The cell membrane plays a crucial role in survival of bacteria and archaea in extreme environments

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University of Groningen

The cell membrane plays a crucial role in survival of bacteria and archaea in extreme
environments
Konings, Wil N.; Albers, Sonja-Verena; Koning, Sonja; Driessen, Arnold J.M.
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Antonie Van Leeuwenhoek: International Journal of General and Molecular Microbiology
DOI:
10.1023/A:1020573408652
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Konings, W. N., Albers, S-V., Koning, S., & Driessen, A. J. M. (2002). The cell membrane plays a crucial
role in survival of bacteria and archaea in extreme environments. Antonie Van Leeuwenhoek: International
Journal of General and Molecular Microbiology, 81(1-4), 61 - 72. https://doi.org/10.1023/A:1020573408652
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AntonievanLeeuwenhoek 81: 61–72,2002. 61
©2002KluwerAcademicPublishers. PrintedintheNetherlands.
The cell membrane plays a crucial role in survival of bacteria and archaea
in extreme environments
WilN. Konings1,∗, Sonja-VerenaAlbers1,SonjaKoning1 &Arnold J.M.Driessen1
1Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University
∗
of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands ( Author for correspondence; E-mail:
[email protected])
Keywords:energytransduction,extremeenvironments,ion-permeability,membrane,solutetransport
Abstract
Thecytoplasmicmembraneofbacteriaandarchaeadeterminetoalargeextentthecompositionofthecytoplasm.
Sincetheionandinparticulartheprotonand/orthesodiumionelectrochemicalgradientsacrossthemembranesare
crucialforthebioenergeticconditionsofthesemicroorganisms,strategiesareneededtorestrictthepermeationof
theseionsacrosstheircytoplasmicmembrane.Theprotonandsodiumpermeabilitiesofallbiologicalmembranes
increase with the temperature. Psychrophilic and mesophilic bacteria, and mesophilic, (hyper)thermophilicand
halophilic archaea are capable of adjusting the lipid composition of their membranes in such a way that the
protonpermeabilityattherespectivegrowthtemperatureremainslowandconstant(homeo-protonpermeability).
Thermophilic bacteria, however, have more difficulties to restrict the proton permeation across their membrane
athightemperaturesandthese organismshaveto rely onthe less permeablesodiumionsfor maintaininga high
sodium-motive force for driving their energy requiring membrane-boundprocesses. Transport of solutes across
the bacterialand archaealmembraneis mainly catalyzed by primaryATP driven transportsystems or by proton
orsodiummotiveforcedrivensecondarytransportsystems.Unlikemostbacteria,hyperthermophilicbacteriaand
archaeapreferprimaryATP-drivenuptakesystemsfortheircarbonandenergysources.Severalhigh-affinityABC
transportersforsugarsfromhyperthermophileshavebeenidentifiedandcharacterized.TheactivitiesoftheseABC
transportersallowtheseorganismstothriveintheirnutrient-poorenvironments.
Abbreviations:PMF–protonmotiveforce;SMF–sodiummotiveforce
Introduction membranesareusuallyimpermeableformostionsand
solutes, a property, which is essential for controlling
Anincreasingnumberofmicroorganismsarefoundto the composition of the cytoplasm. The cytoplasmic
flourish in environments in which the physical para- membrane therefore has a crucial function in main-
meters such as temperature, salinity, pH or pressure, tainingoptimalinternalconditionsformetabolismand
are extreme with respect to the conditions in which energy transduction. Solutes and ions have to pass
eukaryoticorganismslivepreferentially.Mostofthese the membrane for metabolism to proceed and spe-
extreme environments were previously thought to be cific transport proteins catalyze the transfer of these
toohostileforanyformoflife.Theorganismsthatlive compoundsacrossthesemembranes.
in these extreme environments are termed extremo- Membranes are very complex structures. They
philes. Mostextremophilesbelongto thekingdomof consist of a bilayer or monolayer of lipid molecules,
the archaea, but also certain bacteria and even some whichformamatrixinwhichvariousmembranepro-
eukaryacantoleratesomeoftheseextremeconditions. teinsfloat.Thefluid-mosaicmodeldescribesthebasic
Biological cells are surrounded by a cytoplasmic properties of these membranes. The fluidity and per-
membrane, which functions as a barrier between the meability properties of the membranes are mainly
cytoplasm and the extracellular environment. Such determinedbytheirlipidcomposition.Organismsare
62
ableto adjustthe lipid compositionoftheir cytoplas-
micmembraneinresponsetochangesintheenviron-
ment. The different strategies, which extremophiles
use to adapt their membrane and membrane proteins
tothevariousextremeconditionsinwhichtheygrow,
aredescribed.
