Extracellular labeling of nascent polypeptides traversing the membrane of Escherichia coli

Cardiolipin is required in vivo for the stability of bacterial translocon and optimal membrane protein translocation and insertion

Translocation of preproteins across the Escherichia coli inner membrane requires anionic lipids by virtue of their negative head-group charge either in vivo or in situ. However, available results do not differentiate between the roles of monoanionic phosphatidylglycerol and dianionic cardiolipin (CL) in this essential membrane-related process. To define in vivo the molecular steps affected by the absence of CL in protein translocation and insertion, we analyzed translocon activity, SecYEG stability and its interaction with SecA in an E. coli mutant devoid of CL. Although no growth defects were observed, co- and post-translational translocation of α-helical proteins across inner membrane and the assembly of outer membrane β-barrel precursors were severely compromised in CL-lacking cells. Components of proton-motive force which could impair protein insertion into and translocation across the inner membrane, were unaffected. However, stability of the dimeric SecYEG complex and oligomerization properties of SecA were strongly compromised while the levels of individual SecYEG translocon components, SecA and insertase YidC were largely unaffected. These results demonstrate that CL is required in vivo for the stability of the bacterial translocon and its efficient function in co-translational insertion into and translocation across the inner membrane of E. coli.

Chloride promotes refolding of active Vibrio alkaline phosphatase through an inactive dimeric intermediate with an altered interface

Most enzymes are homodimers or higher order multimers. Cold‐active alkaline phosphatase from Vibrio splendidus (VAP) transitions into a dimer with very low activity under mild denaturation conditions. The desire to understand why this dimer fails to efficiently catalyse phosphomonoester hydrolysis led us to investigate interfacial communication between subunits. Here, we studied in detail the unfolding mechanism at two pH values and in the presence or absence of sodium chloride. At pH 8.0, the denaturation model had to include an inactive dimer intermediate and follow the pathway: N₂ → I₂ → 2U. At pH 10.5, the model was of a two‐state nature. Enzyme activity was not recovered under several examined refolding conditions. However, in the presence of 0.5 m NaCl, the enzyme was nearly fully reactivated after urea treatment. Thermal inactivation experiments were biphasic where the inactivation could be detected using CD spectroscopy at 190–200 nm. By incorporating a bimane fluorescence probe at the dimer interface, we could monitor inactivation/denaturation at two distinct sites at the dimer interface. A change in bimane fluorescence at both sites was observed during inactivation, but prior to the global unfolding event. Furthermore, the rate of change in bimane fluorescence correlated with inactivation rates at 40 °C. These results indicate and support the hypothesis that the subunits of VAP are only functional in the dimeric state due to the cooperative nature of the reaction mechanism when proper crosstalk between subunits is facilitated.

SecA inhibitors as potential antimicrobial agents: differential actions on SecA-only and SecA-SecYEG protein-conducting channels

Sec-dependent protein translocation is an essential process in bacteria. SecA is a key component of the translocation machinery and has multiple domains that interact with various ligands. SecA acts as an ATPase motor to drive the precursor protein/peptide through the SecYEG protein translocation channels. As SecA is unique to bacteria and there is no mammalian counterpart, it is an ideal target for the development of new antimicrobials. Several reviews detail the assays for ATPase and protein translocation, as well as the search for SecA inhibitors. Recent studies have shown that, in addition to the SecA-SecYEG translocation channels, there are SecA-only channels in the lipid bilayers, which function independently from the SecYEG machinery. This mini-review focuses on recent advances on the newly developed SecA inhibitors that allow the evaluation of their potential as antimicrobial agents, as well as a fundamental understanding of mechanisms of SecA function(s). These SecA inhibitors abrogate the effects of efflux pumps in both Gram-positive and Gram-negative bacteria. We also discuss recent findings that SecA binds to ribosomes and nascent peptides, which suggest other roles of SecA. A model for the multiple roles of SecA is presented.

The Sec System: Protein Export in Escherichia coli

In Escherichia coli, proteins found in the periplasm or the outer membrane are exported from the cytoplasm by the general secretory, Sec, system before they acquire stably folded structure. This dynamic process involves intricate interactions among cytoplasmic and membrane proteins, both peripheral and integral, as well as lipids. In vivo, both ATP hydrolysis and proton motive force are required. Here, we review the Sec system from the inception of the field through early 2016, including biochemical, genetic, and structural data.

