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.     

Identification of distinct messenger RNAs for nuclear lamin C and a putative precursor of nuclear lamin A

The lamins are the major components of the nuclear matrix and are known as lamins A, B, and C with Mr 72,000, 68,000, and 62,000 when analysed by SDS PAGE. These three polypeptides are very similar, as determined by polypeptide mapping and immunological reactivity. Lamins A and C are so homologous that a precursor-product relationship has been proposed. Using an antiserum against nuclear matrix proteins that specifically immunoprecipitates the three lamins, we examined their synthesis in the rabbit reticulocytes lysate. Four bands of Mr 62,000, 68,000, 70,000, and 74,000 were specifically immunoprecipitated when polysomes or polyadenylated RNA were translated in vitro. By two-dimensional gel electrophoresis, the 68,000- and the 62,000-mol-wt proteins were identified as lamins B and C, respectively, and the 74,000-mol-wt polypeptide had properties of a precursor of lamin A. The mRNAs of lamin C and of the putative precursor of lamin A were completely separated by gel electrophoresis under denaturing conditions, and their respective sizes were determined. These results suggest that lamin A is not a precursor of lamin C.     

Specific purification of elongation factor 2 and isolation of its antibody

Elongation factor 2 (EF-2) was purified from rat liver extracts by affinity chromatography using fragment A of diphtheria toxin as the ligand. Purified EF-2 has a molecular weight of 96,000 and isoelectric point of 6.6-6.8. The sequence of the nineteen N-terminal amino acid is Val-Asn-Phe-Thr-Val-Asp-Gln-Ile-Arg-Ala Ile-Met-Asp-Lys-Lys-Ala-Asn and the C-terminal amino acid is leucine. Purified rat EF-2 modified with ADP-ribose was injected into rabbits to prepare antibodies against EF-2. The anti-EF-2 antibodies can immunoprecipitate with EF-2 from various eukaryotic 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.     

Toxicity of diphtheria toxin-related proteins produced by suppression of nonsense mutations

The Mr = 62,000 diphtheria toxin-related proteins produced from the suppression of nonsense mutations within the tox gene of corynephage beta were purified by affinity chromatography. Except for the toxin 111-sup2-62, the Mr = 62,000 polypeptides were found to have the same specific toxicity as does wild type toxin. 111-sup2-62 was found to have a prolonged lag period prior to the onset of inhibition of protein synthesis and ADP-ribosylation of elongation factor 2. 111-sup2-62 differs from wild type toxin by an amino acid substitution at a site approximatley 47,000 daltons from the NH2 terminus. The data presented provide genetic support for the Boquet-Pappenheimer model (Boquet, P., and Pappenheimer, A. M. Jr (1978) J. Biol. Chem. 251, 5770-5778) of fragment A translocation into the eukaryotic cell cytosol.     

Molecular characterization of the hemolysin determinant of Serratia marcescens.

The nucleotide sequence of a 7.3-kilobase-pair fragment of DNA encoding a hemolytic activity from Serratia marcescens was determined. Two large open reading frames were identified, designated shlA (Serratia hemolysin) and shlB, capable of encoding polypeptides of 165, 056 and 61,897 molecular weight, respectively. Both reading frames were expressed in vivo. The shlB gene product was localized to the outer membrane of Escherichia coli cells harboring the S. marcescens hemolysin determinant. Consistent with this location, a signallike sequence was identified at the N terminus of the polypeptide predicted from the nucleotide sequence of the shlB gene. Hyperexpression of the shlB locus permitted the identification of two shlB-encoded polypeptides of 65,000 and 62,000 molecular weight, respectively. Determination of the N-terminal amino acid sequence of the purified 62,000-molecular-weight protein confirmed that it was the mature form of the ShlB protein initially synthesized as a precursor (65,000-molecular-weight protein). By using polyclonal antisera raised against the purified proteins, ShlA and ShlB were identified in the outer membrane of S. marcescens. The shlA gene product was shown to interact with erythrocyte membranes, confirming it as the hemolysin proper. Both hemolysis and the interaction of ShlA with erythrocyte membranes did, however, require the ShlB function. Progressive deletion of the C terminus of the ShlA protein gradually reduced hemolytic activity until 37% of the amino acids had been removed. Elimination of 54% of the amino acids produced a nonhemolytic protein which, however, was still capable of associating with erythrocyte membranes.     

Gene expression of D-beta-hydroxybutyrate dehydrogenase. II. Incorporation of pre-BDH into mitochondria

beta-hydroxybutyrate dehydrogenase (BDH), a major protein located in the inner mitochondrial membrane is encoded, as most of mitochondrial proteins, in the nuclear genome. It is synthetized on the free polysomes and post-translationally imported into the mitochondria. The neosynthesized protein is a higher molecular weight precursor. The presequence is cleaved by the matrix protease to give the mature protein. The translocation across the mitochondrial membranes needs energy. The results also indicate that cytosolic factors with low molecular weight are essential in the recognition of precursor by mitochondria and to sort out newly synthetized nuclear encoded mitochondrial proteins from others nuclear encoded proteins.     