Thecytoplasmicmembraneandbioenergetics
Biological cells generate metabolic energy by two
basicallydistinctmechanisms.Onemechanismissub-
stratelevelphosphorylation,inwhichchemicalenergy
Figure 1. Energy transduction in the cell membrane of aerobic
released in catabolic processes is stored in ADP to
mesophilic bacteria. The extrusion of protons by the respiratory
form ATP. The second mechanism takes place at the chainresultsinthegenerationofa(cid:2)(cid:3),insidenegative,anda(cid:2)pH,
level of the cytoplasmic membrane. This membrane insidealkaline. TheresultingPMFdrivesthesynthesisofATPby
themembraneboundATPaseandthetransportofsolutesbysecond-
contains energy transducing systems which convert
ary transporters and other metabolic-energy requiring membrane
chemical energy, or in phototrophslight energy, into processes.
electrochemical energy of ions or vice versa. Both
metabolic energy-generating processes are closely
electrochemicalprotongradient contributeequally to
linked and together they determine the energy status
theforceontheprotons.Inmostorganismstheresult-
ofthecell.
ingPMFhasanegativevalueandthedrivingforceon
Energytransductioninthecytoplasmicmembrane
theprotonsisdirectedintothecell. Inorganismsthat
iscatalyzedbyintegralmembraneproteinsthattrans-
live around pH 7 (neutrophiles), both the membrane
locateionsacrossthemembraneintotheexternalme-
potentialandthepH-gradienthaveanegativevalue.
diumattheexpenseofotherformsofenergy.Electron
A numberoforganismsuse sodiumionsascoup-
transfer systems and the membrane bound ATPases
ling ions in energy transduction. In these organisms
are examples of such systems. The activity of these
sodium ionsare pumpedfromthe cytoplasminto the
pumps results in the generation of electrochemical
externalmedium. In analogywith the PMF these so-
gradients of the translocated ions (Figure 1) (Speel-
dium ion pumps can generate a sodium motive force
mans et al. 1993a; Lolkema et al. 1994). These ions
(SMF), which is composed of a membranepotential,
are termed energy-couplingionsand so far only pro-
(cid:2)(cid:3),andthechemicalgradientofthesodiumions.In
tonsandsodiumionshavebeenfoundtocarryoutthis
formula:
function. Proton pumping from the cytoplasm to the
externalmedium results in the generationof an elec- SMF=(cid:2)(cid:3)+2.3RT/Flog[Na+]/[Na+ ])
trochemical gradient of protons which is composed in out
of a pH-gradient, inside alkaline versus acid outside,
ThePMFandtheSMFcanbeusedtodrivemetabolic
andanelectricalpotentialacrossthemembrane,inside
energyrequiringprocessessuchasATPsynthesisfrom
negativeversusoutside.Thesetwogradients,thepH-
ADP and phosphate, the transport of specific solutes
gradient or (cid:2) pH, and the electrical potential across
acrossthemembrane,flagellarrotation,andmainten-
the membrane(also termedmembranepotential), the
ance of the intracellular pH (Booth 1985) and turgor
(cid:2)(cid:3), exert an inward directed force on the protons,
(Figure1).Obviously,thistypeofenergytransduction
theprotonmotiveforce(orPMF).Theformulaofthe
can only operate if the electrochemical gradients of
PMFis: + +
H or Na can be maintained and this will only be
PMF=(cid:2)(cid:3)−2.3(RT/F)(cid:2)pH possible if the biological membranes have a limited
permeabilityfortheseionsandfreeenergyisavailable
The PMF is expressed in mV, in which R is the gas tomaintainthesegradients.