Conditions leading to secretion of a normally periplasmic protein in Escherichia coli

The phosphate-binding protein (PhoS) is a periplasmic protein which is part of the high-affinity phosphate transport system of Escherichia coli. Hyperproduction of PhoS in strains carrying a multicopy plasmid containing phoS led to partial secretion of the protein. By 6 h after transfer to phosphate-limiting medium, about 13% of the total newly synthesized PhoS was secreted to the medium. Kinetic studies demonstrated that this secretion consists of newly synthesized PhoS. This secretion occurs in PhoS-hyperproducer strains but not in a PhoS-overproducer strain. Another type of secretion concerning periplasmic PhoS was observed in both PhoS-hyperproducer and PhoS-overproducer strains. This mode of secretion depended upon the addition of phosphate to cells previously grown in phosphate-limiting medium.

The N-terminal region of Escherichia coli lactose permease mediates membrane contact of the nascent polypeptide chain

Plasmids encoding N-terminal segments of the Escherichia coli lactose permease (also referred to as lactose carrier) have been used to analyze the biosynthesis and membrane insertion of this complex integral protein of the cytoplasmic membrane. Such truncated polypeptides were found to be stably associated with the membrane and to resemble the full-length protein with respect to their solubilization characteristics. Membrane-bound and free cytoplasmic polysomes were prepared from plasmid-bearing cells and incubated in the presence of [35S]methionine to permit completion of polypeptides initiated in vivo. Under these conditions, lactose permease was found to be radiolabeled in the fraction of membrane-bound polysomes; beta-galactosidase, used as a control, was translated almost exclusively by free polysomes. From similar experiments with N-terminal segments of lactose permease, we estimate that at most a polypeptide of 120 amino acid residues emerging from the ribosome is needed to target the nascent chain to the lipid bilayer and to mediate attachment of the ribosome to the membrane during elongation. Additional data support the idea that even shorter N-terminal sequences of 50 and 71 amino acid residues contain sufficient 'information' to provide contact with the membrane.

Biosynthetic arginine decarboxylase in Escherichia coli is synthesized as a precursor and located in the cell envelope

The biosynthetic form of arginine decarboxylase (ADC) catalyzes the synthesis of agmatine, a precursor of putrescine, in Escherichia coli. Selective disruption of the cell envelope and an assessment of ADC activity or immunoprecipitable ADC in various fractions demonstrated its location between the cytoplasmic membrane and peptidoglycan layer. Expression in minicells of the speA gene encoding ADC resulted in the production of two immunoprecipitable species (74 and 70 kilodaltons). Studies in vivo with a pulse and chase of radiolabeled amino acid into the two species suggest a precursor-product relationship. This relationship was corroborated by demonstrating the accumulation of the 74-kilodalton species in a strain of E. coli unable to process signal sequences. Peptide mapping experiments with V8 protease, trypsin, and alpha-chymotrypsin demonstrated that the two species of ADC were very similar except for a minor difference. These data were used to substantiate the compartmentalization hypothesis as to how exogenous arginine can be channeled preferentially into putrescine.

Alkaline phosphatase and OmpA protein can be translocated posttranslationally into membrane vesicles of Escherichia coli

We previously described a system for translocating the periplasmic enzyme alkaline phosphatase and the outer membrane protein OmpA into inverted membrane vesicles of Escherichia coli. We have now optimized and substantially improved the translocation system by including polyamines and by reducing the amount of membrane used. Under these conditions, efficient translocation was seen even posttranslationally, i.e., when vesicles were not added until after protein synthesis was stopped. This was the case not only with the OmpA protein, which is synthesized by free polysomes and hence is presumably exported posttranslationally in the cell, but also with alkaline phosphatase, which is synthesized only by membrane-bound polysomes and has been shown to be secreted cotranslationally in the cells. Prolonged incubation rendered the precursors inactive for subsequent translocation. Posttranslational translocation was impaired, like cotranslational translocation, by inhibitors of the proton motive force and by treatment of the vesicles with protease. Since it appears that E. coli can translocate the same proteins either cotranslationally or posttranslationally, the cotranslational mode may perhaps be more efficient, but not obligatory, for the secretion of bacterial proteins.