Biosynthetic pathway of mitochondrial ATPase subunit 9 in Neurospora crassa

Subunit 9 of mitochondrial ATPase (Su9) is synthesized in reticulocyte lysates programmed with Neurospora poly A-RNA, and in a Neurospora cell free system as a precursor with a higher apparent molecular weight than the mature protein (Mr 16,400 vs. 10,500). The RNA which directs the synthesis of Su9 precursor is associated with free polysomes. The precursor occurs as a high molecular weight aggregate in the postribosomal supernatant of reticulocyte lysates. Transfer in vitro of the precursor into isolated mitochondria is demonstrated. This process includes the correct proteolytic cleavage of the precursor to the mature form. After transfer, the protein acquires the following properties of the assembled subunit: it is resistant to added protease, it is soluble in chloroform/methanol, and it can be immunoprecipitated with antibodies to F1-ATPase. The precursor to Su9 is also detected in intact cells after pulse labeling. Processing in vivo takes place posttranslationally. It is inhibited by the uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP). A hypothetical mechanism is discussed for the intracellular transfer of Su9. It entails synthesis on free polysomes, release of the precursor into the cytosol, recognition by a receptor on the mitochondrial surface, and transfer into the inner mitochondrial membrane, which is accompanied by proteolytic cleavage and which depends on an electrical potential across the inner mitochondrial membrane.     

Thermal stability of different forms of diphtheria toxin

Diphtheria toxin and its enzymatically active A fragment have been examined by differential scanning calorimetry. The thermal stability was measured for different forms of these molecules, including tryptically nicked and intact, nucleotide-bound and free, cysteine alkylated, and cysteine oxidized and reduced. Three ranges of denaturation temperature have been observed among the different forms of diphtheria toxin studied, and there is a correlation of the thermal stability of these different forms with their biological activity. At the concentrations used for measurement, all forms of the 62,000-Da diphtheria toxin are irreversibly denatured by heating to 100 degrees C, while free A chain is reversibly denatured. In vivo and in vitro activities of the samples were measured before and after heating, and all were found to retain significant degrees of activity after heating.     

Dual effects of the ricin A chain on protein synthesis in rabbit reticulocyte lysate. Inhibition of initiation and translocation

Ricin A chain caused inhibition of protein synthesis by reticulocyte lysate with concomitant depurination of 28S rRNA. The partial reaction(s) of protein synthesis inhibited was investigated by following the appearance of [35S]methionine from initiator [35S]Met-tRNA into 40S ribosomal subunits, 80S monosomes and polysomes. Ricin A chain caused an accumulation of [35S]Met in monosomes which did not enter polysomes. In these respects the effects of the ricin A chain resembled those of diphtheria toxin, an inhibitor of elongation-factor-2-catalyzed translocation. This is consistent with the previously proposed site of action of ricin as an inhibitor of elongation. However, the inhibitory effects of the ricin A chain and diphtheria toxin are not equivalent because we observed that the rate of formation of the 80S initiation complex was reduced approximately sixfold with the ricin A chain relative to diphtheria toxin. Analysis of methionine-containing peptides bound to 80S monosomes in ricin-A-chain-inhibited and diphtheria-toxin-inhibited lysates, programmed with globin mRNA, revealed a predominance of Met-Val, suggesting that the elongation cycle is inhibited at the translocation step. Translocation was also implicated as the step blocked in both the ricin-A-chain-inhibited and diphtheria-toxin-inhibited lysates, by the finding that nascent peptide chains were unreactive towards puromycin. It is concluded that ricin-A-chain-modified ribosomes are deficient in two protein synthesis partial reactions: the formation of the 80S initiation complex during initiation and the translocation step of the elongation cycle.     

Entry of diphtheria toxin-protein A chimeras into cells

Fusion proteins consisting of diphtheria toxin and a duplicated Fc-binding domain of protein A were made in vitro after amplification of the DNA template by the polymerase chain reaction. The fusion proteins bound avidly to Vero cells coated with antibodies. A fusion protein containing full-length diphtheria toxin was toxic at lower concentrations than diphtheria toxin alone, apparently due to more efficient binding. The enzymatic part of the fusion protein was translocated across the surface membrane upon exposure to low pH. Like authentic diphtheria toxin, the fusion protein formed cation selective channels at low pH. Excess amounts of unlabeled diphtheria toxin inhibited formation of pronase-protected fragments derived from radiolabeled fusion protein. Furthermore, conditions that down-regulate the diphtheria toxin receptors reduced the sensitivity of the cells to the fusion protein, supporting the notion that authentic diphtheria toxin receptors are required. At temperatures below 18 degrees C the toxicity of the fusion protein was strongly reduced, whereas there was no temperature block for authentic diphtheria toxin. Brefeldin A protected Vero cells against the fusion protein but not against diphtheria toxin. The results indicate that the diphtheria toxin receptor is required for efficient toxin translocation even under conditions where the toxin is bound by an alternate binding moiety, and they suggest that the intracellular routing of the fusion protein is different from that of diphtheria toxin.     

