GroEL and the maintenance of bacterial endosymbiosis

G oEL and he main enance o bac e ial endosymbiosis Ma io Ali Fa es 1 , And e ´s Moya 2 and Eladio Ba io 2 1 Molecula E olu ion and Bioin o ma ics Labo a o y, Biology Depa men , Na ional Uni e si y o I eland, Maynoo h, Co. Kilda e, I eland 2 Ins i u Ca anilles de Biodi e si a i Biologia E olu i a and Depa men o Gene ics, Uni e si a de Vale`ncia, PO Box 2085, 46071 Vale`ncia, Spain Many euka yo ic o ganisms ha e symbio ic associa ions wi h obliga e in acellula bac e ia. The clonal ans- mission o endosymbion s be ween hos gene a ions should lead o he i e e sible ixa ion o sligh ly dele e ious mu a ions in hei non- ecombinan genome by gene ic d i . Howe e , he s abili y o endosymbiosis indica es ha some mechanism is in ol ed in he amelio a ion o he e ec s o hese mu a ions. We p opose ha he chape one G oEL was in ol ed in he acquisi ion o an endosymbio ic li es yle no only by means o i s o e -p oduc ion, as p oposed by Mo an, bu also by i s adap i e e olu ion media ed by posi i e selec ion o imp o e he in e ac ion wi h he uns able endosymbion p o eome. Al hough euka yo es display g ea di e si y and mo pho- logical complexi y, mos o hem show limi ed me abolic capabili ies in compa ison wi h p oka yo es, and some ha e los he abili y o syn hesize he essen ial amino acids and coenzymes ha a e necessa y o hei basic me abolism. Some euka yo es, mainly insec s, ha e sol ed hese limi a ions by associa ions wi h OBLIGATE ENDOSYM- BIONTS (see Glossa y). The acquisi ion o an obliga e in acellula li es yle has imposed gene ic and physiologi- cal ans o ma ions o he bac e ial pa ne . Recen ly, he analysis o la ge genome sequences om di e en insec endosymbio ic bac e ia has e ealed some o he mechan- isms ha a e in ol ed in he ans o ma ion o ee-li ing bac e ia in o obliga e bac e ial endosymbion s, and hei consequences [1]. One consequence o he endosymbio ic li es yle is he ixa ion o SLIGHTLY DELETERIOUS MUTATIONS in he non- ecombining endosymbion genome [2,3]. This ixa ion is due o he s ong e ec o GENETIC DRIFT du ing he s ic e ical ansmission in he hos , om mo he o o sp ing, o a educed numbe o endosymbion s imposing a POPULATION BOTTLENECK [4]. The inc eased MUTATIONAL LOAD o e ime should lead o a decline in he BIOLOGICAL FITNESS, a phenomenon known as MULLER’S RATCHET [5] and he e en ual ex inc ion in he absence o ecombina ion [6,7]. Howe e , he s abili y o endosymbiosis indica es ha compensa ing mechanisms amelio a e he e ec o he mild dele e ious gene ic a ia ion, enabling endosym- bion s o escape om i ness decline. The bac e ial CHAPERONE G oEL, which is in ol ed in he bu e ing o en i onmen al changes in bac e ia, has been pos ula ed as an impo an p o agonis in he acquisi ion o an endosymbio ic li es yle by becoming in ol ed in he amelio a ion o mild dele e ious gene ic a ia ion in he endosymbion p o eome [2]. In his a icle, we p opose ha his new ole was achie ed no only by an inc ease in he exp ession le els o he G oEL-coding gene bu also by he ac ion o POSITIVE SELECTION imp o ing he in e ac ion o G oEL wi h he uns able endosymbion p o eome. Chape ones as essen ial su i ing-machines o he cell Folding, a icking and deg ada ion o p o eins a e p ocesses ha depend on he assis ance o chape ones, also called hea -shock p o eins (Hsp) [8]. Chape ones a e Glossa y Adap i e e olu ion: an e olu iona y p ocess ha is di ec ed by posi i e selec ion, which makes a popula ion be e adap ed o