Proper interaction between at least two components is required for efficient export of proteins to the Escherichia coli cell envelope

An Escherichia coli mutant carrying delta malE12-18, a 21-base pair deletion confined to the coding DNA of the maltose-binding protein signal peptide, is unable to export maltose-binding protein to the periplasm efficiently. Consequently, such a strain is defective for the utilization of maltose as a sole carbon source. We obtained 16 mutants harboring extragenic delta malE12-18 suppressor mutations that exhibit partial restoration of export to the mutant maltose-binding protein. A genetic analysis of these extragenic suppressor mutations demonstrated that 15 map at prlA, at 72 min on the standard E. coli linkage map, and that 1 maps at a new locus, prlD, at 2.5 min on the linkage map. Our evidence indicates that the prlA and prlD gene products play an important role in the normal pathway for export of proteins to the cell envelope. Efficient execution of the secretory process requires that these prl gene products interact properly with each other so that a productive interaction of these gene products with the signal peptide also can occur. Our data suggest that proper assembly of a complex is required for efficient export of E. coli envelope proteins to their various extracytoplasmic compartments.

Both linked and unlinked mutations can alter the intracellular site of synthesis of exported proteins of Escherichia coli

It previously has been demonstrated that synthesis of the periplasmic maltose-binding protein (MBP) and alkaline phosphatase (AP) of Eschericha coli predominantly occurs on membrane-bound polysomes. In this study, signal sequence alterations that adversely affect export of MBP and AP, resulting in their cytoplasmic accumulation as unprocessed precursors, were investigated to determine whether they have an effect on the intracellular site of synthesis of these proteins. Our findings indicate that export-defective MBP and AP are not synthesized or are synthesized in greatly reduced levels on membrane-bound polysomes. In some instances, a concomitant increase in the amount of these proteins synthesized on free polysomes was clearly discerned. We also determined the site of synthesis of MBP and AP in strains harboring mutations thought to alter the cellular secretion machinery. It was found that the presence of a prlA suppressor allele partially restored synthesis of export-defective MBP on membrane-bound polysomes. On the other hand, the absence of a functional SecA protein resulted in the synthesis of wild-type MBP and AP predominantly on free polysomes.

Export of protein in bacteria

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Normal precursors of periplasmic proteins accumulated in the cytoplasm are not exported post-translationally in Escherichia coli

Hyperproduction of phosphate-binding protein, PhoS, in strains carrying a multicopy plasmic containing the phoS gene, resulted in saturation of export sites. As a consequence, pre-PhoS was accumulated both in the inner membrane and in the cytoplasm. This was evidenced both in electron-microscopy and after cell fractionation. Only the membrane-associated precursor could be matured and exported. The signal sequence of the cytoplasmic pre-PhoS was slowly degraded. It was first cleaved about in its middle and then completely removed. Electron microscope studies demonstrated that the cytoplasmic pre-PhoS cannot be exported post-translationally.

Energy-requiring translocation of the OmpA protein and alkaline phosphatase of Escherichia coli into inner membrane vesicles

In developing a reliable in vitro system for translocating bacterial proteins, we found that the least dense subfraction of the membrane of Escherichia coli was superior to the total inner membrane, both for a secreted protein (alkaline phosphatase) and for an outer membrane protein (OmpA). Compounds that eliminated the proton motive force inhibited translocation, as already observed in cells; since protein synthesis continued, the energy for translocation appears to be derived from the energized membrane and not simply from ATP. Treatment of the vesicles with protease, under conditions that did not interfere with subsequent protein synthesis, also inactivated them for subsequent translocation. We conclude that export of some proteins requires protein-containing machinery in the cytoplasmic membrane that derives energy from the proton motive force.