Cotranslational secretion of diphtheria toxin and alkaline phosphatase in vitro: involvement of membrane protein(s).

Crude messenger ribonucleic acid fractions isolated from Corynebacterium diphtheriae and Escherichia coli were translated in an E. coli in vitro protein-synthesizing system and yielded precursors of the secreted proteins diphtheria toxin and alkaline phosphatase, respectively. Addition of inverted E. coli inner membrane vesicles to the system during the initial stages of translation resulted in the intravesicular segregation of mature diphtheria toxin and alkaline phosphatase. Outer membrane vesicles or inner membrane vesicles whose cytoplasmic surfaces had been treated with pronase could not mediate transmembrane transfer of diphtheria toxin or alkaline phosphatase. However, inner membrane vesicles isolated from E. coli spheroplasts which had been treated with pronase and inner membrane vesicles complexed with ribosomes during pronase treatment were functional in transmembrane transfer. At temperatures below the phase transition of E. coli membranes, no intravesicular segregation of alkaline phosphatase or diphtheria toxin was observed. The precursor forms of each protein accumulated free from the vesicles. These results suggest that an inner membrane protein, exposed on the cytoplasmic surface, plays an integral role in secretion.     

Lipid interaction of diphtheria toxin and mutants with altered fragment B. 2. Hydrophobic photolabelling and cell intoxication

The membrane insertion of diphtheria toxin and of its B chain mutants crm 45, crm 228 and crm 1001 has been followed by hydrophobic photolabelling with photoactivatable phosphatidylcholine analogues. It was found that diphtheria toxin binds to the lipid bilayer surface at neutral pH while at low pH both its A and B chains also interact with the hydrocarbon chains of phospholipids. The pH dependence of photolabelling of the two protomers is different: the pKa of fragment B is around 5.9 while that of fragment A is around 5.2. The latter value correlates with the pH of half-maximal intoxication of cells incubated with the toxin in acidic mediums. These results suggest that fragment B penetrates into the bilayer first and assists the insertion of fragment A and that the lipid insertion of fragment B is not the rate-controlling step in the process of membrane translocation of diphtheria toxin. crm 45 behaves as diphtheria toxin in the photolabelling assay but, nonetheless, it is found to be three orders of magnitude less toxic than diphtheria toxin on acid-treated cells, suggesting that the 12-kDa COOH-terminal segment of diphtheria toxin is important not only for its binding to the cell receptor but also for the membrane translocation of the toxin. It is suggested that crm 1001 is non-toxic because of a defect in its membrane translocation which occurs at a lower extent and at a lower pH than that of the native toxin; as a consequence crm 1001 may be unable to escape from the endosome lumen into the cytoplasm before the fusion of the endosome with lysosomes.     

Expression cloning of a diphtheria toxin receptor: identity with a heparin-binding EGF-like growth factor precursor.

A monkey cDNA (pDTS) encoding a diphtheria toxin (DT) sensitivity determinant was isolated by expression cloning in mouse L-M cells. Mouse cells are naturally resistant to DT, because they lack functional cell surface receptors for the toxin. Unlike wild-type L-M cells, pDTS-transfected mouse cells are extremely toxin sensitive and specifically bind radioiodinated DT. Intoxication of the transfected cells requires receptor-mediated endocytosis of the bound toxin. The cDNA is predicted to encode an integral membrane protein that is identical to the precursor of a heparin-binding EGF-like growth factor. The DT sensitivity protein is thus a growth factor precursor that DT exploits as a receptor.     

Attachment of the Synapse-Specific Phosphoprotein Protein I to the Synaptic Membrane: A Possible Role of the Collagenase-Sensitive Region of Protein I

The purified synapse-specific phosphoprotein Protein I was previously shown to be degraded by a bacterial collagenase, through a series of intermediates, to a collagenase-resistant fragment of molecular weight about 48,000 containing a phosphorylated serine residue. In this study, a purified synaptic membrane fraction containing Protein I was treated with Cl. histolyticum collagenase; membrane-bound and membrane-free proteins were then phosphorylated using [gamma-32P]ATP and analyzed by SDS-polyacrylamide gel electrophoresis and autoradiography. It was observed that Protein I bound to the synaptic membrane was susceptible to the collagenase and degraded to fragments of molecular weights about 68,000, 62,000, and 48,000; the 68,000 fragment remained bound to the membrane whereas the 62,000 and 48,000 fragments were dissociated from the membrane. These observations suggest that the peptide moiety of mol. wt. 6000, present in the 68,000 fragment but absent from the 62,000 fragment, may play a crucial role in anchoring Protein I to the synaptic membrane.     