constant, T the absolute temperature (K), and F the Ithasbeenstatedabovethatthecytoplasmicmem-
Faraday constant. The effect of 1 unit pH difference branefunctionsasabarrierbetweenthecytoplasmand
betweencytoplasmandexternalmediumis59mVat theenvironmentandcontrolsthemovementofsolutes
◦ ◦
25 C, and 70mV at80 C. Both componentsofthe (ionsandnutrients)into oroutofthe cell. Biological
63
membranesareusuallycomposedofabilayeroflipids (De Rosa et al. 1991). Most archaea growing un-
inwhichproteinsareembedded.Thelipidshavepolar dermoderateconditionscontainbilayerforminglipids
head groups that stick into the water phase and hy- withaC di-etherlipidcore(Katesetal.1993;Upas-
20
drophobichydrocarbonchainsthatareorientedtothe anietal. 1994;Kates1996).SuchC di-etherlipids
20
interior of the membrane. At the growth temperature also form bilayer membranes just as their bacterial
of a given organism, the membranes are in a liquid and eukaryalcounterparts(Kogaetal. 1993; Yamau-
crystallinestate(Melchior1982).Non-covalentbonds chietal. 1995;Kates1996).Inextremethermophilic
suchasVanderWaalsbondsandelectricinteractions and acidophilic archaea (De Rosa et al. 1991) mem-
mainlyholdthe structureof biologicalmembraneto- branespanning(bolaformamphiphilic)etherlipidsare
gether. The barrier function of the cell membrane foundinwhichthephytanylchainsoftwodietherlip-
is critical for controlling the concentrations of mo- idsarefusedtoaC core.Theseso-calledtetraether
40
leculesandionsinsidethecell.Mostsolutescannotor lipids form a monolayer in which the lipids span
only very slowly cross the lipid bilayer of biological the whole membrane(Figure2). Freeze-fracturingof
membranes by diffusion. Although lipophilic solutes these membranes reveals that cleavage between two
dissolve readily in the membranes these compounds leaflets of the membrane does not occur, indicating
alsocrossthemembranesveryslowly.Specifictrans- that the water facing sides of the membraneare con-
port proteins are needed to catalyze the transfer of nectedandcannotbe separated(Choquetetal. 1992;
thesesolutesacrossthemembraneatratesrequiredfor Elferink et al. 1992; Beveridge et al. 1993). The
growth. observationthattetraetherlipidsfromThermoplasma
Thelipidlayerformsasuitablematrixfortransport acidophilum and Sulfolobus solfataricus form black
proteinsthatcangenerateandmaintainspecificsolute lipidmembraneswithathicknessof2.5-3.0nm(Stern
concentration gradients across the membrane. Under et al. 1992) is in accordance with the length of the
physiological conditions the lipids in the membrane tetraetherlipids(Gliozzietal.1983).
areorderedandinaliquidcrystallinestatethatallows Membranescomposedofetherlipidshaveahigher
optimalfunctioningofthemembraneproteins.Byad- stabilitythanthoseofester-lipids(Elferinketal.1992;
justingthelipidcompositionbacteriaandarchaeacan Thompsonet al. 1992), most likely as a result of the
control the permeability of their cell membrane for reduced segmentary motion of tertiary carbon atoms
solutesandions(seebelow).Alowpermeabilitylim- (i.e., rotationofcarbonatomsthatareboundtothree
its the energy needed for maintaining ion and solute other C-atoms, resulting in kinks in the acyl chain).
gradients. This restriction in hydrocarbon chain mobility may
also result in a reduced permeability of the archaeal
membranes.
Lipidcompositionofbacterialandarchaeal
membranes
Bioenergeticproblemsofextremophiles
Thelipidcompositionofcellmembranesisverycom-
plex and differs strongly between organisms. It is
Extremophiles living in various harsh environments
tightlyregulatedanddependentonenvironmentalcon-
face different problems in maintaining a viable pro-
ditions.Bacterialandeukaryallipidsarecomposedof
ton motive force or sodium motive force across their
two fatty acyl chains that are ester-linked to glycerol
membranes.