li e in a pa icula en i onmen . Biological i ness: he a e age ep oduc i e success o a geno ype in a pa icula en i onmen ela i e o o he geno ypes. Compensa o y mu a ions: in highly in eg a ed gene ic sys ems, mu a ions in any one componen can ha e a dele e ious e ec on i ness; bu mu a ions in se e al componen s could lead o a eco e y o he i ness educ ion o e en lead o an o e all inc ease in i ness compa ed wi h he cu en s a e. Con e gen e olu ion: p ocess by which a simila cha ac e e ol es indepen- den ly om di e en ances o s (also known as con e gence). Gene ic d i : andom changes in he gene equencies. The e ec o gene ic d i is mo e e ec i e in small popula ions han in la ge popula ions. Mulle ’s a che : he p ocess by which he numbe o dele e ious mu a ions inc eases i e e sibly in non- ecombinan asexual popula ions due o he impossibili y o eco e he less mu a ed class. Mu a ional load: a educ ion o he a e age i ness o he membe s o a popula ion because o he accumula ion o dele e ious mu a ions in he popula ion a any gi en ime. Obliga e endosymbion s: bene icial symbion s ha ha e in acellula li es yle in speci ic hos cells and depend on he hos o su i al. Popula ion bo leneck: a se e e educ ion in popula ion size, which andomly educes he gene ic a iabili y o a popula ion. Posi i e selec ion: p ocess by which alleles ha con e a highe ela i e i ness o he geno ypes inc ease hei equency and, ul ima ely, become ixed in he popula ion. Pu i ying selec ion: selec ion ac ing agains dele e ious alleles ha a ise in a popula ion, p e en ing hei inc ease in equency and ensu ing hei e en ual disappea ance. Sligh ly dele e ious mu a ions: dele e ious mu a ions wi h such a small selec i e disad an age (less han he in e se o wice he e ec i e popula ion size) ha hei e olu iona y a e is in luenced mo e by gene ic d i han selec ion. Co esponding au ho : Eladio Ba io (eladio.ba io@u .es). A ailable online 20 July 2004 www.sciencedi ec .com 0168-9525/$ - see on ma e Q2004 Else ie L d. All igh s ese ed. doi:10.1016/j. ig.2004.07.001 Opinion TRENDS in Gene ics Vol.20 No.9 Sep embe 2004 p esen in all euka yo ic and p oka yo ic cells whe e hey a e exp essed cons i u i ely. They a e in ol ed in he bu e ing o en i onmen al changes by inc easing hei exp ession unde s ess ul condi ions p omo ed by hea -shock, oxida i e s ess, nu i ional de iciencies, ul a iole i adia ion, chemicals, i al in ec ions, and so on. [8]. The equi emen o molecula chape ones o old he newly syn hesized p o eins app op ia ely has also been p o en du ing he pas decade [9,10]. In con as o small p o eins ha a e able o old spon aneously in i o, mul idomain p o eins a e p one o mis olding and agg e- ga ion because o hyd ophobic in e ac ions be ween hei unshielded exposed su aces [11]. Molecula chape ones a e he mos success ul olding machines ha cells ha e de eloped o p e en p o ein agg ega ion. The h ee majo chape ones ha a e esponsible o p o ein olding a e G oEL, Hsp70 and Hsp90. Hsp70, called DnaK in bac e ia, is he mos ubiqui ous and is loca ed in he cy osol and in he cellula compa men s o euka yo ic cells and is also ound in a chaea. By con as , G oEL, he bes -cha ac e ized chape one o da e [12–14], is ound speci ically in bac e ia and in he o ganelles o euka yo ic cells. Hsp90 is a specialized chape one ha is only p esen in euka yo ic cells. The high unc ional lexibili y o G oEL The e is a la ge body o s uc u al and expe imen al da a demons a ing ha G oEL binds o a wide a ie y