Biosynthesis of the covalently linked diglyceride in murein lipoprotein of Escherichia coli

Mass spectrometry analysis of the photosystem II assembly factor Psb27 revealed variations in its lipid modification

The assembly of large, multi-cofactor membrane protein complexes like photosystem II (PSII) requires a high level of coordination. The process is facilitated by a large network of auxiliary proteins that bind transiently to unassembled subunits, preassembled modules or intermediate states of PSII, which are comprised of a subset of subunits. However, analysis of these immature, partially assembled PSII complexes is hampered by their low abundance and intrinsic instability. In this study, PSII was purified from the thermophilic cyanobacterium Thermosynechococcus elongatus via Twin-Strep-tagged CP43 and further separated by ion exchange chromatography into mature and immature complexes. Mass spectrometry analysis of the immature Psb27-PSII intermediate revealed six different Psb27 proteoforms with distinct lipid modifications. The maturation and functional role of thylakoid localized lipoproteins are discussed.

Characterizing Membrane Association and Periplasmic Transfer of Bacterial Lipoproteins through Molecular Dynamics Simulations

Escherichia coli lipoprotein precursors at the inner membrane undergo three maturation stages before transport by the Lol system to the outer membrane. Here, we develop a pipeline to simulate the membrane association of bacterial lipoproteins in their four maturation states. This has enabled us to model and simulate 81 of the predicted 114 E. coli lipoproteins and reveal their interactions with the host lipid membrane. As part of this set we characterize the membrane contacts of LolB, the lipoprotein involved in periplasmic translocation. We also consider the means and bioenergetics for lipoprotein localization. Our calculations uncover a preference for LolB over LolA and therefore indicate how a lipoprotein may be favorably transferred from the inner to outer membrane. Finally, we reveal that LolC has a role in membrane destabilization, thereby promoting lipoprotein transfer to LolA.

In the absence of Lgt, lipoproteins are shed from Streptococcus uberis independently of Lsp

The role of lipoprotein diacylglyceryl transferase (Lgt) and lipoprotein signal peptidase (Lsp) responsible for processing lipoproteins was investigated in Streptococcus uberis, a common cause of bovine mastitis. In the absence of Lgt, three lipoproteins [MtuA (SUB0473), Hap (SUB1625) and an extracellular solute-binding protein (SUB0365)] were detected in extracellular locations. All were shown by Edman degradation analysis to be cleaved on the carboxy side of the LXXC lipobox. Detection of MtuA, a lipoprotein shown previously to be essential for infectivity and virulence, was used as a surrogate lipoprotein marker to locate and assess processing of lipoproteins. The absence of Lgt did not prevent location of MtuA to the cell membrane, its location in the wild-type strain but, in contrast to the situation with wild-type, did result in a widespread location of this protein. In the absence of both Lgt and Lsp, MtuA was similarly released from the bacterial cell. In such strains, however, the cell-associated MtuA represented the full-length gene product, indicating that Lsp was able to cleave non-lipidated (lipo)proteins but was not responsible for their release from this bacterium.

Deletion of internal twenty-one amino acid residues of Escherichia coli prolipoprotein does not affect the formation of the murein-bound lipoprotein

Mutation pgsA affecting the phosphatidylglycerol phosphate synthesis is lethal for all but certain E. coli strains such as strains deleted for the lpp gene or strains containing unmodifiable prolipoprotein like lppD14. Strain SD312 pgsA3 is tolerant to pgsA mutation, which suggests the lpp alleles in strain SD312 pgsA3 and its parental strain SD12 may be defective. DNA sequence analysis of the lpp genes in Escherichia coli strains SD12 and SD312 pgsA using asymmetric polymerase chain reaction showed that the lpp alleles in these two strains contained a 63 base pair deletion corresponding to the 37th to 57th codons of the wild-type lpp gene. [3H]Palmitate labeling of strains SD12 and SDS312 showed that the mutant lipoprotein in SD12 strain was modified with lipid, while the prolipoprotein in SD312 was not modified. The shortened mature lipoprotein in SD12 and the lipid-modified prolipoprotein in globomycin-treated SD12 were found to be covalently attached to the peptidoglycan, while the unmodified prolipoprotein in SD312 did not form significant amounts of murein-bound lipoprotein.