(Figure 2). The third hydroxylgroup of the glycerol
islinkedtohydrophilicphospho-orglyco-containing
headgroups.Theselipidsareorganizedinabilayerso
thatthepolarhead-groupsstickintothewaterphases Temperature
whilethealkanechainsaredirectedtowardstheinner
sideofthemembrane. Bacteriaandarchaeacanrespondtochangesinambi-
In contrast to bacterial and eukaryal lipids, ar- enttemperaturethroughadaptationsofthelipidcom-
chaeallipidsconsistoftwophytanylchainswhichare position of their cytoplasmic membranes (Gaughran
linkedviaanetherbondto glycerolorotheralcohols 1947). These changes are needed to keep the mem-
like nonitol (Figure 2). The acyl chains of these ar- braneinaliquidcrystallinestate(Russelletal.1990)
chaeal lipids are usually fully saturated isoprenoids and to limit the proton permeation rates. At higher
64
Figure2. Lipidsfromarchaeaandbacteria.(A)Bilayerforminglipidsinbacteria:Phosphatidylglycerol(PG)fromEschericiacoli.Theacyl
chaincanbebranched,containacyclohexylgroupattheendoftheacylchain,orcontainoneormoreunsaturatedbonds.Theconnectionof
theacylchainwiththeheadgroupisanester.(B)Bilayerformingdietherlipidsfromarchaea.Theconnectionofthephytanylchainwiththe
headgroupisanetherlinkage.Thephytanylchaincontainsisoprenoid-likebranches.Someacidophilictetraetherscontaincyclopentanerings.
(C)Monolayerformingtetraetherlipidsinthermoacidophilicarchaea.
temperatures,thiscanbedoneinbacteriabyincreas- oid in the tetraether lipids increases with the growth
ingthechainlengthof thelipidacylchains, the ratio temperatures (De Rosa and Gambacorta 1988). In
◦
of iso/anteiso branching and/or the degree of satur- Thermoplasmacellsgrownat40 Ctheratioofacyc-
ation of the acyl chain (Reizer et al. 1985; Prado lic/monocyclic/bicyclicchainis62/37/1and25/50/24
◦
et al. 1988; Svobodová and Svobodová, 1988). The for cells grown at 60 C (Langworthy 1982). By in-
archaealSulfolobalesspecies, Sulfolobussolfataricus creasing the cyclization of the C isoprenoid chains
40
and Thermoplasma acidophilum, contain a high per- thelipidscanbepackedmoretightly,whichresultsin
centageoftetraetherlipidsintheirmembranes(above amorerestrictedmotionofthelipidsandpreventsthat
90%). The degree of cyclization of the C isopren- the membrane becomes too fluid. In the euryarchae-
40
65
ote Methanococcus janaschii an increase of growth
temperature results in a decrease of the diether lipid
contentandanincreaseofthecontentofthemorether-
mostable tetraether lipids (Sprott et al. 1991). Also,
in this organism cyclization of the isoprenoid chains
tends to decrease the motion of the lipids and there-
fore contributes to acceptable membrane fluidity at
elevatedgrowthtemperature.
Theincreasedmotionofthelipidmoleculesinthe
membranes causes an increased proton permeability
of the membranes at high temperature. Due to this
increased motion more water molecules are trapped
in the lipid core of the membranes. Protons can hop
from one water molecule to the other and the higher Figure 3. Temperature dependency of the proton permeability of
watercontentofthemembranethusleadstoa higher liposomal membranes derived from various archaea and bacteria
thatliveatdifferenttemperatures.Themembraneofpsychrophilic
permeation of protons. Unlike protons, other ions
andmesophilicbacteriaandofmostarchaeahasattheirgrowthtem-
cross the membrane by diffusion. This is a temper- peratures aprotonpermeability thatfallswithinanarrowwindow
ature dependent process and results in an increased (greybar).ThemembraneofthethermophilicbacteriaT.maritima
andB.stearothermophilus, however, hasattheirgrowthtemperat-
ion-permeabilityofthemembraneathighertemperat-
ure, amuchhigherprotonpermeability thantheotherorganisms.
ures.Whenthemembranepermeationofthecoupling
Squaresindicatetheprotonpermeabilityatthegrowthtemperature.