o p o ein con o ma ions and sizes, including in e media es a di e en olding s ages [15,16], comple ely un olded p o eins [17], mol en-globule-like p o eins [18],in addi ion o helical and ex ended s uc u es [19,20]; he e o e, i is di icul o es ablish a common ea u e o G oEL p o ein subs a es. Despi e he lack o appa en subs a e speci ici y o G oEL, i s p e e ence o exposed hyd ophobic egions and i s lexibili y sugges a common mechanism o he in e ac ion wi h i s di e se clien p o eins [21]. The ema kable abili y o G oEL o bo h old and un old p o eins has been demons a ed [17,22]. Un olding o kine ically apped con o ma ions would enable hei deg ada ion o p o ide he oppo uni y o p oduc i e olding [23]. Gi en he high lexibili y o G oEL (i.e. binding o p o ein- olding in e media es and hal ing he non-speci ic agg ega ion o un olded p o eins), i is easy o deduce how his chape one migh p o ec he cell om p o ein con o ma ions ha ha e accumula ed des abilizing amino-acid eplacemen s [24]. The abili y o G oEL o amelio a e he e ec o mu a ions is suppo ed by he obse a ion ha plasmids ha o e exp ess he ope on g oE (also called mopBA), which encodes G oEL and i s co- chape one G oES, can supp ess empe a u e-sensi i e mu a ions [25,26]. G oEL masks he e ec s o ha m ul mu a ions in he endosymbio ic bac e ia o insec s As men ioned p e iously, obliga e endosymbio ic bac e ia a e expec ed o accumula e sligh ly dele e ious mu a ions as a esul o hei dis inc i e popula ion s uc u e. In ac , as ixa ion a es o amino-acid subs i u ions due o gene ic d i ha e been demons a ed in se e al p o ein-coding genes om he aphid endosymbion Buchne a [2,3].The ixa ion by gene ic d i o poin mu a ions in p o ein- coding genes migh p oduce amino-acid eplacemen s a ec ing he unc ional con o ma ion o p o eins. In his way, he he modynamic ins abili y o Buchne a p o eins has been demons a ed by a compa ison o he no malized ene gy gap (a) o p o eins om endosymbio ic bac e ia, pa hogenic in acellula bac e ia and hei ee-li ing ela i es [27]. The abili y o G oEL o escue non- p oduc i e p o ein con o me s, which enables mul iple chances o ad ance o he mos s able s a e, p o ides a compensa ion mechanism o amelio a e he e ec s o poin mu a ions inducing loss o p o ein con o ma ion un il compensa ing mu a ions accumula e o a new ene gy minimum is eached. This ole, o iginally p oposed by Mo an [2], is suppo ed by se e al obse a ions. The g oE (mopBA) ope on is he mos highly cons i u- i ely exp essed loci in insec endosymbio ic bac e ia, e en du ing non-s ess condi ions [28]. Thus, G oEL cons i u- es 10% o he o al p o ein in Buchne a [29] and is also o e p oduced in o he endosymbio ic bac e ia o di e en insec s [30–33] and Amoeba p o eus [34]. Du ing hea -s ess condi ions, g oE and o he hea - shock genes show a low ansc ip ional inc ease in Buchne a compa ed wi h he ee-li ing ela i e E. coli [28]. Bu his modes s ess esponse ails o escue hea - s essed Buchne a bo h in labo a o y condi ions [35] and in na u e [36] – because he o e p oduced G oEL is p obably de o ed o he escue o non- unc ional p o eins ha a e des abilized by he accumula ion o sligh ly dele e ious mu a ions [28]. Finally, in an expe imen mimicking he popula ion dynamics o endosymbio ic bac e ia [37], he cons i u i e o e exp ession o g oE is esponsible o he signi ican eco e y o biological i ness in Esche ichia coli s ains ha had accumula ed dele e ious mu a ions du ing 3240 gene a ions unde s ong bo leneck condi