Synthesis of pigment-binding protein in toluene-treated Rhodopseudomonas capsulata and in cell-free systems

Pigment-binding protein of the facultatively phototrophic bacterium Rhodospeudomonas capsulata could be selectively synthesized in toluene-treated cells as well as in homologous and heterologous cell-free translation systems by isolated polysomes. It is shown that the pigment-binding polypeptides of the light-harvesting complexes are encoded by messenger RNA of extreme longevity. The dependence of their synthesis on the concomitant synthesis of tetrapyrroles was demonstrated in the toluene-treated cells. The large Mr-28 000 polypeptide of the reaction center and the Mr-10 000 pigment-binding polypeptide of the light-harvesting complex II were found to be synthesized by free (water-soluble) polysomes without a cleavable 'leader' or 'signal' peptide [reviewed by W. Wickner (1979) Annu. Rev. Biochem. 48, 23-45]. The Mr-10 000 polypeptide, as synthesized in vitro, was studied in more detail. Unlike the membrane-assembled polypeptide in vivo it was insoluble in an organic solvent mixture (chloroform/methanol 1:1, v/v). After detergent denaturation in the presence of membrane isolated from the organism it became organic-solvent-soluble. Obviously the polypeptide could be induced to assume alternative conformations in which its apolar residues were either exposed to the solvent or buried within. These findings, in agreement with Wickner's hypothesis, indicate that the Mr-10 000 polypeptide may enter the lipid bilayer by a 'membrane-triggered' conformational change.

Mechanism of protein excretion by gram-negative bacteria: Pseudomonas aeruginosa exotoxin A

Excretion of proteins by a cell with a double membrane may involve mechanisms different from secretion across a single membrane. We studied this problem with Pseudomonas aeruginosa exotoxin A. This 68,000-dalton protein was released as rapidly as it was completed; even after short pulse-labeling the cells contained neither the toxin nor a larger precursor. Excretion is evidently cotranslational, since in fractionated lysates the toxin was formed (almost entirely in the mature form) by the membrane-polysome complexes but not by the free polysomes. When the membrane was perturbed by 10% ethanol, the cells stopped excreting the toxin and they accumulated an immunoprecipitable, enzymatically active precursor of 71,000 daltons. The precursor was located entirely in the outer membrane on its outer surface. On removal of the ethanol, the cells again excreted mature toxin, but they did not process or release the previously accumulated precursor. Based on these data, a model for the excretion of exotoxin A is presented.

Localization of the major dehydrogenases in two methylotrophs by radiochemical labeling

The localization of prominent proteins in intact cells of two methylotrophic bacteria, Hyphomicrobium sp. strain X and bacterium W3A1, was investigated by radiochemical labeling with [14C]isethionyl acetimidate. In bacterium W3A1, trimethylamine dehydrogenase was not labeled by the reagent and is, therefore, an intracellular protein, whereas the periplasmic location of the methylamine and methanol dehydrogenases was evidenced by being readily labeled in intact cells. Similarly, an intracellular location of the trimethylamine and dimethylamine dehydrogenases in Hyphomicrobium sp. strain X was indicated, whereas methanol dehydrogenase was periplasmic.

Proteolytic systems in lactic acid bacteria

The proteolytic systems of lactic acid bacteria are important as a means of making protein and peptide N available for growth and as part of the curing or maturation processes which give foods their characteristic rheological and organoleptic properties. The proteolytic systems of lactic acid bacteria are described in relation to their growth and their functions in protein-rich foods. Their role in the manufacture of milk products is discussed.

Proteins of ribosome-bearing and free-membrane domains in Bacillus subtilis

In lysates of Bacillus subtilis a free-membrane fraction without ribosomes can be separated from the denser membrane-ribosome complexes. As determined by one-dimensional sodium dodecyl sulfate gel electrophoresis, these two fractions differ markedly in protein composition; at least six major bands (molecular weights, 130,000, 92,000, 68,000, 64,000, 45,000, and 31,000) are essentially unique to the complexed-membrane fraction (CM proteins), and two are unique to the free-membrane fraction. After growth was slowed, the proportion of the free-membrane fraction increased, but the composition of this fraction was the same, whereas after puromycin treatment, which abruptly increased the proportion of the free-membrane fraction, this fraction contained CM proteins. Thus, it appears that the two fractions recovered from growing cells represent topographically and functionally distinct domains. In addition, the effect of growth rate suggests that formation of the complexed domain is regulated at least roughly in parallel with the formation of ribosomes. The separation of these membrane fractions should facilitate the study of protein secretion, membrane topography, and morphogenesis in bacteria.