ions, protons or sodium ions, becomes too high, the
organism will encounter difficulties in establishing a
PMForSMFofsufficientlyhighmagnitude.Theper- The proton permeability of the membrane of
meabilityofthecytoplasmicmembranethereforeisa psychrophilic and mesophilic bacteria and archaea is
major factor in determining the maximum temperat- controlledbythelipidcomposition.Thishomeostasis
ure of growth. To determine the proton and sodium ofprotonpermeabilityhasbeentermed‘homeo-pro-
ion permeability’s of the cell membrane of bacteria ton permeability adaptation’. Strong support for the
andarchaea,thelipidshavebeenextractedfromtheir homeo-proton permeability theory was supplied by
isolatedmembranesandusedfortheformationoflipo- studies of Bacillus subtilis, grown at and within the
somes (van de Vossenberg et al. 1995). The proton boundaries of its growth temperature range: 13 to
permeability of these liposomal membranes can be 50 ◦C (van de Vossenberg et al. 1999a). The proton
determined by following the fluorescence change of permeability of the membranes of B. subtilis grown
theexternallypresentpHprobepyranineuponproton at different temperatures remained constant over the
influx, while the sodium ion permeability can be as- whole growth temperature range (Figure 4). Interest-
sessedbyfollowingtheleakageofradioactivesodium ingly,incontrasttotheprotonpermeabilitythefluid-
ions from preloaded liposomes (van de Vossenberg ity of the membranes is not maintained constant but
et al. 1999b). In all liposomes studied, the proton increasessignificantlywithtemperature.Theseobser-
permeability increases with temperature (Figure 3). vationsstronglyindicatethatthegrowthtemperature-
However, a most important finding of these studies dependentalterations in fatty acyl chain composition
was that the proton permeability of membranesfrom aremainlyaimedatmaintainingtheprotonpermeab-
psychrophilic and mesophilic bacteria and of all ar- ilityofthecytoplasmicmembraneataratherconstant
chaea studied so far is maintained within a narrow level. The observations also clearly demonstrate that
window (H+-permeability coefficient near 10−9 cm theuppertemperatureofgrowthisdeterminedbythe
s−1) at their growth temperature. Above this growth protonpermeabilityoftheirmembranes.
temperaturethemembranesbecomehighlypermeable The situation in thermophilic bacteria is signific-
forprotons.Theseorganismscanapparentlyuse pro- antlydifferentfromthoseinpsychrophilicandmeso-
tons at their growth temperature as couplingions for philic bacteria and archaea. In thermophilic bacteria,
energytransduction.However,athighertemperatures the membraneproton permeabilityat their respective
theprotonpermeationbecomestoohighandaPMFof growth temperatureshas foundto be extremelyhigh.
viablemagnitudecannotbemaintened. Asa result, theseorganismshaveextremedifficulties
in generating a PMF of viable magnitude (Figure 3)
66
Figure4. Thetemperaturedependencyoftheprotonpermeability
ofmembranesofBacillus subtilisgrownatdifferent temperatures
◦ ◦
intherangefrom13to50 C.Onlythedatafor13and50 Care Figure5. Temperaturedependency ofthesodiumpermeability of
shown. The proton permeabilities of all membranes fall within a liposomesderivedfromvariousbacteriaandarchaea. P.immobilis
narrowwindow. sp((cid:1)),M.barkeri((cid:8)),E.coli((cid:1)),B.stearothermophilus((cid:2)),T.
maritima((cid:2)),andS.acidocaldarius((cid:3)).
(van de Vossenberg et al. 1995). Evidently, thermo-
philic bacteria, such as B. stearothermophilus, Ther- SMF with the help of proton/sodium exchange sys-
motoga maritima and Calaromator fervidus, are not tems.InthiswayahighSMFcanbegeneratedthatcan
able to restrict the protonpermeabilityof their mem- subsequentlybeusedto driveenergyrequiringmem-
brane at the high growth temperatures by adjusting brane processes such as secondary solute transport
thelipidcomposition.Thesodiumionpermeabilityof (Figure6).
themembraneofallbacteriaandarchaeastudiedalso Mostthermophilicbacteria, however,havetorely
increases exponentially with temperature (Figure 5). completelyonthelesspermeablesodiumionascoup-
However,theincreaseofthesodium-ionpermeability ling ion for energy transduction (Figure 7). This
is the same for all organisms. These results indicate strategy is used by Caloramator fervidus (previously
that the lipid composition of the membrane affects Clostridium fervidus) (Speelmans et al. 1993a,b), an
the membrane permeability of sodium ions only to a organismthatcangrowatahighertemperaturethanB.