ions. Howe e , a compa a i e analysis o he e olu ion o he g oE ope on in endosymbio ic and ee-li ing bac e ia showed ha he G oEL-encoding gene, in con as o o he endosymbio ic genes, is mainly subjec ed o PURIFYING SELECTION [38]. The lowe equency o sligh ly dele e ious mu a ions in g oE compa ed wi h o he endosymbion genes [39] suppo s he essen ial ole o his chape one in he main enance o he endosymbion p o eome because mu a ions nega i ely a ec ing i s unc ion a e emo ed by pu i ying selec ion. Howe e , he me hods used o analyse G oEL e olu ion in di e en endosymbion s also de ec ed speci ic amino-acid eplacemen s in his chape one d i en by posi i e selec ion [38]. Thus, a concen a ion o posi i e-selec ed eplacemen s in key amino-acid posi ions in ol ed in he in e ac ions be ween G oEL, G oES and hei clien p o eins was de ec ed, indica ing ADAPTIVE EVOLUTION in G oEL, which imp o es i s in e ac ion wi h he uns able endosymbion p o eome. We ha e also es ed he hypo hesis o CONVERGENT EVOLUTION o G oEL masking he e ec o sligh ly dele e - ious mu a ions in endosymbion s belonging o wo dis an ly ela ed bac e ial phyla, P o eobac e ia and Opinion TRENDS in Gene ics Vol.20 No.9 Sep embe 2004414 www.sciencedi ec .com Bac e oide es. Rema kably, he same G oEL amino- acid posi ions we e iden i ied ha ix amino-acid subs i u ions by posi i e selec ion in bo h g oups o endosymbio ic bac e ia. These posi i ely selec ed amino-acid a ia ions ha e occu ed in bo h endosym- bio ic g oups a he same key amino-acid posi ions ha a e in ol ed in he binding o non-na i e p o ein con o me s and G oES. These esul s clea ly indica e ha he dele e ious mu a ion bu e ing ole o G oEL migh ha e been con e gen ly acqui ed in, a leas , wo phylogene ically dis an bac e ial lineages wi h endosymbio ic li es yle. Conclusions The unc ion o G oEL should be iewed in he con ex o he gene ic cha ac e is ics o he genome o endosymbio ic bac e ia. We p opose ha a e he acquisi ion o an endosymbio ic li es yle, in such a s able en i onmen , G oEL changed i s ole om one ha bu e ed he en i onmen al changes o one ha led o he amelio a ion o mild dele e ious gene ic a ia ion a ec ing he s abili y o he endosymbion p o eome, as p oposed by Mo an [2]. The e olu ion o his new ole was a ou ed no only by he cons i u i e o e exp ession o g oE bu also by he ac ion o posi i e selec ion on hose G oEL amino-acid posi ions ha a e in ol ed in he in e ac ion wi h clien p o eins. Two g oundb eaking publica ions ha e also demon- s a ed ha ano he chape one, Hsp90, is in ol ed in he s abili y o ma u e con o ma ions o signal ansduce s in euka yo es. An inc ease in empe a u e o he pha maco- logical inhibi ion o Hsp90 p o ided e idence o he high pheno ypic a ia ion ha is silenced by his chape one in D osophila melanogas e [40] and in A abidopsis haliana [41]. The bu e ing capaci y migh be a uni e sal unc ion o chape ones bu , unlike G oEL, euka yo ic chape ones, especially in mul icellula o ganisms, can be mo e speci ic in hei bu e ing abili y. Finally, we can no disca d he possibili y ha o he mechanisms could ac syne gis ically wi h G oEL in he main enance o he in acellula li es yle. Mo eo e , COMPENSATORY MUTATIONS ixed in he genome o hese bac e ia migh also educe he e ec o sligh ly dele e ious mu a ions. Re e ences 1 We neg een, J.J. (2002) Genome e olu ion in bac e ial endosymbion s o insec s. Na . Re . Gene . 3, 850–861 2 Mo an, N.A. 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