A quantitative assay to determine the amount of signal peptidase I in E. coli and the orientation of membrane vesicles

The number of Signal Peptidase I (SPasel) molecules per E. coli cell was determined using western blot techniques. Different strains were found to contain approximately 1000 SPasel molecules per cell during exponential growth. Based on the activity of SPasel in vitro it could be estimated that this amount is sufficient to process all translocated precursors. SPasel did not appear to be under growth phase dependent control, but was constitutively expressed. The quantitative western blot technique was also used to establish the orientation and intactness of isolated inner membrane vesicles.

Characterization of the import process of a transit peptide into chloroplasts

In order to get insight into the functioning of transit sequences in chloroplast protein transport, the import of the full-length transit peptide of ferredoxin (trfd) was investigated. trfd rapidly associated with chloroplasts under import conditions and becomes protected against externally added proteases. Import of radiolabeled trfd is inhibited equally efficiently by nonlabeled trfd as well as by the intact precursor of ferredoxin. This strongly suggests that trfd enters the general import pathway of proteins into chloroplasts. trfd import was stimulated by ATP, which is the first demonstration that ATP is involved in membrane translocation of a targeting signal. Imported trfd was membrane-associated but was also partially degraded by internal proteases, most likely present in the stroma, indicating that the membrane-associated fraction of trfd is en route to its functional localization. The degradation products are exported out of the organelle. In contrast to the import of the precursor of ferredoxin, the import of trfd was independent of protease-sensitive components on the chloroplast surface, indicating that the initial binding of precursor proteins may be facilitated by transit sequence-lipid interactions.

PhoE signal peptide inserts into micelles as a dynamic helix-break-helix structure, which is modulated by the environment. A two-dimensional 1H NMR study

Proteins that are destined for export out of the cytoplasm of Escherichia coli cells are synthesized as precursor proteins with N-terminal extensions or signal sequences, which are essential for translocation of the protein across the inner membrane. Signal sequences contain very little primary sequence homology, and therefore recognition of these sequences is thought to involve specific folding. To assess the conformational flexibility of signal sequences, we have studied the signal peptide of PhoE (MKKSTLALVVMGIVASASVQA) by two-dimensional nuclear magnetic resonance and circular dichroism in different membrane mimetic environments. The secondary structure of the PhoE signal peptide was analyzed via interresidue nuclear Overhauser enhancement measurements, chemical shifts of backbone protons, and by measuring amide proton exchange. The membrane mimetic environments studied were trifluoroethanol (TFE) and micelles of sodium dodecyl sulfate (SDS) or dodecylphosphocholine (DPC). In all systems alpha-helix formation was observed. In TFE, the alpha-helix stretches from the positively charged N-terminus to Ser18. In SDS and DPC micelles, the N- and C-terminal alpha-helical half are separated from each other by a kink at the Gly12 position, with the helical content being higher at the N-terminus and lower at the C-terminus. In zwitterionic DPC micelles, the C-terminal region has a less regular or more flexible structure compared to SDS. The insertion of the PhoE signal peptide into the hydrophobic environment of the micelles was demonstrated by the effect of spin-labeled 12-doxylstearate on the line widths of the peptide proton resonances.(ABSTRACT TRUNCATED AT 250 WORDS)

Anionic phospholipids can mediate membrane insertion of the anionic part of a bound peptide

The effect of anionic lipids on the membrane insertion of a carboxyl group on a specially designed palmitoylated peptide was studied, using tryptophan fluorescence. It is demonstrated that the negatively charged membrane surface of mixed phosphatidylcholine/phosphatidylglycerol small unilamellar vesicles enhances the protonation of the C-terminal carboxyl group, and the subsequent insertion of that part of the peptide.

Palmitoylation-induced conformational changes of specific side chains in the gramicidin transmembrane channel

To gain insight into the structural consequences of acylation for membrane proteins, we have covalently attached palmitic acid to the ethanolamine end of gramicidin A (gA), which functions as a well-characterized cation-selective membrane channel. Next, we investigated by NMR methods the effect of acylation on the side chains of Trp9, Leu10, and Trp11, which are expected to be close to the acyl chain, and of Val7, which is expected to be far from the acyl chain. Two-dimensional NMR spectroscopy in a sodium dodecyl sulfate (SDS) environment suggests that one of the beta-hydrogens of Leu10 of gA is severely shielded by a nearby aromatic ring. This shielding disappears upon acylation. Deuterium NMR spectra for labeled samples in hydrated dimyristoylphosphatidylcholine (DMPC) bilayers show that, for the major gA conformation, the (deuterated) side chains of Trp9 and Leu10 are markedly influenced by acylation, whereas the side chains of Val7 and Trp11 are essentially unaffected. The NMR results in both environments suggest that the indole ring of Trp9 is situated near the side chain of Leu10 and moves away upon acylation. We propose that acylation provides a subtle mechanism to modulate protein and lipid interactions and to regulate the stability and function of proteins within membranes.

