Amino acid sequence homology among the major outer membrane proteins of Escherichia coli

Analysis of amino acid sequences reported for the major outer membrane proteins of Escherichia coli, including the porins (OmpF, OmpC, and PhoE), the phage lambda receptor (LamB), and another protein (OmpA), revealed several regions of local homology that is statistically significant. The implications of this observation are discussed in relation to the evolutionary origins of these proteins, as well as to the mechanism of export of these proteins to the outer membrane.

A distinct signal peptidase for prolipoprotein in Escherichia coli

We have previously demonstrated the modification and processing of Escherichia coli prolipoprotein (Braun's) in vitro ( Tokunaga M, Tokunaga H, Wu HC: Proc Natl Acad Sci USA 79:2255, 1982). Using this in vitro assay of prolipoprotein signal peptidase and globomycin selection, we have isolated and partially characterized an E coli mutant which contained a higher level of prolipoprotein signal peptidase activity. In contrast, the procoat protein signal peptidase activity was not increased in this mutant as compared to the wild-type strain. Furthermore, E coli strains containing cloned procoat protein signal peptidase gene were found to contain elevated levels of procoat protein signal peptidase, but normal levels of prolipoprotein signal peptidase. These two signal peptidase activities were also found to exhibit different stabilities during storage at 4 degrees C. Thus biochemical, immunological, and genetic evidence clearly indicate that prolipoprotein signal peptidase is distinct from procoat protein signal peptidase in E coli.

Biosynthesis of Bacillus licheniformis penicillinase in Escherichia coli and in Bacillus subtilis

Modified prepenicillinase was accumulated in both Escherichia coli and Bacillus subtilis treated with globomycin. Although the inhibitions of processings of prepenicillinase and prolipoprotein by globomycin in E. coli are qualitatively similar, they differ in the degree of inhibition at given concentrations of globomycin. The processing of prepenicillinase proceeds much more rapidly in E. coli than in B. subtilis.

Isolation and characterization of an Escherichia coli clone overproducing prolipoprotein signal peptidase

Based on the rationale that Escherichia coli cells containing increased levels of prolipoprotein signal peptidase would be highly resistant to globomycin, a specific inhibitor of the prolipoprotein signal peptidase, we have isolated a clone from the Carbon-Clarke collection, plasmid pLC3-13, which is globomycin-resistant and contains an increased level of prolipoprotein signal peptidase activity. The plasmid pMT521, a subclone of pLC3-13 in pBR322, conferred on its host cells approximately 20 times overproduction of prolipoprotein signal peptidase and an extremely high level of resistance against globomycin. The overproduced prolipoprotein signal peptidase was completely inhibited by the presence of globomycin in the in vitro assay, and the overproduced activity was found in the cell envelope fraction. Several lines of biochemical and genetic evidence suggest that the gene contained in pLC3-13 and its derivative clones is most likely the structure gene (lsp) for prolipoprotein signal peptidase.

Temporal behavior of abrin in the intoxication of Chinese hamster cells (line CHO)

Abrin, a potent cytotoxin, was utilized as a probe to elucidate the mechanism by which external proteins are delivered to the cytoplasm of mammalian cells. Abrin bound rapidly to the surface receptors of the Chinese hamster cells (line CHO) and appeared to be internalized immediately without any significant lag. The maximum level of abrin internalization was achieved within eight minutes, based on both biochemical and electron microscopic autoradiographic studies with [125I]abrin. About 10% of the silver grains of internalized [125I] abrin were associated with vesicular structures, irrespective of the incubation time. Inhibition of protein synthesis began 30 minutes postincubation, and this latent period was not dependent on extracellular toxin concentration. SDS-polyacrylamide gel electrophoresis of the internalized [125I]abrin indicated that internalized abrin molecules remained intact even after two hours of incubation.

