Use of gene fusion to study secretion of maltose-binding protein into Escherichia coli periplasm

Helicobacter pylori ABC transporter: effect of allelic exchange mutagenesis on urease activity

Helicobacter pylori urease requires nickel ions in the enzyme active site for catalytic activity. Nickel ions must, therefore, be actively acquired by the bacterium. NixA (high-affinity nickel transport protein)-deficient mutants of H. pylori retain significant urease activity, suggesting the presence of alternate nickel transporters. Analysis of the nucleotide sequence of the H. pylori genome revealed a homolog of NikD, a component of an ATP-dependent nickel transport system in Escherichia coli. Based on this sequence, a 378-bp DNA fragment was PCR amplified from H. pylori genomic DNA and used as a probe to identify an H. pylori lambda ZAPII genomic library clone that carried these sequences. Four open reading frames of 621, 273, 984, and 642 bp (abcABCD) were revealed by sequencing and predicted polypeptides of 22.7, 9.9, 36.6, and 22.8 kDa, respectively. The 36.6-kDa polypeptide (AbcC) has significant homology (56% amino acid sequence identity) to an E. coli ATP-binding protein component of an ABC transport system, while none of the other putative proteins are significantly homologous to polypeptides in the available databases. To determine the possible contribution of these genes to urease activity, abcC and abcD were each insertionally inactivated with a kanamycin resistance (aphA) cassette and allelic exchange mutants of each gene were constructed in H. pylori UMAB41. Mutation of abcD resulted in an 88% decrease in urease activity to 27 +/- 31 mumol of NH3/min/mg of protein (P < 0.0001), and a double mutant of nixA and abcC resulted in the near abolishment of urease activity (1.1 +/- 1.4 mumol of NH3/min/mg of protein in the double mutant versus 228 +/- 92 mumol of NH3/min/mg of protein in the parent [P < 0.0001]). Synthesis of urease apoenzyme, however, was unaffected by mutations in any of the abc genes. We conclude that the abc gene cluster, in addition to nixA, is involved in production of a catalytically active urease.

Involvement of stress protein PspA (phage shock protein A) of Escherichia coli in maintenance of the protonmotive force under stress conditions

The expression of specific PhoE mutant proteins leads to induction of the expression of the psp operon of Escherichia coli and the export of various plasmid-encoded precursors is retarded in a pspA mutant strain. Here, we have investigated the specific role of various Psp proteins in the export process. PspB and PspC are both inner membrane proteins that are involved in the regulation of the transcription of the psp operon. Precursor PhoE translocation was retarded in a pspB mutant strain to a similar extent as in a pspA mutant strain. The reduced translocation efficiencies in the various psp mutants could be complemented by expression of PspA from a plasmid, indicating that only PspA is required for efficient translocation. Mutant prePhoE proteins that can be translocated independently of the deltamu H+ appeared to translocate equally efficiently in a wild-type and in a pspA mutant strain. Furthermore, quantitative in vivo determination of the deltamu H+ showed that it specifically decreased in a pspA mutant strain upon expression of plasmid-encoded (mutant) prePhoE protein. Apparently, the translocation defects observed in a psp mutant strain are caused by a decrease of the delta mu H+ and PspA functions by maintaining the delta mu H+ under these conditions.

Overproduction of NlpE, a new outer membrane lipoprotein, suppresses the toxicity of periplasmic LacZ by activation of the Cpx signal transduction pathway

The LamB-LacZ-PhoA tripartite fusion protein is secreted to the periplasm, where it exerts a toxicity of unknown origin during high-level synthesis in the presence of the inducer maltose, a phenotype referred to as maltose sensitivity. We selected multicopy suppressors of this toxicity that allow growth of the tripartite fusion strains in the presence of maltose. Mapping and subclone analysis of the suppressor locus identified a previously uncharacterized chromosomal region at 4.7 min that is responsible for suppression. DNA sequence analysis revealed a new gene with the potential to code for a protein of 236 amino acids with a predicted molecular mass of 25,829 Da. The gene product contains an amino-terminal signal sequence to direct the protein for secretion and a consensus lipoprotein modification sequence. As predicted from the sequence, the suppressor protein is labeled with [3H]palmitate and is localized to the outer membrane. Accordingly, the gene has been named nlpE (for new lipoprotein E). Increased expression of NlpE suppresses the maltose sensitivity of tripartite fusion strains and also the extracytoplasmic toxicities conferred by a mutant outer membrane protein, LamBA23D. Suppression occurs by activation of the Cpx two-component signal transduction pathway. This pathway controls the expression of the periplasmic protease DegP and other factors that can combat certain types of extracytoplasmic stress.