◦
minorextentandthatthetemperatureisthemainde- stearothermophilus,i.e., 70 C(Esser&Souza1974;
+
terminingfactoroftherateofsodiumionpermeation. Pateletal.1987).C.fervidushasaNa -translocating
However,sincethesodiumpermeabilityisseveralor- ATPase that excretes sodium ions at the expense of
dersofmagnitudelowerthantheprotonpermeability, ATP. These V-type ATPases have interesting proper-
thegenerationofahighSMFispossibleinallbacteria tieswhichwillnotbefurtherdiscussedinthisreview
andarchaeaevenathightemperatures. (Speelmanset al. 1993a; Boekema et al. 1999). As a
Aerobic bacteria, such as Bacillus stearothermo- result,aSMFisgeneratedthatisthedrivingforcefor
philus, which live at the lower range of thermophilic energyrequiringprocessessuchassolutetransport.
◦
temperatures(around60 C), can compensateforthe The high proton leakage of the membranes of C.
highprotonleakageoftheirmembranesbydrastically fervidushasalsootherconsequentiesforgrowth.Due
increasingtherateofrespirationandconsequentlythe to the high proton permeability of its membrane, C.
rate of proton excretion (de Vrij et al. 1988). Since fervidus is not able to control its intracellular pH.
thepermeabilityofthemembranesforsodiumionsis Consequently, growth of C. fervidus is confined to
severalfoldlowerthanforprotons, metabolicenergy a narrow niche, i.e., an environment with a pH near
can be saved by rapidlytransducingthe PMFinto an neutrality.
67
Figure6. EnergytransductioninthemembranesofBacillusstearo-
thermophilus. Therespiratory chainpumps atahighrate protons
Figure 8. Energy transduction in the membrane from the ther-
fromthecytoplasmtotheexternalmediumthusgeneratingaPMF.
mophilic archaea such as Solfolobus solfataricus and Picrophilus
ThisPMFdrivesthesynthesisofATPviathemembraneboundAT-
oshimae. The proton pumping respiratory chain extrudes protons
Pase.Thehighpassiveinfluxofprotonsresultsinrapiddissipation
+ + andgeneratesaPMFandincreasestheinternalpHto4.5.Inorderto
ofthePMF.ANa /H antiporterconvertsthePMFintoaSMFand +
lowerthePMFtoviablevaluescation/H symportsystemstranslo-
thisSMFissubsequentlyusedforotherenergyrequiringprocesses
catepositivechargesfromoutsidetoinsideandgenerateareversed
suchassecondarytransport.
(cid:2)ϕ, inside positive. The PMF drives energy requiring processes
suchassecondarytransport.
gottschalkii (Thermoalkalibacter bogoriae) the low
PMFistheresultofareversed(cid:2)pH(insideacid)and
anormal(cid:2)(cid:3) (insidenegative).Thisreversed(cid:2)pHis
mostlikelygeneratedbythepassiveinfluxofprotons
+
inresponsetothe(cid:2)(cid:3) generatedbyNa pumping.
Salt
Halophiles, such as Halobacterium salinarum, gen-
erate an electrochemical proton gradient across the
membranebyrespirationand/orthelightdrivenpump
bacteriorhodopsin.Thisorganismkeepsthecytoplas-
mic concentration of sodium ions low by using the
PMF to drive sodium ions out of the cells via a
+ +
H /Na antiporter. In doing so also a high SMF is
Figure7. EnergytransductioninthemembraneofCalaromatorfer-
generated.
