Assembly of outer membrane lipoprotein in an Escherichia coli mutant with a single amino acid replacement within the signal sequence of prolipoprotein

We have compared the rate of assembly of outer membrane proteins including the lipoprotein in a pair of isogenic mlpA+ (lpp+) and mlpA (lpp) strains by pulse-chase experiments. The rate of assembly of the mutant prolipoprotein into the outer membrane was slightly slower than that of the wild-type lipoprotein. The rate of assembly of protein I and protein H-2 was similar in the wild type and the mutant, whereas the rate of assembly of protein II into the outer membrane was slightly reduced in the mutant strain. The organization of outer membrane was slightly reduced in the mutant strain. The organization of outer membrane proteins in the mutant cells appeared not to be grossly altered, based on the apparent resistance (or susceptibility) of these proteins toward trypsin treatment and their resistance to solubilization by Sarkosyl. Like the wild-type lipoprotein, the mutant prolipoprotein in the outer membrane was resistant to trypsin. On the other hand, the prolipoprotein in the cytoplasmic membrane fraction of the mutant cell envelope was susceptible to trypsin digestion. We conclude from these data that proteolytic cleavage of prolipoprotein is not essential for the translocation and proper assembly of lipoprotein into outer membrane.

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.

Lipoprotein synthesis in Escherichia coli spheroplasts: accumulation of lipoprotein in cytoplasmic membrane

Synthesis of cell envelope proteins was studied in ethylenediaminetetraacetic acid-lysozyme spheroplasts of Escherichia coli ML30. The rate of incorporation of [3H]arginine into proteins in spheroplasts was about 30% of that of intact cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins synthesized in spheroplasts revealed the preferential synthesis of five polypeptides, one of which has been identified as the free form of murein lipoprotein. Lipoprotein synthesized in spheroplasts was found to be of same molecular size as that of mature lipoprotein. No prolipoprotein was observed even with a short pulse-labeling with [3H]arginine. On the other hand, significant accumulation of newly synthesized lipoprotein in the cytoplasmic membrane fraction of spheroplasts was observed. These results suggest that the processing of prolipoprotein occurs in the cytoplasmic membrane fraction of the cell envelope.

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.

Physiological characterization of an Escherichia coli mutant altered in the structure of murein lipoprotein

Studies using isogenic transductant strains mlpA+ and mlpA as well as reversion analysis suggested that the physiological consequences of a structural gene mutation in murein lipoprotein include (i) increased sensitivity toward chelating agents ethylenediaminetetraacetic acid and ethyleneglycol-bis (beta-aminoethyl ether)-N,N-tetraacetic acid, (ii) leakage of periplasmic enzyme ribonuclease, (iii) weakened association between the outer membrane and the rigid layer accentuated by Mg2+ starvation, resulting in the formation of outer membrane blebs, and (iv) decreased growth rate in media of low ionic strength or low osmolarity. It is suggested that the bound form of lipoprotein plays an important role in the maintenance of the structural integrity of the outer membrane of the Escherichia coli cell envelope. Other outer membrane components may also contribute to the anchorage of outer membrane to the rigid layer, probably through ionic interactions with divalent cations. Using the phenotype of ribonuclease leakage as an unselected marker in a three-factor cross with P1 transduction, we were able to establish the gene order of man mlpA aroD pps on the E. coli chromosome.

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

Biosynthesis of the diglyceride moiety of murein lipoprotein in Escherichia coli was studied by pulse-labeling with [2-3H]glycerol and subsequent chase. The evidence strongly suggests that the precursor of the glycerol moiety in lipoprotein is one of the major phospholipid species in E. coli. Studies of biosynthesis of lipoprotein in cerulenin-treated cells indicated that the nonacylated glycerol moiety of phosphatidylglycerol is the donor for the formation of a thioether linkage in the glycerylcysteine residue of the lipoprotein. This is supported by the observation that carbon 1 rather than carbon 3 of sn-glycerol is involved in this thioether linkage. We propose that the biosynthesis of lipoprotein proceeds as follows: apolipoprotein + phosphatidylglycerol or acyl phosphatidylglycerol leads to lipoprotein + phosphatidic acid.

Amino acid replacement in a mutant lipoprotein of the Escherichia coli outer membrane

The primary structure of a mutant lipoprotein of the outer membrane of Escherichia coli was investigated. This mutant was previously described as a mutant that forms a dimer of the lipoprotein by an S-S bridge (H. Suzuki et al., J. Bacteriol. 127:1494-1501, 1976). The amino acid analysis of the mutant lipoprotein revealed that the mutant lipoprotein had an extra cysteine residue, with concomitant loss of an arginine residue. From the analysis of the mutant lipoprotein revealed that the mutant lipoprotein had an extra cysteine residue, with concomitant loss of an arginine residue. From the analysis of tryptic peptides, it was found that the arginine residue at position 57 was replaced with a cysteine residue. The amino terminal structure of the mutant lipoprotein was found to be glycerylcysteine, as in the case of the wild-type lipoprotein. The present results show that the mutation that was previously determined to map at 36.5 min on the E. coli chromosome occurred in the structure gene (lpp) for the lipoprotein. This was further confirmed by the fact that a merodiploid carrying both lpp+ and lpp produces not only the wild-type lipoprotein but also the mutant lipoprotein.