In vitro construction and characterization of phoA-lacZ gene fusions in Escherichia coli

Using recombinant DNA techniques, we have constructed phoA-lacZ gene fusions. Two of the fusions encode hybrid proteins containing approximately half of alkaline phosphatase at the amino terminus joined to beta-galactosidase. For the one fusion strain analyzed in detail, it was shown that the hybrid protein is found in the membrane fraction of cells. In its membrane location, the beta-galactosidase activity of the hybrid is not sufficient to support cell growth on lactose. Unexpectedly, fusions containing phoA and lacZ joined in the wrong translational reading frame were also obtained. These fusions direct the phosphate-regulated synthesis of beta-galactosidase, apparently via a translation restart mechanism. Thus, when gene fusions are constructed, the presence of properly regulated beta-galactosidase activity does not necessarily indicate that a hybrid protein is being produced.

Role of primary structure and disulfide bond formation in beta-lactamase secretion

Plasmid pBR322-encoded beta-lactamase was shown to contain a single disulfide bond, which caused the protein to migrate faster in sodium dodecyl sulfate-polyacrylamide gels than the fully reduced form. A similar difference in mobility of the in vitro synthesized precursor before and after reduction indicates that it also contained a disulfide bond. Formation of the disulfide bond in vivo, however, occurred concomitant with processing. In vivo accumulation of the precursor by inhibition of secretion did not allow disulfide bond formation to occur. This result is consistent with post-translational translocation of the precursor. Synthesis of a fragment of beta-lactamase lacking the carboxy terminus was obtained by insertion of a foreign DNA segment into the PstI site of bla. Processing and secretion of the protein did not appear to be greatly affected, indicating that the carboxy terminus is not required for secretion.

Membrane potential (delta psi) depolarizing agents inhibit maturation

Precursor forms of exported proteins were first accumulated in the envelope of phenethyl alcohol (PEA)-treated cells. After removal of PEA, a complete processing could be obtained in a few minutes. In this work, we demonstrate that colicins A and E1, that act on the electrical gradient in the cytoplasmic membrane, prevent the processing of precursor forms previously accumulated. Concentrations of colicins accounting for approximately 1 killing unit (50--3000 molecules/cell) were found to be sufficient for inhibition of processing. Therefore our results strongly suggest that in intact cells the electrical gradient across the cytoplasmic membrane is required for maturation of exported proteins.

Cloning of mglB, the structural gene for the galactose-binding protein of Salmonella typhimurium and Escherichia coli

From libraries of EcoRI fragments of Salmonella typhimurium and Escherichia coli DNA in lambda gt7, phages could be isolated that carry mglB, the structural gene of the galactose-binding protein as well as other mgl genes. Lysogenization of an E. coli mutant carrying a defective galactose-binding protein with lambda gt7 mglB (Salmonella) restores full galactose transport and galactose chemotaxis. Both the E. coli mutant protein as well as the wild-type Salmonella galactose-binding protein are synthesized in this strain. The EcoR1 fragments of both organisms carrying the mgl genes were 6 Kb long. They were subcloned into the multicopy plasmid pACYC184. The hybrid plasmid containing the Salmonella mgl DNA gives rise to the synthesis of large amounts of galactose-binding protein in the periplasm of E. coli. The protein can be precipitated by antibodies against the E. coli binding protein and is identical to the fully processed protein isolated from Salmonella typhimurium LT2. In vitro protein synthesis (Zubay-system) with either lambda gt7 mgl phages as well as the hybrid plasmid as DNA matrix produces the galactose-binding protein mainly in precursor form that is precipitable by specific antibodies.