The specificity of glycolipid-preferredoxin interaction: requirements for membrane binding

Preferredoxin (prefd) is a precursor protein that is imported into chloroplasts. Monolayer experiments have shown that prefd has a high affinity for monogalactosyldiglyceride (MGaIDG) isolated from chloroplasts, which contains polyunsaturated fatty acid constituents and is therefore in a liquid-expanded state, but has been found to interact also with MGaIDG with long-chain saturated fatty acids, which exist in a gel state. For an optimal interaction, the fatty acid chain length and the extent of unsaturation are also important parameters, whereas the conformation of the sugar moiety, the sugar-glycerol or glycerol-hydrocarbon chain linkages are of little influence on the pressure changes measured in monomolecular layers. Conversely, steric hindrance of a methyl group at position 3 of the sugar largely inhibits the interaction. Quantification of the interaction with radiolabelled prefd shows that only a small part of the molecule is able to penetrate MGaIDG in the gel state, whereas a nearly four-times larger part is able to penetrate MGaIDG isolated from chloroplasts. It is likely that interactions of the transit sequence of prefd with the glycolytic head group MGaIDG are involved in targeting and binding to the chloroplast membrane.

The C terminus of SecA is involved in both lipid binding and SecB binding

Using C-terminal deletion mutations in secA, we localized the previously proposed (Breukink, E., Keller, R.C. A., and de Kruijff, B. (1993), FEBS Lett. 331, 19-24) second lipid binding site on SecA. Since removal of these residues completely abolished the property of SecA to cause aggregation of negatively charged phosphatidyl-glycerol vesicles, we conclude that the C-terminal 70 amino acid residues of SecA are involved in lipid-binding. The C-terminal 70 amino acid residues of SecA are important for efficient in vitro translocation of the SecB-dependent precursor of PhoE across inverted inner membrane vesicles. Moreover, in vivo studies showed that this region is essential for growth. SecB and a SecB-precursor complex were shown to inhibit the SecA-mediated lipid vesicle aggregation, suggesting that the overall acidic SecB protein binds at or near the second lipid binding site on SecA. This together with the observation that the SecA mutant protein lacking the C-terminal 70 residues had a strongly reduced ability to mediate binding of SecB-precursor complexes to inverted inner membrane vesicles demonstrates that the C terminus of SecA is also involved in SecB binding.

Transit sequence-dependent binding of the chloroplast precursor protein ferredoxin to lipid vesicles and its implications for membrane stability

The binding of the transit peptide (trfd) and precursor of the chloroplast protein ferredoxin (prefd) to large unilamellar lipid vesicles was investigated in relation to the lipid composition of the bilayer. Prefd binds with a dissociation constant of 0.27 microM to vesicles with a composition corresponding to the chloroplast envelope outer membrane. Binding is mediated by the transit sequence. From an analysis of binding to vesicles containing the individual lipid components it could be concluded that anionic lipids are mainly responsible for binding, emphasizing the importance of electrostatics for the transit sequence-lipid interaction. Binding is also mediated by the specific chloroplast glycolipid monogalactosyldiacylglycerol. Monolayer experiments revealed that in this case a more extended domain of the transit sequence inserts into the lipid layer. Precursor binding does not result in a loss of vesicle barrier function. However, high concentrations of trfd do cause release of vesicle-enclosed carboxyfluorescein. The results are discussed in the light of the chloroplast protein import process, with special emphasis on the role of monogalactosyldiacylglycerol.

Functional domains of the ferredoxin transit sequence involved in chloroplast import

In order to analyze the information content of a chloroplast transit sequence, we have constructed and analyzed by in vitro assays seven substitution and 20 deletion mutants of the ferredoxin transit sequence. The N-terminal part and the C-terminal part are important for targeting, and in addition the C-terminal region is required for processing. A third region is important for translocation but not for the initial interaction with the envelope. A fourth region is less essential for in vitro import. Purified precursors were tested for their ability to compete for the in vitro import of radiolabeled wild-type precursor, which confirmed the important role in chloroplast recognition of both the N- and the C-terminal domain of the transit sequence. Monolayer experiments showed that the N terminus was mainly involved in the insertion into mono-galactolipid-containing lipid surfaces whereas the C terminus mediates the recognition of negatively charged lipids. A sequence comparison to other transit sequences suggests that the domain structure of the ferredoxin transit sequence can be extended to these sequences and thus reveals a general structural design of transit sequences.