Prolipoprotein signal peptidase in Escherichia coli is distinct from the M13 procoat protein signal peptidase

We have previously reported a signal peptidase activity in Escherichia coli cell envelope which processes prolipoprotein modified with glyceride (Tokunaga, M., Tokunaga, H., and Wu, H. C. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2255-2259). To ascertain whether the processing enzyme for prolipoprotein is distinct from the signal peptidase for M13 procoat protein purified by Zwizinski and Wickner (Zwizinski, C., and Wickner, W. (1980) J. Biol. Chem. 255, 7973-7977), we have used antibody against purified procoat protein signal peptidase to study the processings of prolipoprotein and M13 procoat protein in vitro. the signal peptidase for modified prolipoprotein remained fully active in solubilized membrane preparations which had been treated with antibody against purified procoat protein signal peptidase whereas the activity towards procoat protein was completely abolished by immunoadsorption. Furthermore, both unmodified and glyceride-modified prolipoprotein were not cleaved by the highly purified signal peptidase preparation provided by Wickner. These data clearly indicate that prolipoprotein signal peptidase is distinct from the M13 procoat protein signal peptidase.

Receptors for a cytotoxic lectin, abrin and their role in cell intoxication

The nature of binding of abrin to Chinese hamster ovary cells was examined in relation to the ensuing intoxication of the treated cells. Approx. 20% of [125I] abrin bound to CHO cells at 37 degree C was found to be resistant to the addition or presence of 0.1 M lactose. The extent of lactose-resistant binding depended inversely upon the temperature of incubation. Among various proteins, lectins and sugars, only non-labeled abrin could strongly inhibit the lactose-resistant binding of [125I] abrin. Lactose-resistant binding could lead to an inhibition of cellular protein synthesis and to a loss of cell viability. Abrin molecules bound at the lactose-sensitive and lactose-resistant binding sites apparently have an equal probability of being internalized by CHO cells. Binding of approx. 3.10(3) abrin molecules per CHO cell was required to elicit 50% loss of cell viability regardless of whether the binding occurs in the presence or absence of lactose. The result of a cross-linking experiment suggested that a membrane protein with an Mr of about 45 000 may be responsible for the lactose-resistant binding of abrin.

Chinese hamster ovary cell mutants defective in the internalization of ricin

Chinese hamster ovary mutants simultaneously resistant to ricin and Pseudomonas toxin have been isolated. Two mutant cell lines (4-10 and 11-2) were found to retain normal levels of binding of both ricin and Pseudomonas toxin. They were defective in the internalization of [125I]ricin into the mutant cells, as measured by both a biochemical assay for ricin internalization and electron microscopic autoradiographic studies. Although pretreatment of Chinese hamster ovary cells with a Na+/K+ ionophore, nigericin, resulted in an enhancement of the cytotoxicities of ricin and Pseudomonas toxin in the wild-type Chinese hamster ovary cells, preculture of the mutant cells did not alter the susceptibility of the mutant cells to either toxin. These results provide further evidence that there is a common step in the internalization process for ricin and Pseudomonas toxin.

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.

Internalization of ricin in Chinese hamster ovary cells

Internalization of ricin into Chinese hamster ovary cells has been investigated. Combined treatment with galactose and pronase at 0 degrees C resulted in a complete release of surface-bound [125I]ricin into the media. Galactose-pronase-resistant cell-bound [125I]ricin represents internalized ricin molecules inside the cells. The internalization process is time, temperature, and concentration dependent. The pH optimum of internalization of ricin is about pH 7. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis has revealed that intact ricin molecules are internalized. Neither reduction nor proteolytic processing of ricin is required for the entry of ricin into Chinese hamster ovary cells.

Bacillus licheniformis penicillinase synthesized in Escherichia coli contains covalently linked fatty acid and glyceride

DNA sequence analysis of the structural gene for Bacillus licheniformis penicillinase has revealed a tetrapeptide sequence of Leu-Ala-Gly-Cys within the NH2-terminal part of the precursor form of penicillinase (penicillin amido-beta-lactamhydrolase, EC 3.5.2.6). The same tetrapeptide occurs in the signal sequence of the prolipoprotein of Escherichia coli, and the cysteine residue in the tetrapeptide of prolipoprotein is modified to form glyceride-cysteine which becomes the NH2 terminus of Braun's lipoprotein. On the basis of labeling, with [2-3H]glycerol, [3H]palmitate, [35S]methionine, and [35S]sulfuric acid, of an E. coli strain lysogenic for a lambda vector containing the penicillinase gene from B. licheniformis and of immunoprecipitation with rabbit antisera against purified B. licheniformis penicillinase, we conclude that B. licheniformis penicillinase synthesized in E. coli contains covalently linked glyceride and fatty acid. These results strongly suggest the operation of a modification system in E. coli, and presumably in other Gram-negative bacteria, which results in the formation of a glyceride-cysteine residue if the proper peptide sequence is present in the signal sequence of membrane proteins.