Exclusive localization of colicin A in cell cytoplasm of producing bacteria

The production of colicin A in Citrobacter freundii and in Escherichia coli was studied. After induction with low concentrations of mitomycin C, these organisms differed with regards to cell growth, cell viability, and kinetics of colicin A biosynthesis. Despite these differences, immunoferritin labelling on ultra-thin sections of induced frozen cells demonstrated that colicin A was located exclusively within the cell cytoplasm in both types of bacteria. By using protein markers, it was shown that at no time after induction was colicin A accumulated in the periplasmic space or in inner or outer membranes. These results were confirmed by a biochemical approach. For at least 3 h after induction, colicin A remained associated with producing cells and no colicin A activity was found in the periplasmic space. These results are discussed with reference to the synthesis and export of other bacteriocins.

Bacillus licheniformis penicillinase: cleavages and attachment of lipid during cotranslational secretion

The penicillinase of Bacillus licheniformis is shown to be secreted cotranslationally. In extracts it was formed by membrane-associated but not by free polysomes; and after extracellular labeling of cells, followed by completion of the growing chains on polysomes in vitro, labeled penicillinase could be immunoprecipitated. This product contained electrophoretic peaks of Mr 36,000, 33,000, and 29,000, which correspond to previously reported forms of the enzyme. The Mr 36,000 form exhibits moderate hydrophobicity, as expected of a precursor with an NH2-terminal signal sequence for secretion. In addition, part of the Mr 33,000 fraction evidently contains a lipid: it is even more hydrophobic, and [2-3H]glycerol was found to be incorporated into these molecules but not into the other forms of the enzyme. These findings renew the earlier, discarded suggestion that the Mr 33,000 membrane-bound penicillinase in the cells contains lipid. The incorporation of lipid and two different cleavages can evidently all occur during growth of the penicillinase chain. Moreover, the resulting terminal regions are all accessible to extracellular labeling on growing chains. Several additional, unidentified lipoproteins also incorporate lipid during chain growth.

Biosynthesis and export of colicin A in Citrobacter freundii CA31

Synthesis of colicin A after induction with mitomycin C was studied. Specific inhibition of chromosomal protein synthesis occurred very shortly after mitomycin addition. There was no coordinate synthesis of colicin A (61000 Mr) and low-molecular-weight protein. Free and membrane-bound polysome fractions were isolated from cells induced with mitomycin C. Colicin A is synthesized in vitro in the free polysomes and not in the membrane-bound polysomes. Conditions are described which allow a practically specific labelling of colicin A in vivo. By using this system it was possible to demonstrate that colicin A is not transferred cotranslationally across the cytoplasmic membrane. In contrast, this protein leaves the cell where it was made long after synthesis. Preliminary evidence, suggesting that pauses occur during synthesis of colicin A, is presented.

Energy is required for maturation of exported proteins in Escherichia coli

It has been established in numerous cases that proteins which are exported from Escherichia coli are synthesized on membrane-bound polysomes in precursor forms which are proteolytically cleaved to generate the mature species. Here we present evidence that at least one step in the export of proteins requires energy. Energy requirements for processing of the precursors of both the M13 coat protein [Date, T., Zwizinski, C., Ludmerer, S., and Wickner, W. (1980) Proc. Natl Acad. Sci. USA, 77, 827-831; Date, T., Goodman, J. M., and Wickner, W. T. (1980) Proc. Natl Acad. Sci. USA, 77, 4669-4673] and the B subunit of heat-labile enterotoxin [Palva, T., Hirst, T. R., Hardy, S. J. S., Holmgren, J., and Randall, L. L. (1981) J. Bacteriol. in the press] have been demonstrated previously. An energy requirement for the proteolytic processing of an additional five exported proteins is reported here. Studies utilizing an uncA mutant suggest that the form of energy required is proton-motive force. Thus an energized membrane is probably essential for export of most periplasmic and outer membrane proteins.

