandLH2.TbiaentbediameteraofLIilandLXi26xtbe .suratheclosestp~a-rkhiQ distala ef their &seat approach.TIE? intereompIex distances to fadlit& requirement that LH2 eompl@xes &icient energy transfer.
the E&50and B375 &hl a m&c&s. These His residues are found at the same place in a port&n ofhydr&&ie !ransmembrane peptide. which puts them at the same depth in the mem-
cmdd have been achieved with neuM amino acids. There are 110 equivalent amino acids in the LHl sequence and it would bepredi*.sd that no speeik interactions are made with LH2. Rotation of of LH2 Eomplexes about the LHl centre does not seem to be Ritical, a5 the requirement that LEE complexes only just touch forces a radial distance for La2 that resuks in ne&ibls interacttons with LHI. Helixhelix interactions am achieved by mtatig LH2 about its own membrane normal until el0.S contact8 are made a helix on the neighbaring LIE This plvxedure .P&omaticalIy Seems to achieve close interaction between the gmups of-0 and Asp17 amino acids
thering
with
tit
&tempted; It is therefore estimated that the likely error of the rot&km of L&I2about the membrane normal, running through its centre, is about 5”.
eight LHZ: Fig. 3) is very striking in ap pearanca. An RC complex is surrounded by an LHI complex, which is in turn surrmmded by eight JA-E complexes. The view ewmp&ng a narrow layer kbaut 5 A) of the membrane around the ringa of R&l a pigments reveals the overall geometry of tbe eIwgy transfer partofttephot~~yntb&icunit (Fig. 3a).
content
Tbe total B&l a cd the pIlotssynthetic unit is 239, quite close. to the value of 259.399 stained fium purple
kact&,&asRb..~des,gmwn in low lightmnditioDE’? There iea
superiicial resemblance
betmen tbis mcdelandmeoftbemcdebprqmed by Tab&~. This latter model wan mmpcm.zd of B hemmer of m& motifs, forming qr& rings within which an RC complex could be situated. In tII m&I described in this article, the number of a@mits invohred is very different, and sn important role ia played by pigmenepigment and pigmen& protein interactions in determinin3 the immt shwhlmI interactions that
d#3&inatetheilight~~ complex structures. The &se+.t Mg-Mg atom distances between Bchl a pigme& in differ+ mmpIexea are: Ia? t+H2, about 19A: uI2toIHl~abcut24A;aLHl~oQe of gle SPxmI pair of pi@Ents, about 43 A. There is a sienifcant diEeremx b&weenringsofliehlapigmentsin LIZlandL?I2.InLIB,tbean&s bekwn bacteric&Ioti planes of the BcbIapaimk.tbwEicbIanioIecuIes assoeiatedwitbthe~ameapopmteiopair) are about 140; the angks between pairs are about 26”. In contrast, the an3llIar dispIamments bekveeacbIa p+enta within LIIl are 8” and 14’. The angle titbintbetruestnlctureofLHIi8 pmbabIy around ll”, which is half the angle between a@ apopmtein pairs. Tbis more reg&r arraugement leads support to the idea that LII2 mmpkxes were later and more speck&d additions to pbotosyntbetic mmpkxes, and may also explain the lower ahwption energg of LHl, &semed at 875 nn5 cnmpaIEdtatbatofLH2atS59rlm. Contacts between LHl and LH2 mmplexes are sparse - tb=eOnIy pIace whemtbeyammadeisattbeperipIa5 mie side, near the B850 and B875 pigments. Tbia is where the p &in &&c?s lean ouward to acmdaetherin@ofp&ments.TbeLH2 amino acid residues Phe31 and Phe34 make van du WaaIs contacts with VaI31 and Trp34 of LEIl. In park&r, Trp34 of Ix1 is bvgE enan3b fcr cIw.e rIw overlap with Pbe34 of LIE?. TIE LH2mmpIexbaBmanymolwbydm phobic intertions aIon the len&h of its h&es that rue partiaIIy wrapped around one another. The only chai%ed tranaaembmne amim acids a&found at Asp17 and A&O. Specitic irksactions between Asp17 and AC@0 cm p cbaio apopmteins of different IB?