The pgpA and pgpB genes of Escherichia coli are not essential: evidence for a third phosphatidylglycerophosphate phosphatase

To further define the genes and gene products responsible for the in vivo conversion of phosphatidylglycerophosphate to phosphatidylglycerol in Escherichia coli, we disrupted two genes (pgpA and pgpB) which had previously been shown to encode gene products which carried out this reaction in vitro (T. Icho and C. R. H. Raetz, J. Bacteriol. 153:722-730, 1983). Strains with either gene or both genes disrupted had the same properties as the original mutants isolated with mutations in these genes, i.e., reduced in vitro phospholipid phosphatase activities, normal growth properties, and an increase in the level of phosphatidylglycerophosphate (1.6% versus less than 0.1% in wild-type strains). These results demonstrate that these genes are not required for either normal cell growth or the biosynthesis of phosphatidylglycerol in vivo. In addition, the total phosphatidylglycerophosphate phosphatase activity in the doubly disrupted mutant was reduced by only 50%, which indicates that there is at least one other gene that encodes such an activity and thus accounts for the lack of a dramatic effect on the biosynthesis of anionic phospholipids in these mutant strains. The phosphatidic acid and lysophosphatidic acid phosphatase activities of the pgpB gene product were also significantly reduced in gene-interrupted mutants, but the detection of residual phosphatase activities in these mutants indicated that additional genes encoding such phosphatases exist. The lack of a significant phenotype resulting from disruption of the pgpA and pgpB genes indicates that these genes may be required only for nonessential cell function and leaves the biosynthesis of phosphatidylglycerophosphate as the only step in E. coli phospholipid biosynthesis for which a gene locus has not been identified.

Lipoproteins in bacteria

Covalent modification of membrane proteins with lipids appears to be ubiquitous in all living cells. The major outer membrane (Braun's) lipoprotein of E. coli, the prototype of bacterial lipoproteins, is first synthesized as a precursor protein. Analysis of signal sequences of 26 distinct lipoprotein precursors has revealed a consensus sequence of lipoprotein modification/processing site of Leu-(Ala, Ser)-(Gly, Ala)-Cys at -3 to +1 positions which would represent the cleavage region of about three-fourth of all lipoprotein signal sequences in bacteria. Unmodified prolipoprotein with the putative consensus sequence undergoes sequential modification and processing reactions catalyzed by glyceryl transferase, O-acyl transferase(s), prolipoprotein signal peptidase (signal peptidase II), and N-acyl transferase to form mature lipoprotein. Like all exported proteins, the export of lipoprotein requires functional SecA, SecY, and SecD proteins. Thus all precursor proteins are exported through a common pathway accessible to both signal peptidase I and signal peptidase II. The rapidly increasing list of lipid-modified proteins in both prokaryotic as well as eukaryotic cells indicates that lipoproteins comprise a diverse group of structurally and functionally distinct proteins. They share a common structural feature which is derived from a common biosynthetic pathway.

Suppression of the lethal effect of acidic-phospholipid deficiency by defective formation of the major outer membrane lipoprotein in Escherichia coli

The Escherichia coli pgsA3 allele encoding a defective phosphatidylglycerophosphate synthase is lethal for all but certain strains. Genetic analysis of such strains has revealed that the lethal effect is fully suppressed by the lack of the major outer membrane lipoprotein that consumes phosphatidylglycerol for its maturation.

Molecular cloning of the cls gene responsible for cardiolipin synthesis in Escherichia coli and phenotypic consequences of its amplification