vidus. ATP is generated by substrate level phosphorylation. ATP
hydrolysis by the membrane-bound V-type ATPase results in the The proton and sodium permeabilities of mem-
extrusionofsodiumionsandthegenerationofaSMF.ThisSMFis branes from the halophile Halobacterium salinarium
usedtodriveenergy-requiringmembraneprocessessuchassecond- andthehaloalkaliphileHalorubrumvacuolatumhave
arysolutetransport. TheinternalpHequilibrateswiththeexternal
found to be very similar to those of non-halophilic
pHbythepassiveinfluxofprotons.
organismslivingat the same temperature. In contrast
tomembranesofE.colithoseofhalophilesandhalo-
Attheirgrowthtemperature,thethermophilicbac- alkaliphilesremainstableupto4MNaClorKCland
teria have a high SMF, but a very low PMF. In C. alsoathigherpHvalues.Obviouslythesemembranes
fervidusboththe(cid:2)(cid:3)andthe(cid:2)pHarelow.Inotheror- are well adapted to the halophilic conditions (van de
ganisms such as the thermoalkaliphileAnaerobranca Vossenbergetal.1999b).
68
pH
Extremeacidophilesmaintainan internalpH closeto
neutrality. As a consequence, these organismsexper-
ience a very high (cid:2)pH (inside alkaline) across their
cell membrane. This (cid:2)pH can be up to 4 pH units
in organisms such as Picrophilus oshimae that grow
optimallyatmediumpH-valuesof0.5–1(Schleperet
al. 1995). Such a large (cid:2)pH can only be maintained
withamembranethathasverylowprotonpermeabil-
ity.Theveryhigh(cid:2)pHintheseorganismsneedstobe
compensatedby aninverted(cid:2)(cid:3) (negativeoutsidevs
positiveinside)inordertokeepthePMFwithinviable
Figure9. Energytransductioninthecellmembraneofaerobical-
values(Figure8).Thisinversionofthe(cid:2)(cid:3) ismainly kaliphiles.Therespiratorychainpumpsprotonsfromthecytoplasm
realizedbypotassiumionuptakesystems. to the external medium. This results in a rapid generation of a
(cid:2)(cid:3), inside negative. This(cid:2)(cid:3) drives ATP-synthesis and electro-
Some thermophilic archaea can grow at very low
genicH+/Na+exchange, inaratio>1, resulting inadecrease of
pH-values.AnorganismsuchasSulfolobussolfatari- theinternal pHandinthegeneration ofaSMF.ThisSMFdrives
◦
cus can grow up to 90 C at pH values of 2.5–3.5. secondarytransportprocesses.
Ithasbeen describedabovethatthe protonpermeab-
ility of the membranes of these thermo-acidophilic
archaea, is at the high growth temperature as low as
1999).Thegenomesequencesofbacteriaandarchaea,
thatofmembranesofmesophilicbacteriagrownatthe
whicharenowavailable,indicatethatupto30%ofthe
mesophilicgrowthtemperatures.
genesencodefor membraneproteinsand that a large
Alkaliphilesalsohavetomaintainanintracellular
fraction of these proteins (up to 20%) are transport
pH close to neutrality in their very alkaline environ-
proteins. Very few studies have been carried out on
ment.TheinternalpHisthereforesignificantlylower
transportsystemsofextremophiles.
thantheexternalpH.Sincethe(cid:2)pHisreversed(inside
The transportsystemsofsolutesacrossbiological
acidvs. outside),averyhigh(cid:2)(cid:3) (insidenegative)is
membraneswhich have been described so far can be
neededtocreateasufficientlyhighPMF(Figure9).To
classified according to their molecular architectures
keeptheinternalpHclosetoneutrality,aerobicmeso-
and their driving force of transport. Five classes of
+ +
philicalkaliphilicbacteriauseNa /H -antiportersin
transport systems have been discriminated: (i) chan-
+
combinationwithH -coupledrespiration.Manyalka-
nels(Figure10);(ii)secondarytransporterswhichuse
liphileslive in sodium-richenvironments.Asa result
ofelectrochemicalgradientsofprotonsorsodiumions
ofthe highchemicalgradientof sodiumionsandthe
to drive transport of substrates across the membrane
high(cid:2)(cid:3) intheseorganismsahighSMFisgenerated.