Cloning and characterization of the Escherichia coli gene coding for alkaline phosphatase

The Escherichia coli structural gene for alkaline phosphatase, phoA, and a promoter-like mutant of phoA, called pho-1003(Bin) phoA+, were cloned by using plasmid vectors. Initially, these genes were cloned on deoxyribonucleic acid fragments of 28.9 kilobases (kb). Subsequently, they were subcloned on fragments and 4.8 and then 2.7 kilobases. A restriction map was developed, and phoA was localized to a 1.7-kb region. The promoter end of the gene was inferred by its proximity to another gene cloned on the same deoxyribonucleic acid fragment, proC. The stability of the largest plasmid (33.3 kb) was found to be recA dependent, although the subcloned plasmids were stable in a recA+ strain. Synthesis of alkaline phosphatase directed by the phoA+ and pho-1003(Bin) phoA+ plasmids in a phoA deletion strain was assayed under repressing and derepressing levels of phosphate. These data were compared with the copy numbers of the plasmids. It was found that synthesis of alkaline phosphatase was tightly regulated, even under derepressing conditions: a copy number of 17 enabled cells to synthesize only about twofold more enzyme than did cells with 1 chromosomal copy of phoA+. Enzyme levels were also compared for cells containing pho-1003(Bin) phoA+ and phoA+.

Ultrastructural localization of the maltose-binding protein within the cell envelope of Escherichia coli

Logarithmically growing cells of Escherichia coli were fixed with glutaraldehyde and incubated with antimaltose-binding protein Fab coupled to horseradish peroxide (molecular weight of the complex 80,000). The position of this complex within the cell envelope was determined by reacting with diaminobenzidine-H2O2, staining with osmium tetroxide and processing for thin section electron microscopy. The following observations were made: (i) induction of the maltose-binding protein resulted in swelling and staining of the outer membrane; (ii) the swelling and staining was more prominent in short cells, less prominent or absent in long cells; (iii) rare examples exhibited granular staining in the space between the plasma membrane and the peptidoglycan layer. These stainings were observable mainly in pole caps; (iv) a mutant lacking the receptor for phage lambda showed altered staining pattern. Treatment of glutaraldehyde-fixed cells with EDTA-lysozyme prevented the specific labelling of the maltose-binding protein.

ATP-stimulated endoprotease is associated with the cell membrane of E. coli

Despite knowledge of the physiological significance and regulation of protein degradation in bacteria, the pathway of proteolysis and the responsible enzymes are still not known. Degradation of cell proteins in bacterial and animal cells requires continuous ATP production, inhibition of which in Escherichia coli prevents the degradation of normal proteins in growing cells, accelerated breakdown of such proteins in starving cultures and the very rapid breakdown of abnormal proteins. Intracellular proteolysis proceeds by repeated endoproteolytic steps and ATP is required for the initial cleavages of the substrate. We have recently demonstrated ATP stimulation of proteolysis in extracts of bacterial and animal cells. These ATP-stimulated systems seem to be responsible for the rapid degradation of abnormal proteins in vivo, but they may also be involved in the catabolism of normal cell proteins, limited proteolysis, such as the processing of precursors for secreted or membrane proteins, and the selective inactivation of specific proteins, as occurs in the ATP-dependent cleavage of the lambda repressor by the recA protein. We report here that membrane fragments contain an ATP-stimulated protease that degrades cell proteins to large peptides (of molecular weight (MW) 71,500) which are then rapidly hydrolysed to amino acids by soluble ATP-independent enzymes.

K88-mediated binding of Escherichia coli outer membrane fragments to porcine intestinal epithelial cell brush borders

We have examined the interactions between various radiolabeled membrane fractions obtained from an enterotoxigenic Escherichia coli strain and brush borders isolated from porcine intestinal epithelial cells. Outer membrane fragments containing the K88 attachment factor bound tightly to brush borders, whereas cytoplasmic membrane vesicles did not. Three different types of outer membrane preparations were tested: (i) cellular outer membranes isolated from lysozyme spheroplasts, (ii) medium vesicles or outer membrane fragments released into the medium during growth, and (iii) periplasmic vesicles, or outer membrane fragments which were released from the cells during spheroplast formation and were therefore isolated in the periplasmic fraction. Of these fractions, which were heterogeneous, it was always the outer membrane subfraction which bound tightly to brush borders. This binding, which was K88 dependent, may have some physiological significance in view of the association between outer membrane fragments and enterotoxin. Thus, released outer membrane fragments equipped with attachment factors may function as enterotoxin carriers which increase the efficiency with which enterotoxin can be delivered to intestinal epithelial cells.