SecA restricts, in a nucleotide-dependent manner, acyl chain mobility up to the center of a phospholipid bilayer

The effects of SecA-lipid interactions on lipid mobility were studied by electron spin resonance (ESR) spectroscopy in bilayer systems containing phospholipids spin-labeled at different positions along the acyl chain. The SecA protein, which functions in protein translocation at the cytosolic side of the E. coli inner membrane, was found to decrease the mobility of the lipids upon its interaction with the membrane. The restriction of lipid motion, at all chain positions measured, reflects the ability of SecA to penetrate the membrane. At a 49:1 lipid/protein molar ratio, a second, motionally more restricted component is observed in ESR spectra of phospholipids spin-labeled close to the methyl ends of the chains (12th and 14th positions). Furthermore, SecA was found to eliminate the order-to-disorder phase transition of 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol bilayers. A remarkably strong reduction in the ability of SecA to penetrate the membrane was found when the nucleotides ATP and ADP+P(i) were present. The presence of the non-hydrolyzable analogue AMP-PNP had no effect. These results clearly demonstrate that SecA perturbs, in a nucleotide dependent manner, lipid mobility upon insertion into the bilayer. The implications of these findings for translocation of precursor proteins across the E. coli inner membrane are discussed.

Precursor-mediated opening of translocation pores in chloroplast envelopes

Direct electrical measurements on native chloroplast envelopes reveal that a full-length chloroplast precursor protein causes an increase in the conductivity of the envelope membranes, due to its transit sequence. The conductivity is not influenced by a truncated precursor protein incapable of efficient translocation, suggesting precursor-mediated opening of translocation pores in chloroplast envelopes.

Transport studies of doxorubicin in model membranes indicate a difference in passive diffusion across and binding at the outer and inner leaflets of the plasma membrane

The kinetics of passive transport of the anticancer drug doxorubicin were analyzed in relation to membrane composition in large unilamellar vesicles in which DNA was enclosed. Special attention was paid to lipids that are typical for the inner and outer leaflet of the plasma membrane of mammalian cells: Phosphatidylethanolamine and anionic phosphatidylserine versus phosphatidylcholine, sphingomyelin, and cholesterol, respectively. The presence of anionic phospholipids results in a highly efficient incorporation of the drug into biological and model membranes [de Wolf, F. A., et al. (1993) Biochemistry 32, 6688-6695]. Therefore, the effect of drug binding on the amount of free, transportable drug was explicitly taken into account. However, even after correction for binding the permeability coefficient was about 35% lower in membranes containing 50 mol % of the anionic phosphatidylserine than in membranes consisting only of zwitterionic phospholipids (0.71-0.79 versus 1.18-1.25 microns s-1). This shows that drug binding and insertion also affect the intrinsic transport characteristics of the membranes. As compared to pure phosphatidylcholine, binding was not influenced by the incorporation of sphingomyelin or cholesterol, but equimolar amounts of sphingomyelin and cholesterol in phosphatidylcholine membranes decreased the rate of doxorubicin transport by 60% and 80%, respectively. The inhibitory effect of these two lipids is probably due to a closer packing of the membranes. In accordance, after the acyl chain order was decreased by adding the anaesthetic-like phenethyl alcohol (0.5% v/v), transport was stimulated more than 4-fold. The implications of our findings for the functioning and rate of drug pumping by the multidrug resistance-conferring P-glycoprotein in cancer cells are discussed.