(Figure 10); (iii) binding-protein-dependent second-
ary transporters (TRAP transporters), which consist
of a periplasmic binding protein and a membrane
Solutetransportproteins translocator. These systemsuse the PMF or the SMF
to drive uptake of solutes (Figure 10); (iv) primary
Membrane proteins form a large part of the mass transporters, which use light energy or the chemical
of prokaryotic membranes (up to 60% w/w). Many energy of ATP or of other compounds to drive sub-
of these membrane proteins are involved in energy strate translocation. Well-studied examples are ion-
transducing processes, such as electron transfer pro- translocatingrespiratorychainsandbacteriorhodopsin
teins(cytochromes,etc.),ATPasesandsolutetransport and the ABC- (ATP-binding cassettes) transporters.
systems. Membrane proteins have been studied ex- The latter transporters consist of two transmembrane
tensively in bacteria and to a much lesser extent in proteins, that form the translocation pathway, and
archaeaandhardlyinthermophilicandhyperthermo- two cytosolic ATP-binding proteins. ABC transport-
philicarchaea.Membraneproteinsinvolvedinenergy ers which catalyse the uptake of solutes contain in
transducing processes, such as cytochrome oxidases addition a high-affinity periplasmic binding protein
andATPases, havebeendescribedand characterized, (Figure 10); (v) the grouptranslocationsystems, i.e.,
e.g.fromS.acidocaldarius(reviewedinSchäferetal. the phosphoenolpyruvate (PEP) dependent phospho-
69
Table1. Describedsolutetransportersinextremophiles
ABC-transporter Substrate Kmforuptake Kdforsolute Reference
(nM) bindinga
(nM)
Archaea
T.litoralis maltose/trehalose 22/17 160 Xavieretal.1996;Horlacheretal.1998
S.solfataricus glucose 2000 480 Albersetal.1999
cellobiose+cellooligomers –b – Elferinketal.2001
trehalose – – "
maltose/maltotriose – – "
arabinose – 130 "
P.furiosus cellobiose+cellooligomers 175 45 Koningetal.2001
maltodextrin – 270 Evdokimovetal.2001
H.volcanii glucose(anaerobic) – – Wanneretal.1999
molybdate – – "
Inorganicanions – – "
A.fulgidus glycinebetaine,prolinebetaine – – Holtmannetal.2000
M.thermoautrophicum phosphate 25 – Kruegeretal.1986
Bacteria
T.maritima maltose/maltotriose/trehalose 300 Wassenbergetal.2000
T.ethanolicus39E maltose/maltotriose/trehalose 40 270 Jonesetal.2000
A.acidocaldarius Maltose/maltodextrin 1500 1500 Hülsmannetal.2000
Secondarytransporter Substrate Kmforuptake Coupling Reference
(µM) ion
Archaea
H.volcanii glucose –b Na+ Tawara&Kamo1991
+
H.halobium glutamate – Na Kamoetal.1988
+
allaminoacidsexceptcysteineand – Na Greene&MacDonald1984
aspartate
Bacteria
T.thermophilus nitrate/nitrite – – Ramirezetal.2000
+
C.fervidus aminoacids – Na Speelmansetal.1989
+
BacillusTA2.A1 glutamate 290 Na Peddieetal.1999
+
sucrose 33 Na Peddieetal.2000
+
B.acidocaldarius methylthio-β-galactoside – H Krulwichetal.1978
+
A.gottschalkiiLBS3 leucine – Na Proweetal.1996
aSolutebindingtobindingprotein.
bNotdetermined.
transferasesystems(PTS),whichcouplethetransport this kind of transporter is absent. PTS-systems seem
ofsugarstophosphorylation(Figure10). to be restrictedto mesophilicbacteria, whichmaybe
Members of each of these classes of transport- anindicationthatthesesystemsevolvedrelativelylate.
ers have been identified in extremophiles, except for Thetransportsystems,whichhavebeenidentified
PTSsystems.Innoneofthecompletedgenomesfrom so far in extremophiles, are listed in Table 1. Most
archaeagenesencodingmembranecomponentsPTS- secondary transporters listed couple the transport of
systems have been found, and also in the hyperther- a solute to sodium ions. This property of the ther-
mophilic bacteria T. maritima and Aquifex aeolicus mophilic bacteria, studied so far, is consistent with