Protein translocation by bacterial toxin channels: a comparison of diphtheria toxin and colicin Ia

Regions of both colicin Ia and diphtheria toxin N-terminal to the channel-forming domains can be translocated across planar phospholipid bilayer membranes. In this article we show that the translocation pathway of diphtheria toxin allows much larger molecules to be translocated than does the translocation pathway of colicin Ia. In particular, the folded A chain of diphtheria toxin is readily translocated by that toxin but is not translocated by colicin Ia. This difference cannot be attributed to specific recognition of the A chain by diphtheria toxin's translocation pathway because the translocation pathway also accommodates folded myoglobin.     

Topology of diphtheria toxin in lipid vesicle membranes: a proteolysis study

The diphtheria toxin (DT) membrane topology was investigated by proteolysis experiments. Diphtheria toxin was incubated with asolectin liposomes at pH 5 in order to promote its membrane insertion, and the protein domains located outside the lipid vesicles were digested with proteinase K (which is a non-specific protease). The protected peptides were separated by electrophoresis and identified by microsequence analysis. Their orientation with respect to the lipid bilayer and their accessibility to the aqueous phase were determined by attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). These data, combined with those provided by proteolytic cleavage with a specific protease (endoproteinase Glu-C), led us to propose a topological model of the N-terminal part of the diphtheria toxin B fragment inserted into the lipid membrane. In this model, two a-helices adopt a transmembrane orientation, with their axes parallel to the lipid acyl chains, while a third o-helix could adopt a transmembrane topology only in a small proportion of DT molecules.     

Lipid interaction of diphtheria toxin and mutants with altered fragment B. 1. Liposome aggregation and fusion

The interaction of diphtheria toxin and its cross-reacting mutants crm 45,228 and 1001 with small unilamellar vesicles has been followed by a turbidity assay, electron microscopy, fluorescence energy transfer and membrane permeability. All toxins at pH lower than 6 induce the aggregation and fusion of liposomes containing negatively charged phospholipids; crm 45 and crm 1001 are less potent than diphtheria toxin. Isolated diphtheria toxin fragment B is very effective while isolated fragment A is ineffective. Liposome fusion induced by the toxins at low pH occurs without release of the internal content implying that fusion does not involve vesicle breakage and resealing. The pH dependence of the membrane interaction of diphtheria toxin monitored by turbidity is in close agreement with that monitored by fluorescence energy transfer. It shows that diphtheria toxin can alter the lipid bilayer structure in the pH interval 5-6. This pH range occurs in endosomes and suggests that histidyl and carboxyl residues are likely to be involved in the conformational change of diphtheria toxin triggered by acidic pH.     

Biosynthesis of mitochondrial porin and insertion into the outer mitochondrial membrane of Neurospora crassa

Mitochondrial porin, the major protein of the outer mitochondrial membrane is synthesized by free cytoplasmic polysomes. The apparent molecular weight of the porin synthesized in homologous or heterologous cell-free systems is the same as that of the mature porin. Transfer in vitro of mitochondrial porin from the cytosolic fraction into the outer membrane of mitochondria could be demonstrated. Before membrane insertion, mitochondrial porin is highly sensitive to added proteinase; afterwards it is strongly protected. Binding of the precursor form to mitochondria occurs at 4 degrees C and appears to precede insertion into the membrane. Unlike transfer of many precursor proteins into or across the inner mitochondrial membrane, assembly of the porin is not dependent on an electrical potential across the inner membrane.     

Characterization of purified Shiga toxin from Shigella dysenteriae 1

Shiga toxin was purified from the culture supernatant of Shigella dysenteriae 1 by ammonium sulfate fractionation, DEAE-cellulose column chromatography and repeated chromatofocusing column chromatography. About 1.6 mg of purified Shiga toxin was obtained from 15 liters of culture with a yield of about 27%. The molecular weight of purified Shiga toxin was estimated to be 62,000. The toxin consisted of A and B subunits with molecular weights of about 30,000 and 5,000-6,000, respectively. The isoelectric point of purified Shiga toxin was 7.0. Purified Shiga toxin showed the following biological activities: lethal toxicity to mice when injected intraperitoneally with an LD50 of 28 ng per mouse; cytotoxicity to Vero cells, killing about 50% of the cells at 1 pg and all of the cells at 10 pg; and fluid accumulation in rabbit ileal loops at a concentration of more than 1 microgram.