Regulation of lipid polymorphism is essential for the viability of phosphatidylethanolamine-deficient Escherichia coli cells

Escherichia coli strain AD93 is unable to synthesize the nonbilayer lipid phosphatidylethanolamine and requires high concentrations of specific divalent cations for growth. Previous studies suggested that in this strain, cardiolipin in combination with divalent cations functionally replaces phosphatidylethanolamine, reflecting polymorphic regulation of membrane lipid composition. However, it is also possible that divalent cations are required for regulation of lipid packing or membrane surface potential. 2H NMR was employed to measure the effect of different divalent cations on lipid packing in aqueous dispersions of lipid extracts isolated from AD93 and the wild type parental strain W3899, which were grown with [11,11-2H2]oleic acid. The results indicate that a range of acyl chain order is compatible with growth and that Ba2+, which cannot support growth of AD93, can increase chain packing to the wild type level. By means of microelectrophoresis, it was shown that the growth-promoting cations and Ba2+ have a strong and comparable ability to screen the surface charge of large unilamellar vesicles prepared from AD93 lipid extracts. Therefore, it is unlikely that the growth-promoting capacity of divalent cations is primarily due to their effect on lipid packing or their potency to decrease the surface potential. Furthermore, the addition of small amounts of Ba2+ to a AD93 lipid dispersion with excess Mg2+ diminished HII phase formation. This observation can explain the growth arrest in AD93 cultures upon the addition of Ba2+ and further supports the conclusion that the cation requirement of this strain arises mainly from polymorphic regulation of lipid composition.

Membrane location of spin-labeled apocytochrome c and cytochrome c determined by paramagnetic relaxation agents

The mitochondrial precursor protein horse heart apocytochrome c was spin-labeled on the cysteine residue at position 14 or 17 in the N-terminal region, and the mature protein yeast cytochrome c was similarly labeled on the single free cysteine residue at position 102 at the C-terminal. The proteins were bound to negatively charged phospholipid bilayers, and the accessibility of the spin-labeled cysteine residues to lipid-soluble molecular oxygen and to the lipid-impermeant chromium oxalate anion was determined from the saturation properties of the ESR spectra. Binding of the protein was found to have a considerable effect on the local oxygen concentrations within the lipid bilayer. The accessibilities of the spin-labeled proteins relative to those obtained for phospholipids spin-labeled either in the headgroup or at positions in the sn-2 acyl chain, in the presence of unlabeled protein, identify the position of the spin-labeled cysteine residues in the phospholipid bilayer. The spin label on apocytochrome c bound to phosphatidylglycerol bilayers lies between the 5- and 14-C positions of the lipid acyl chain. Admixture of > or = 75 mol % phosphatidylcholine induces an additional surface-associated apocytochrome c population. The spin label on native and heat-denatured cytochrome c is located at the membrane surface. These different extents of membrane penetration correlate also with the reduction in local oxygen concentration experienced by spin-labeled phospholipids on binding of apo- and holocytochrome c. The possible biological implications of the data are discussed.

The transit sequence of a chloroplast precursor protein reorients the lipids in monogalactosyl diglyceride containing bilayers

The interaction of the chloroplast precursor protein of ferredoxin with mixed model membranes composed of 2H chain labeled monogalactosyl diacylglycerol and phosphatidylcholine was studied by 2H and 31P NMR. The bilayers were found to have special chain packing properties which most likely are the result of a specific arrangement of head groups at the interface. The precursor and not the corresponding apoprotein induced a bilayer-->isotropic transition in lipid organization as a result of the transit sequence-lipid interaction. The implications of these observations for proteins import into chloroplasts are indicated.

The full length of a mitochondrial presequence is required for efficient monolayer insertion and interbilayer contact formation

The peptide specificity of both presequence-monolayer interactions and the ability of presequences to induce interbilayer contacts between large unilamellar vesicles was investigated. A range of different synthetic peptides that are documented for their mitochondrial protein import abilities were used for this purpose. Both monolayer insertion and vesicle aggregation were found to be strongly dependent on the primary structure of the studied presequence peptides. The combination of monolayer data and results of vesicle aggregation experiments leads to the overall suggestion that monolayer insertion and interbilayer contact formation are mechanistically related. For maximal effects the full length of a presequence peptide is required. The cardiolipin specificity of presequence-induced interbilayer contact formation previously reported was found to be a more general property among presequence peptides. The peptide's ability to induce vesicle-vesicle contacts seems to parallel the efficiency of its import ability into mitochondria. These results lead to an extended hypothesis on the role of presequence-induced contact site formation during the mitochondrial protein import process.