The cls gene responsible for cardiolipin synthesis in Escherichia coli K-12 was cloned in a 5-kilobase-pair DNA fragment inserted in a mini-F vector, pML31, and then subcloned into a 2.0-kilobase-pair fragment inserted in pBR322. The initial selection of the gene was accomplished in a cls pss-1 double mutant that had lesions in both cardiolipin and phosphatidylserine synthases and required either the cls or the pss gene product for normal growth at 42 degrees C in a broth medium, NBY, supplemented with 200 mM sucrose. The cloned gene was identified as the cls gene by the recovery and amplification of both cardiolipin and cardiolipin synthase in a cls mutant as well as by the integration of a pBR322 derivative into its genetic locus at 27 min on the chromosome of a polA1 mutant. The maxicell analysis indicated that a protein of molecular weight 46,000 is the gene product. The cls gene is thus most likely the structural gene coding for cardiolipin synthase. Hybrid plasmids of high copy numbers containing the cls gene were growth inhibitory to pss-I mutants under the above selective conditions, whereas they inhibited neither the growth of pss-I mutants at 30 degrees C nor that of pss+ strains at any temperature. Amplification of cardiolipin synthase activity was observed, but was not proportional to the probable gene dosage (the enzyme activity was at most 10 times that in wild-type cells), and cardiolipin synthesis in vivo was at the maximum 1.5 times that in wild-type strains, implying the presence in E. coli cells of a mechanism that avoids cardiolipin overproduction, which is possibly disadvantageous to proper membrane functions.

Duplication of Escherichia coli during inhibition of net phospholipid synthesis

In Escherichia coli BB26-36, the inhibition of net phospholipid synthesis during glycerol starvation affected cell duplication in a manner that was similar in some respects to that observed during the inhibition of protein synthesis. Ongoing rounds of chromosome replication continued, and cells in the D period divided. The initiation of new rounds of chromosome replication and division of cells in the C period were inhibited. Unlike the inhibition of protein synthesis, however, the accumulation of initiation potential in dnaA and dnaC mutants at the nonpermissive temperature was not affected by the inhibition of phospholipid synthesis. Furthermore, proteins synthesized during the inhibition of phospholipid synthesis can be utilized later for division. The results are consistent with a dual requirement for protein and phospholipid synthesis for both the inauguration of new rounds of chromosome replication and the initiation of septum formation. Once initiated, both processes progress to completion independent of continuous phospholipid and protein synthesis.

Correlation of 3,4-dihydroxybutyl 1-phosphonate resistance with a defect in cardiolipin synthesis in Escherichia coli

Escherichia coli treated for 1 h with 100 microM rac-3,4-dihydroxybutyl 1-phosphonate (DBP), a glycerol-3-phosphate analog, die when sorted at 5 degrees C, whereas the viability of untreated cells is relatively unaffected. This observation formed the basis of a selection procedure that was used to isolate mutants that are partially resistant to DBP. One such mutant, strain 6204, is constitutive for DBP transport, exhibits a particularly high degree of cold resistance, has the same doubling time as the parent, and is similar to the parent strain in terms of incorporation of DBP into the lipid fraction. Glycerol-3-phosphate and phosphatidylglycerol phosphate synthetases obtained from strain 6204 and its parent were identical in terms of DBP recognition. The parent strain is killed when incubated in the presence of a combination of 70 microM rac-DBP and 0.25% deoxycholate, whereas strain 6204 continues to grow, albeit more slowly, in the presence of this combination. Strain 6204 can be distinguished from the parent strain on agar plates (low phosphate minimal medium with glucuronate as the sole carbon source) containing 15 microM rac-DBP. The insertion of Tn10 near the 6204 mutation has facilitated genetic manipulations. All phenotypic effects attributed to strain 6204 appear to be due to a single mutation. Genetic analysis indicates that Tn10, inserted near the gene responsible for DBP resistance, maps in the vicinity of 27 min. Three-factor crosses reveal a gene order of hemA-Dbpr-Tn10(zch)-trp. The only gene for phosphoglyceride metabolism known to map in this region is the gene associated with cardiolipin synthetase, cls. Genetic results suggest that the mutation responsible for DBP resistance maps in or very near cls. Analysis of the lipids isolated from untreated strain 6204 (and from each of the transductants prepared by P1 vir-mediated transfer of DBP resistance of wild-type strains) reveals that cardiolipin synthesis is defective. These results strongly suggest that the mutation responsible for DBP resistance has its primary effect on cardiolipin synthesis. To further test this hypothesis, strains with an authentic cls mutation were constructed and examined for resistance to DBP. These strains had growth properties that were identical with those of strain 6204. Wild-type strains and mutants defective in cardiolipin synthesis were treated with DBP and 20 mM magnesium or calcium chloride. Simultaneous treatment of either cell type with DBP and divalent cation not only failed to stimulate growth but, quite the contrary, had a marked synergistic growth inhibitory effect.