Cholera toxin is synthesized in precursor form on free polysomes in Vibrio cholerae 569B

Membrane-bound and free polysomes have been isolated from Vibrio cholerae 569B. Nacent polypeptide chains were completed in a cell-free translation mixture containing Escherichia coli S-300 extracts and [3H]leucine or [35S]methionine. Cholera toxin-related polypeptides synthesized in vitro were immunologically detected after treatment with either anti-subunit A or anti-subunit B serum. Immunoreactive translation products were removed from reaction mixtures with formalinized Cowan's strain of Staphylococcus aureus, electrophoresed on sodium dodecyl sulfate-polyacrylamide gels, and visualized by fluorography. Anti-subunit A serum precipitated two major polypeptide species (molecular weights 52,000 and 45,000) from translation mixtures programed with free polysomes, whereas anti-subunit B serum precipitated only the 45,000-molecular-weight polypeptide. No cholera toxin-related polypeptides were detectable in translation mixtures programed with membrane-bound polysomes. Purified subunit A and cholera toxin competed for anti-subunit A binding sites and blocked the immunoprecipitation of the 35S-labeled 52,000- and 45,000-dalton polypeptides from in vitro translation mixtures. The data presented suggest that cholera toxin is synthesized in the cytoplasm in a precursor form on free polysomes and is secreted post-translationally.

A mechanism of protein localization: the signal hypothesis and bacteria

We are studying the molecular mechanism of cellular protein localization. The availability of genetic techniques, such as gene fusion in Escherichia coli, has made this problem particularly amenable to study in this prokaryote. We have constructed a variety of strains in which the gene coding for an outer membrane protein is fused to the gene coding for a normally cytoplasmic enzyme, beta-galactosidase. The hybrid proteins produced by such strains retain beta-galactosidase activity; this activity serves as a simple biochemical tag for studying the localization of the outer membrane protein. In addition, we have exploited phenotypes exhibited by certain fusion strains to isolate mutants that are altered in the process of protein export. Genetic and biochemical analyses of such mutants have provided evidence that the molecular mechanism of cellular protein localization is strinkingly similar in both bacteria and animal cells.

Kinetics and regulation of cell-free alkaline phosphatase synthesis

Regulation of alkaline phosphatase (EC 3.1.3.1) synthesis in a cell-free system from Escherichia coli has been observed. Synthesis from transducing phage deoxyribonucleic acid templates carrying phoA+ occurred in S30 fractions from wild-type or alkaline phosphatase-constitutive mutants. It did not occur in S30) fractions from alkaline phosphatase-negative mutants (phoB). The hybrid gene phoA-lacZ was also subject to phoB control, implying that phoA transcription is regulated. The yield of active alkaline phosphatase per phoA+ gene copy from cell-free synthesis was similar to that of beta-galactosidase. Alkaline phosphatase activity took longer to appear than beta-galactosidase activity. Synthesis of alkaline phosphatase subunits was not delayed, suggesting that a minimum number of subunits are synthesized before formation of active alkaline phosphatase occurs.

Some characteristics of the outer membrane material released by growing enterotoxigenic Escherichia coli

The high-molecular-weight material released into the medium by Escherichia coli AP1, an enterotoxigenic strain of porcine origin, has been isolated and resolved into two clearly distinct fractions, based on sucrose density gradient and differential centrifugation, chemical analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and freeze-fracture electron microscopy. These two fractions, referred to as "medium vesicles" and "medium lipopolysaccharides", were compared with the cellular outer and cytoplasmic membranes, the periplasmic fraction, and the cytoplasmic fraction. The medium vesicles closely resembled outer membrane and accounted for 3 to 5% of the total cellular outer membrane. They contained most of the heat-labile enterotoxin (LT) activity released into the medium by E. coli AP1. The medium lipopolysaccharide consisted mostly of lipopolysaccharide and a small amount of outer membrane and contained relatively little LT activity. Based on experiments with E. coli K-12 strains, in which about 5% of the newly synthesized outer membrane is lost from areas of outer membrane synthesis, it is proposed that enterotoxigenic E. coli strains release LT as part of such newly synthesized outer membrane fragments and that released outer membrane fragments may function as physiologically significant LT carriers.