Interaction of spin-labeled apocytochrome c and spin-labeled cytochrome c with negatively charged lipids studied by electron spin resonance

Apocytochrome c has been spin-labeled with a nitroxide derivative of maleimide on a cysteine residue at either position 14 or position 17 in the N-terminus. Yeast cytochrome c was spin-labeled with the same maleimide derivative on its single free cysteine residue at position 102 in the C-terminus. The ESR spectra of spin-labeled apocytochrome c have been characterized in different environments with respect both to the conformation of the protein and to its association with lipid. In buffer, the spectrum of spin-labeled apocytochrome c indicates high mobility, characteristic of the unfolded structure of the apoprotein, and that of spin-labeled cytochrome c is only slightly less mobile, suggesting that the site labeled is situated at the surface of the folded holoprotein. Upon binding the spin-labeled protein to negatively charged lipid membranes composed of dioleoylphosphatidylglycerol (DOPG), the ESR spectra of apocytochrome c evidence a large reduction in the mobility of the spin-label group, as also do those of yeast cytochrome c. In the case of apocytochrome c, this immobilization most likely arises from both an increase in secondary structure and a partial penetration of the protein into the lipid bilayer, in addition to the electrostatic interaction with the lipid headgroups, whereas for cytochrome c the immobilization observed arises primarily from an intimate association with the membrane surface. When the spin-labeled holocytochrome c is denatured by heating and is bound to DOPG bilayer membranes, a rather mobile ESR spectrum is observed, which demonstrates that the spin-label is located at the surface of the membrane in this case. The ESR spectra of spin-labeled apocytochrome c bound to mixed bilayers of dimyristoylphosphatidylglycerol and dimyristoylphosphatidylcholine (DMPC) consist of both an immobile and a mobile component. The proportion of the mobile component is increased by increasing the mole fraction of the zwitterionic DMPC in the mixed bilayers. The mobile component represents a localization of apocytochrome c at the membrane surface, whereas the immobile component most probably represents the penetration of the precursor protein into the membrane interior. The immobile component assigned to membrane penetration of the precursor protein is still present at negatively charged lipid contents comparable to those in the native mitochondrial system. The results are discussed in relation to the conformation of apocytochrome c, its interaction with lipid, and the import of the apoprotein into mitochondria.

Requirement for conformational flexibility in the signal sequence of precursor protein

According to the "unlooping" model (de Vrije, T., Batenburg, A. M., Killian, J. A., and de Kruijff, B. (1990) Mol. Microbiol. 4, 143-150), proposed to explain how signal sequences serve to target proteins into the secretory pathway, the initial interaction of the signal sequence with the membrane in a helix-turn-helix conformation (spanning half of the bilayer) plays an important role in the initiation of the translocation reaction. To test this model we have introduced 2 cysteines (at positions -5 and -19) in the signal sequence of the Escherichia coli outer membrane protein PhoE. The mutations did not influence the translocation of precursor PhoE in vivo or in vitro. The 2 cysteines were oxidized to form a disulfide bridge. In vitro translocation of the looped precursor into inner membrane vesicles was disturbed. The looped precursor competed with translocation of wild type precursor PhoE, and looped precursor that was first bound to inner membrane vesicles could be translocated after the addition of dithiothreitol. Apparently, the mutant precursor with a disulfide bridge in the signal sequence is arrested as a very early intermediate in the translocation process. All of these results are consistent with the proposed unlooping model and show that, besides the primary structure characteristics of a signal sequence, conformational flexibility is needed to initiate the translocation reaction.

Anionic phospholipids and protein translocation

Anionic phospholipids determine, in diverse ways, the membrane interaction of proteins involved in or undergoing membrane insertion or translocation. How these lipids modulate protein localization, organization, folding and membrane insertion is herein summarized and generalized, leading to a proposal for the function of anionic lipids in cellular transport of newly synthesized proteins.

Presequence-mediated intermembrane contact formation and lipid flow. A model membrane study

The ability of a synthetic peptide corresponding to the presequence of cytochrome c oxidase subunit IV from yeast to cause intermembrane contacts was investigated using monolayer techniques. The presequence inserted efficiently into the monolayer with a specificity for the mitochondrial cardiolipin. In the inserted form, the peptide strongly promoted the formation of close contacts with large unilamellar lipid vesicles present in the subphase, a property which was also specific for cardiolipin. The contacts formed were stable and tight and resulted in the flow of lipids from the vesicles to the monolayer. These results led to new suggestions on the involvement of intermembrane contact formation in mitochondrial protein import and membrane biogenesis.