Post-translational modification and processing of outer membrane prolipoproteins in Escherichia coli

This mini review is primarily concerned with the biosynthesis of the major outer membrane lipoprotein of Escherichia coli. The lipoprotein is composed of fifty-eight amino acid residues, one glyceride residue and one acyl residue being at the amino terminal cysteine residue. About one-third of the lipoprotein is covalently bound to the underlying peptidoglycan layer and plays an important role in the assembly of the outer membrane on the peptidoglycan layer. The lipoprotein is first synthesized on ribosomes as a precursor form having twenty extra amino acid residues (signal peptide) at the amino terminus. During secretion through the cytoplasmic membrane, the modification at the cysteine residue that is to be the amino terminus of the mature lipoprotein and cleavage of the signal peptide take place successively. These events are then followed by N-acetylation of the terminal cysteine residue, translocation to the outer membrane, and covalent binding to the peptidoglycan layer of the lipoprotein. Digestion of the cleaved signal peptide also takes place upon cleavage of the signal peptide. In this review these chemical and topographical events are discussed step by step especially in relation to the process of protein secretion across the cytoplasmic membrane.

Transfer of the phosphatidyl moiety of phosphatidylglycerol to phosphatidylethanolamine in Escherichia coli

Phosphatidylglycerol was pulse-labeled with radioactive lipid precursors in a serine auxotroph of Escherichia coli. Most of the radioactivity of phosphatidylglycerol labeled in a serine-depleted medium was transferred to phosphatidylethanolamine during a chase in the presence of L-serine, but not in its absence. Metabolism of fatty acyl moieties labeled with [1-14C]acetate, acylated glycerol moieties labeled with [2-3H]glycerol, and phosphate moieties labeled with 32Pi, followed by a chase in the presence of cerulenin, showed that the intact phosphatidyl moiety of phosphatidylglycerol was transferred to phosphatidylethanolamine. The composition of phosphatidylethanolamine molecular species was unaltered and not perturbed by the transfer of the phosphatidyl moiety of phosphatidylglycerol. The increase of phosphatidylethanolamine with a concomitant decrease of phosphatidylglycerol was not coupled with the postulated turnover of phosphatidylglycerol to membrane-derived oligosaccharides and lipoprotein. It is suggested that phosphatidylglycerol is capable of providing its phosphatidyl moiety for the production of phosphatidylethanolamine in response to the relief of serine limitation by addition of L-serine.

Post-translational modification and processing of Escherichia coli prolipoprotein in vitro

Escherichia coli strain MM18 cells containing malE-lacZ hybrid protein was reported to accumulate prolipoprotein when they were induced with maltose [Ito, K., Bassford, P. J. & Beckwith, J. (1981) Cell 24, 707-717]. We have shown that the prolipoprotein accumulated in maltose-induced MM18 cells is not modified, lacking covalently linked glyceride. When the cell envelope of MM18 containing unmodified prolipoprotein was incubated in the presence of detergent with [2-3H]glycerol-labeled cell envelope of strain JE5505 lacking murein lipoprotein, incorporation of [2-3H]glycerol radioactivity into both prolipoprotein and processed mature lipoprotein was observed. Likewise, when [3H]-palmitate-labeled JE5505 cell envelope was incubated with the MM18 cell envelope containing unmodified prolipoprotein in the presence of detergent, [3H]palmitate radioactivity was incorporated into prolipoprotein by ester linkage and into mature lipoprotein by both ester and amide linkages. These results indicate that our in vitro system contains activities of prolipoprotein modification and processing enzymes, including glyceryltransferase, O-acyltransferase, signal peptidase, and N-acyltransferase. The signal peptidase activity in our in vitro system was completely inhibited by globomycin. At pH 5.0, glyceryltransferase was inactive. Signal peptidase was active at pH 5.0, provided that prolipoprotein had been modified by glyceryltransferase (O-acyl-transferase) during a prior incubation at pH 9.1. These results strongly suggest that the modification of prolipoprotein by glyceryltransferase (and O-acyltransferase) precedes, and may in fact be a prerequisite for, the processing of prolipoprotein by signal peptidase.