Genetic characterization of an Escherichia coli mutant altered in the structure of murein lipoprotein

Mutants defective in the structure, biosynthesis, and assembly of murein lipoprotein have been isolated. One of these mutants has been shown to synthesize a structurally altered lipoprotein. The biochemical features of the mutant lipoprotein (lipid deficiency, dimer formation, and a reduced, bound form of lipoprotein) could be attributed to a single mutation (or closely linked mutations) located at 36.4 min of the Escherichia coli map. We propose that this mutant is altered in the structural gene for murein lipoprotein (mlpA). Biochemical studies carried out with a heterogenote, mlpA/F'mlpA+, revealed the biochemical codominance of the wild-type and mutant genes.

Biochemical characterization of a mutant lipoprotein of Escherichia coli

A lipoprotein mutant of E. coli K-12 has been characterized. The mutant lipoprotein was found to differ from the wild-type lipoprotein in the following respects: (i) it is present in an appreciable amount in the soluble fraction (275,000 X g supernatant); (ii) it lacks the covalently-linked diglyceride; (iii) it contains an unmodified cysteine which can be carboxymethylated in vitro; (iv) it undergoes dimerization and the dimer can be converted into monomeric form by reduction with 2-mercaptoethanol; (v) both the monomeric form and especially the dimeric form of the mutant lipoprotein migrate more slowly than the corresponding forms of wild-type lipoprotein in sodium dodecyl sulfate/urea polyacrylamide gel electrophoresis; and (vi) the mutant lipoprotein is not assembled into the murein sacculi, and this results in a greatly reduced amount of bound-form lipoprotein in the mutant. These data strongly suggest that the mutation has affected the primary structure of lipoprotein, in such a way that it is not modified normally, leading to the production of a structurally-altered lipoprotein deficient in covalently-linked lipid as well as a defective assembly of the altered lipoprotein into the rigid layer of the cell envelope.

Escherichia coli mutants altered in murein lipoprotein

Mutants with alterations in the structure, biosynthesis, or assembly of murein lipoprotein were selected by a procedure based on radiation suicide of wild-type organisms by [3H]arginine under conditions where the radioactive arginine was preferentially incorporated into lipoprotein. Further screening for the potential mutants among the survivors of [3H]arginine suicide was carried out by using a sensitive immunodiffusion test, followed by radioactive double-labeling experiments. Three mutants were obtained and partially characterized.

Biosynthesis and assembly of envelope lipoprotein in a glycerol-requiring mutant of Salmonella typhimurium

A glycerol-requiring mutant of Salmonella typhimurium was used in a study of the biosynthesis and assembly of a structural lipoprotein in the cell envelope of gram-negative bacteria. Upon removal of glycerol from the growth medium, the biosynthesis of lipoprotein, as measured by radioactive arginine incorporation, was reduced by the same extent as that of other envelope proteins, the cumulative incorporation of arginine being 20% of that of the unstarved control cells. However, the incorporation of radioactive palmitate into lipoprotein was more severely curtailed after glycerol starvation, the cumulative rate of which was 8% of that observed in the unstarved cells. It was further observed that the lipoprotein synthesized in the glycerol-starved cells was more enriched in unmodified cysteine, which is known to be the N-terminal amino acid of lipoprotein, than that synthesized in the unstarved cells. We conclude that the synthesis of the apoprotein portion of Braun's lipoprotein proceeds independently of the attachment of diglyceride to the sulfhydryl group of the N-terminal cysteine and may, in fact, precede the incorporation of the diglyceride moiety.

Purification and properties of beta-N-acetylglucosaminidase from Escherichia coli

beta-N-acetylglucosaminidase (EC 3.2.1.30) has been purified from Escherichia coli K-12 to near homogeneity based on polyacrylamide gel electrophoresis in both 0.5% sodium dodecyl sulfate and in 6 M urea at pH 8.5. The purified enzyme shows a pH optimum of 7.7 and the Km for p-nitrophenyl-beta-D-2-acetamido-2-deoxyglucopyranoside is 0.43 mM. The molecular weight of this enzyme, determined by both Sephadex gel filtration and by sodium dodecyl sulfate gel electrophoresis, is equivalent to 36,000. It is shown to be a soluble cytoplasmic enzyme. Studies on the substrate specificites of the purified enzyme indicate that this enzyme is an exo-beta-N-acetylglucosaminidase.

Isolation of Escherichia coli K-12 mutants with altered level of beta-N-acetylglucosaminidase

Eight mutants with less than 25% of the wild-type level of beta-N-acetylglucosaminidase activity have been isolated from Escherichia coli K-12. Studies on these mutants suggest that less than 1% of the wild-type level of this enzyme may be adequate for the normal growth and division of E. coli cells.

Escherichia coli mutants permissive for T4 bacteriophage with deletion in e gene (phage lysozyme)

Escherichia coli mutants have been isolated that are permissive for the infection by T4 phage with deletion in the cistron for the phage lysozyme, the e gene. Some, but not all, of these mutants are simultaneously permissive for the infection by T4 phage defective in the t gene, the product of which has also been implicated in the release of progeny phages. Most of these mutants shared the following properties: temperature sensitivity in growth and cell division, increased sensitivity towards a number of unrelated antibiotics and colicins, and increased sensitivity towards anionic detergents (sodium dodecyl sulfate and sodium deoxycholate). The possible biochemical basis for these phenotypes is discussed.