The cryoprotectant trehalose destabilises the bilayer organisation of Escherichia coli-derived membrane systems at elevated temperatures as determined by 2H and 31P-NMR

In this study, 2H and 31P-NMR techniques were used to study the effects of trehalose and glycerol on phase transitions and lipid acyl chain order of membrane systems derived from cells of E. coli unsaturated fatty acid auxotroph strain K1059, which was grown in the presence of [11,11-2H2]-oleic acid or [11,11-2H2]-elaidic acid. From an analysis of the temperature dependence of the quadrupolar splitting it could be concluded that neither 1 M trehalose or glycerol generally had any significant effect on the temperature of the lamellar gel to liquid-crystalline phase transition. In the case of the oleate-containing hydrated total lipid extract, glycerol but not trehalose caused a 5 degrees C increase of this transition temperature. In general, both cryoprotectants induced an ordering of the acyl chains in the liquid-crystalline state. Trehalose and glycerol both decrease the bilayer to non-bilayer transition temperature of the hydrated lipid extract of oleate-grown cells by about 5 degrees C, but only trehalose in addition induces an isotropic to hexagonal (HII) phase transition. In the biological membranes, trehalose and not glycerol destabilised the lipid bilayer, and in the case of the E. coli spheroplasts, part of the induced non-bilayer structures is ascribed to a hexagonal (HII) phase in analogy with the total lipids. Interestingly, 1 mM Mg2+ was a prerequisite for the destabilisation of the lipid bilayer. In the hydrated total lipid extract of E. coli grown on the more ordered elaidic acid, both transition temperatures were shifted about 20 degrees C upwards compared with the oleate-containing lipid, but the effect of trehalose on the lipid phase behaviour was similar. The bilayer destabilising ability of trehalose might have implications for the possible protection of biological systems by (cryo-)protectants during dehydration, in that protection is unlikely to be caused by preventing the occurrence of polymorphic phase transitions.

Effect of divalent cations on lipid organization of cardiolipin isolated from Escherichia coli strain AH930

Escherichia coli strain AH930 is a lipid biosynthetic mutant, which is unable to synthesize phosphatidylethanolamine. Instead it produces large amounts of phosphatidylglycerol and cardiolipin and has an absolute requirement for certain divalent cations. Cardiolipin was isolated from this mutant strain and its interaction with divalent cations was studied by various biophysical techniques. Monolayer measurements showed that the cations decrease the molecular surface area of cardiolipin in the order Ca2+ approximately Mg2+ > Sr2+ > Ba2+. 31P-NMR and X-ray diffraction measurements demonstrated a comparable sequence for the ability of the cations to promote HII phase formation in dispersions of the E. coli cardiolipin: Ca2+ and Mg2+ induced HII phase formation at 50 degrees C, Sr2+ at 75 degrees C, while Ba2+ was found to be unable to promote HII phase formation in the temperature range measured. Furthermore, all divalent cations were found to increase the temperature at which the transition to the liquid-crystalline phase takes place, which was below 5 degrees C for the lipid in the absence of divalent cations. In the presence of Sr2+, Mg2+ and Ba2+ and at 25 degrees C two lamellar phases were observed, one corresponding to a liquid-crystalline phase, the other to either a gel or a crystalline phase. In the presence of Ca2+ at 25 degrees C and even at 45 degrees C no evidence for a liquid-crystalline phase was obtained and only a crystalline phase could be observed. The ability of the different cations to promote HII phase formation in the isolated E. coli cardiolipin was found to correlate with their ability to support growth of the mutant strain (De Chavigny, A., Heacock, P.N., Dowhan, W. (1991) J. Biol. Chem. 266, 5323-5332), suggesting that cardiolipin with divalent cations can replace the role of phosphatidylethanolamine in the mutant strain, and that this role involves the preference of these lipids for organization in non-bilayer lipid structures.

A dual role for phosphatidylglycerol in protein translocation across the Escherichia coli inner membrane

The involvement of phosphatidylglycerol in the SecA-independent translocation of M13 procoat in Escherichia coli was demonstrated. Processing of procoat to mature coat protein was retarded when the level of phosphatidylglycerol was reduced. In vitro translocation experiments using inner membrane vesicles isolated from a strain with inducible synthesis of phosphatidylglycerol, showed that translocation of procoat and of a SecA-dependent procoat analog was proportional to the content of phosphatidylglycerol. Moreover, introduction of phosphatidylglycerol by means of a lipid transfer method into phosphatidylglycerol-depleted inner membrane vesicles, efficiently restored procoat translocation. The phosphatidylglycerol dependence in both the SecA-dependent and -independent translocation pathway indicates that phosphatidylglycerol plays a dual role in translocation. We suggest that besides membrane binding of SecA this lipid has a direct interaction with the M13 procoat in translocation across the inner membrane.