Biogenesis of membrane lipoproteins in Escherichia coli

Globomycin-resistant mutants of Escherichia coli have been isolated and partially characterized. Approximately 2-5% of these mutants synthesize structurally altered Braun's lipoprotein. The majority of these mutants contain unprocessed and unmodified prolipoprotein. One mutant is found to contain modified, processed, but structurally altered lipoprotein. Mutants containing lipid-deficient prolipoprotein or lipoprotein also show increased resistance to globomycin. These results suggest that the inhibition of processing of modified prolipoprotein by globomycin may require fully modified prolipoprotein as the biochemical target of this novel antibiotic. Our failure to isolate mutant containing cleaved but unmodified lipoprotein among globomycin-resistant mutants is consistent with the possibility that modification of prolipoprotein precedes the removal of signal sequence by a unique signal peptidase. Recent evidence indicates that the minor lipoproteins in the cell envelope of E. coli are also synthesized as lipid-containing prolipoproteins and the processing of these prolipoproteins is inhibited by globomycin. These results suggest the existence of modifying enzymes in E. coli which would transfer glyceryl and fatty acyl moieties to cysteine residues located in the proper sequences of the precursor proteins. This speculation is confirmed by our demonstration that Bacillus licheniformis penicillinase synthesized in E. coli as well as in B. licheniformis is a lipoprotein containing glyceride-cysteine at its NH2-terminus.

Lipid-synthesis-dependent biosynthesis (or assembly) of major outer-membrane proteins of Escherichia coli

Upon inhibition of fatty acid synthesis in the presence of cerulenin, the uptake of 5' AMP and of other nutrients using similar pore systems can be inhibited as much as 70%. The same effect was observed upon inhibition of phospholipid synthesis after glycerol deprivation in a mutant strain defective in sn-glycerol-3-phosphate acyltransferase. Resumption of both fatty acid synthesis and phospholipid synthesis restores a normal uptake of 5' AMP. The protein composition of the outer membranes, analyzed after pulse radiolabelling by [35S]methionine, was mainly altered in OmpF and OmpC proteins. These proteins are the main porins used by most nutrients like 5' AMP. Whereas the synthesis or the assembly of OmpF protein seems to be more inhibited that that of OmpC protein after inhibition of fatty acid synthesis, the reverse case was observed after inhibition of phospholipid synthesis. No protein produced during inhibition of fatty-acid or phospholipid synthesis is reincorporated into the outer membrane after resumption of these syntheses. These results are discussed with regard to the biosynthesis and assembly of these proteins.

Isolation and characterization of outer and inner membranes of Selenomonas ruminantium: lipid compositions

The isolation procedure and characterization of the outer and inner membranes from Selenomonas ruminatium cells, a strictly anaerobic bacterium, are described. The metabolic fate of [14C]decanoate incorporated into the outer and inner membranes was examined. The percent distribution of radioactivities in the outer and inner membranes was about 40 and 50% of the total incorporated activity, respectively. Approximately 47% of the radioactivity incorporated into the outer membrane was recovered in the phospholipid fraction, and the remaining radioactivity was found in both aqueous and phenol layers when the outer membrane was treated with phenol-water. In contrast to [14C]decanoate, the percent distribution of [3H]glycerol in the outer and inner membranes was about 25 and 70% of the total incorporated activity, respectively. Most of the assimilated 3H was located in the phospholipid fraction of both membranes. However, no significant label was detected in either the protein or cell wall fraction. The following observations were made concerning lipid compositions in the outer and inner membranes by chemical and isotopic analyses. (i) The outer and inner membranes contained no detectable phosphatidyl glycerol or cardiolipin. (ii) A prominent radioactive compound, designated band III lipid, was found mainly in the outer membrane as a major radioactive spot when cells were grown with [14C]decanoate. This lipid contained phosphorus, 2-keto-3-deoxyoctulosonic acid and 3-OH fatty acid but no detectable glycerol. This lipid was identified tentatively to be 2-keto-3-deoxyoctulosonic acid-lipid A. (iii) Although the ubiquity of phosphatidyl ethanolamine plasmalogen in both outer and inner membranes was confirmed, the occurrence of the molecular species of phosphatidyl ethanolamine plasmalogen was quite different in the outer and inner membranes.