Bcbl 2t-m~~ RC-LHl mmplexes sbmvs hexagonally packed units of
&UtlZOAdi@i?&~~~~.ThiS~~ interpreted as an RC surrounded by sii or 12 c@ pairs. The six- m 12fold symmetry may have been impos?d on theimagesbythepzn2efaofimage enharmement. In any case the msolution is not sacient to resolve the true building blwks of the complex. There has not been any published work, on tbe intact membrane. to mnl&m or disprove our model of the photcwnthetic unit. The observed ratio of 25@-300 R&l a Ic&cul~ per RC, for bacteria gmwn in low light canditions, requires photosynthetic units to be packed closely to&her. A hexagonal lattice ia the mast likely regular packing of photosynthetic units of equal size. On lint wnsideration it would seem to be a problem for a cornplex with eightfold symmetry in pack in anays of sixfold symmetry. Careful mtideration suggest4 that this may not be so. The LIE! cQmplexes arrqed anmad I&I1 have near&neiebiwnn anenlar intervals of 135”
Witi;tkP~~liUkSkhV~~
and A@?, on each side of an LIT2 compla spaced hy an angle of 120”. A hexa& network of pbo~synthetic tits can be built (Fig. 4a), with dis tarted sixfold symmetry, that is cornlaib1.e with local cctagonal symmetry, while at the same tie maserving tbe Arg-Asp Ii&.. This is possible because these amino acids repeat witbin LII3
must aeeom&d& the oct&dral symme!~ oftbe pmposed photasystem ifclosepackiugistohemaintajned. Tbiscanbeacbievedthmugllhmughdistorted hexagonaI packing amongst the Bulk’ MI3 mmplema. In reality, the lattice can only be appmximately regular 68 it must incorporate other membmne pmt&u, such as the protondriven ATP syntbetase and cytwbmme b/c, corn$.2X Even in this more dis‘xdered model Al!-Asp links can &ill be made between Buflicient numbem of LIB complexes (Fig. 4b) to link the pbtp synthetic unita cl&y together. There is sufficient mnformational tlexlbiliti intbesidechtiofArgandAsp~ accommodate the.92 distortions. The recent determination of a crystal stmctute of LR.2 from Rs. moltihianum revealed it is an wtamer (a&i.). It isalsopxslbletbatwithinagiven species there may be d&r& LID rine sizes oresent in the native mem&cH~l,~awmulLlItmay well be, therefore, that tbe baeiful, regularUlOd&presentedinRg.4aand 4b are nahuallv dintatted bv the additionofafew Ii& of dilTei-& diameter. This could then easily allow the intact systeminthephc&+heticmembmne to aeemnmodate other important manbrane pm&ins. The packing arrange ment is consistent with the lake m&l of energy tmnster~~.~*, for which erperimental evidelxe aists to indicate that at least ten photcaynthetic units are energetically linked together. The zg--&i lb&s pm@ here give a mechanism by which these l&scanformandbestabili&.Aring of large ammatic residues has been ob. semiI the membrane hydrophobic hvdi-wbilic interfacP in Dxin. the RC abd s&e eukaryatic III;& pm. teins.Itisposhdatedtbattheeraidws help to stabilie and p&ion proteins in the lipid bilayer. The absence of suchlwgetidueswithinLH3mayindieate that pmtein-prG&in ilIt.?& am more important than pmteb+lipid interactions, and ia con&tent with our model of the photosyntb&ic unit.
+&at
at
p1exandthepmpoaedmoftbebacterial photasyntbetic unit explain the kinetinr oc energy tansfer deb?rmined by time-resolved spectroscopy? A
transfer-%mc from LIIltotbeRCthatisaboutanwderof magnitude slower than transfer frw LI-IZ to LHI. The eme~ntauv ob
distance, tranaition&pole-~~~ pbemsneaon-Fm_nuggtraoafaR. l%isbasallR”dependeme,whereRis thediaancebetweendomandaseptor molecules. It baa been shown that it t&~s.sprortbeeneqytobe+Jalw fen-al fmm R&i0 to R875 (i.e. LHZ to LHlP? Since the ring8 of Rcbl a pigments am 105&d at the mme depth in the membrane. the diatawe between tbemcaIlbeminimwand,tberefare,