Enhanced internalization of ricin in nigericin-pretreated Chinese hamster ovary cells

Biochemical and electron microscopic autoradiographic studies with [125I] ricin have revealed that nigericin-pretreated Chinese hamster ovary cells are more efficient than untreated cells in the internalization of the toxin into the cells. These results suggest that the enhanced rate of internalization of ricin in nigericin-pretreated cells may account for the enhancement of cytotoxicity of ricin in Chinese hamster ovary cells by nigericin.

Enhancement of cytotoxicities of ricin and Pseudomonas toxin in Chinese hamster ovary cells by nigericin

Nigericin and monensin, ionophores for Na+ and K+, have been found to enhance the cytotoxicities of abrin, ricin, and Pseudomonas aeruginosa exotoxin A in Chinese hamster ovary (CHO) cells. They do not affect the cytotoxicity of diphtheria toxin in the same cell line. Maximal sensitization of the CHO cells toward ricin and Pseudomonas toxin requires preculture of CHO cells in the presence of nigericin. Inhibition of protein synthesis in CHO cells by ricin or Pseudomonas toxin is also enhanced by preculture of CHO cells in the presence of nigericin. These results suggest a common step in the intoxication process of ricin and Pseudomonas toxin, the rate of which is facilitated by pretreatment with nigericin. This step is, however, not shared by the intoxication of CHO cells with diphtheria toxin.

Globomycin sensitivity of Escherichia coli and Salmonella typhimurium: effects of mutations affecting structures of murein lipoprotein

The sensitivity of strains of Escherichia coli and Salmonella typhimurium to globomycin is increased in mutants defective in the lipopolysaccharide structure. E. coli mutants altered in the structures or biosynthesis of murein lipoprotein are more resistant to globomycin than the parental strains.

Incorporation of acyl moieties of phospholipids into murein lipoprotein in intact cells of Escherichia coli by phospholipid vesicle fusion

The biosynthesis of the acyl moieties in murein lipoprotein was studied by fusion of [3H]palmitate-labeled phospholipid vesicles with intact cells of an fadD mutant of Escherichia coli. A linear increase in the incorporation of [3H]palmitate radioactivity into both the ester- and amide-linked fatty acids in lipoprotein was observed during a 3-h chase after the fusion. Addition of chloramphenicol completely prevented the incorporation of [3H]palmitate from phospholipids to lipoprotein. These results strongly support our hypothesis that the acyl moieties in phospholipids are the precursors for the fatty acids in murein lipoprotein of E. coli. Among the major glycerophosphatides in E. coli, no specificity was observed regarding the efficacy of the donor.

Determination of transcriptional units and gene products from the ftsA region of Escherichia coli

Lambda transducing phage gamma 16-2 carries the genes envA, ftsZ, ftsA, ddl, and murC and directs the synthesis of six unique proteins in ultraviolet-irradiated cells. Various derivatives of gamma 16-2 carrying smaller segments of the bacterial deoxyribonucleic acid have also been analyzed for their capacity to direct protein synthesis in ultraviolet-irradiated cells. These results, in combination with genetic results, have allowed the gene product of each of these genes to be assigned. In addition, an unidentified gene was located counterclockwise to murC between murC and murF. Analysis of the direction of transcription indicates that murC, ddl, ftsA, and ftsZ are transcribed clockwise on the Escherichia coli genetic map, and envA is transcribed counterclockwise. In addition, it is shown that each of the genes envA, ftsZ, and ftsA can be expressed independently.