Outer membrane proteins and cell surface structure of Selenomonas ruminantium

The protein compositions of the membrane preparations from Selenomonas ruminantium grown in glucose or lactate medium were determined by sodium dodecyl sulfate- and two-dimensional (first, isoelectric focusing; second, sodium dodecyl sulfate) polyacrylamide slab gel electrophoresis. The outer membrane from both glucose- and lactate-grown cells contained two major proteins with apparent molecular weights of 42,000 and 40,000. These proteins existed as peptidoglycan-associated proteins in the outer membrane. The critical temperature at which they were dissociated completely into the monomeric subunits of 42,000 and 40,000 daltons was found to be 85 degrees C. The amount of each protein varied considerably depending upon the cultural conditions. The absence of the lipoprotein of Braun in S. ruminantium was suggested in our preceding paper (Y. Kamio, and H. Takahashi, J. Bacteriol. 141:888--898, 1980), and the possible absence of the protein components corresponding to the Braun lipoprotein in this strain was confirmed by electrophoretic analysis of the outer membrane and the lysozyme-treated peptidoglycan fractions. Examination of the cell surface of S. ruminantium by electron microscopy showed that the outer membrane formed a wrinkled surface with irregular blebs, some of which pinched off forming vesicles of various sizes. Rapid cell lysis occurred with the addition of a low level of lysozyme to the cell suspension. These findings led us to conclude that the physiological and morphological properties of this strain were similar to those of "deep rough" and mlp or lpo mutants of Escherichia coli K-12, respectively.

Biosynthesis of murein lipoprotein in Escherichia coli: effects of 3,4-dihydroxybutyl-1-phosphonate

The effects of 3,4-dihydroxybutyl-1-phosphonate, a four-carbon analog of sn-glycerol 3-phosphate, on the biosynthesis of the glyceryl moiety in murein lipoprotein of Escherichia coli were studied. The compound at a concentration of 55 microM strong inhibits in the incorporation of [2-3H]glycerol radioactivity into lipoprotein by virtue of its inhibition of the synthesis of phosphatidylglycerol. On the other hand, the incorporation of prelabeled [2-3H]glycerol radioactivity into lipoprotein was only partially inhbited by 3,4-dihydroxybutyl-1-phosphonate even at a much higher concentration (1 mM). These data were consistent with the postulated pathway for the biosynthesis of the lipid moiety in lipoportein: cysteine-lipoprotein + phosphatidylglycerol leads to glycerylcystein-lipoprotein + phosphatidic acid.

Incorporation of phosphatidylglycerol into murein lipoprotein in intact cells of Salmonella typhimurium by phospholipid vesicle fusion

The biosynthesis of the diglyceride moiety of murein lipoprotein was studied by fusion of labeled phospholipid vesicles with intact cells of Salmonella typhimurium. Phosphatidylglycerol was found to be an excellent donor for the glyceryl moiety in lipoprotein, whereas phosphatidylethanolamine and cardiolipin were not. The incorporation of radioactivity from monoacyl-phosphatidylglycerol into lipoprotein can be attributed to its conversion to phosphatidylglycerol. The results strongly support our hypothesis that the glyceryl residue covalently linked to murein lipoprotein is derived from the nonacylated glycerol moiety of phosphatidylglycerol.

An Escherichia coli mutant with an amino acid alteration within the signal sequence of outer membrane prolipoprotein

Lipoprotein has been purified from an Escherichia coli strain carrying a mutation in the structural gene for murein lipoprotein (mlpA). Amino acid analysis of the purified mutant lipoprotein indicates that the mutant lipoprotein corresponds to the uncleaved prolipoprotein with a single amino acid replacement of glycine with aspartic acid. Automated Edman degradation has established the precise location of this amino acid substitution to be at the 14th residue of the prolipoprotein. This alteration in the signal sequence of prolipoprotein results in a failure of the mutated prolipoprotein to be processed. Furthermore, the structural alteration in the mutant lipoprotein appears also to have affected its topological localization in the mutant cell. Whereas lipoprotein in the wild-type strain is exclusively located in the outer membrane of the cell envelope, the membrane-bound lipoprotein in this mutant is recovered in both the inner and outer membranes of the cell envelope. The data suggest, however, that proteolytic cleavage of prolipoprotein to form mature lipoprotein is not essential for the translocation and assembly of lipoprotein into the outer membrane.