Beta-galactosidase is inactivated by intermolecular disulfide bonds and is toxic when secreted to the periplasm of Escherichia coli

The wild-type LamB-LacZ hybrid protein inhibits the export machinery upon induction when assayed by biochemical and genetic techniques, a phenotype referred to as hybrid protein jamming. This hybrid protein also renders cells sensitive to growth in the presence of the inducer maltose, presumably because of the jamming. We constructed a new version of this fusion by adding alkaline phosphatase, encoded by phoA, to the C terminus of the LamB-LacZ hybrid protein. This tripartite protein, LamB-LacZ-PhoA, is as toxic to cells as the hybrid LamB-LacZ; however, it does not jam at temperatures greater than 33 degrees C. Extreme C-terminal sequences of LacZ function as a critical folding domain and are therefore responsible for stabilizing the LacZ structure. To determine if this region of LacZ is important for jamming, we recombined a late nonsense mutation (X90) onto the hybrid construct. We found the toxicity of this new hybrid, LamB-LacZX90, to be nearly identical to that of the full-length protein, but it also does not jam the secretion machinery. This suggests that jamming is caused by LacZ folding. We found no inhibition of secretion in the tripartite and X90 fusion strains at 37 degrees C, suggesting that the toxicity of the new fusions is novel. Under these conditions, the tripartite and X90 fusion proteins form disulfide-bonded aggregates with high molecular weights in the periplasm. Accordingly, we believe that LacZ disrupts some essential function(s) in the periplasm.

Inefficient membrane targeting, translocation, and proteolytic processing by signal peptidase of a mutant preproparathyroid hormone protein

A preproparathyroid hormone allele from a patient with familial isolated hypoparathyroidism was shown to have a single point mutation in the hydrophobic core of the signal sequence. This mutation, changing a cysteine to an arginine codon at the -8 position of the signal peptide, was associated with deleterious effects on the processing of preproparathyroid hormone to proparathyroid hormone in vitro. To examine the biochemical consequence(s) of this mutation, proteins produced by cell-free translation of wild-type and mutant cRNAs were used in assays that reconstitute the early steps of the secretory pathway. We find that the mutation impairs interaction of the nascent protein with signal recognition particle and the translocation machinery. Moreover, cleavage of the mutant signal sequence by solubilized signal peptidase is ineffective. The consequence of this mutation on processing and secretion of parathyroid hormone is confirmed in intact cells by pulse-chase experiments following transient expression of the mutant protein in COS-7 cells. The inability of the mutant signal sequence, however, to interfere with the targeting and processing of other secreted proteins does not support obstruction of the translocation apparatus as the mechanism underlying the dominant mode of inheritance of hypoparathyroidism in this family.

Translational regulation of a recombinant operon containing human platelet-derived growth factor (PDGF)-encoding genes in Escherichia coli: genetic titration of the peptide chains of the heterodimer AB

A new strategy is described for the production of recombinant heteromultimeric proteins using Escherichia coli as host. A recombinant operon was constructed containing modified cDNA sequences encoding the mature A and B chains of human platelet-derived growth factor (PDGF). The relative expression rates of the PDGF genes were varied over a range equivalent to A:B ratios from 0.8 to 3.7 by means of translational regulation. This was achieved using two different translational initiation sequences (TIS) upstream from the respective coding regions, one derived from the E. coli atpE translational initiation region, and the other containing a sequence with extended complementarity to the 3' end of the 16S rRNA. The generation of mature PDGF A and B chains in different relative amounts in E. coli provided the basis for developing a novel procedure for the production of the biologically active PDGF heterodimer AB in large quantities. The general strategy is applicable to the preparation of a wide range of heteromultimeric complexes. Moreover, the described PDGF operon constitutes a compact and versatile model system for studies of the posttranscriptional regulation of gene expression.

Secretion of eukaryotic growth hormones in Escherichia coli is influenced by the sequence of the mature proteins

We report the construction of secretion plasmids expressing the fusion proteins, OmpA::pGH (pSpGH.01) and OmpA::hGH (phGH.01), and compare the secretion of mature porcine growth hormone (pGH) and human growth hormone (hGH) employing Escherichia coli. E. coli [phGH.01] secreted 10-15 micrograms hGH/ml/A600 cells into the periplasmic space, representing 30% of total periplasmic proteins. E. coli [pSpGH.01], however, secreted 30-fold less mature pGH. On the basis that both pSpGH.01 and phGH.01 are stably maintained in E. coli and in vitro transcription/translation data showed equivalent expression of OmpA::pGH and OmpA::hGH precursors, we attribute the higher secretion of hGH to the translocation-competent OmpA::hGH protein configuration. Two OmpA::GHF (growth hormone fusion) precursors, OmpA::GHF.02 and OmpA::GHF.03, both with hGH helix 3/helix 4 together instead of the pGH equivalent, secreted mature proteins as efficiently as OmpA::hGH. We propose that hGH helices 3 and 4 in these OmpA::GHF precursors play a major role in the folding of the precursor to a translocation-competent state, mimicking the translocation-competent nature of the OmpA::hGH precursor.

Molecular characterization and expression of p23 (OspC) from a North American strain of Borrelia burgdorferi

We have found that sera from patients with early stages of Lyme disease contain predominant immunoglobulin M reactivity to a major 23-kDa protein (p23) from Borrelia burgdorferi 2591 isolated in Connecticut. To characterize this immunodominant antigen, we cloned and sequenced p23 and found it to be 83% identical by nucleotide sequence and 75% identical by amino acid sequenced to pC (recently renamed OspC), an abundantly expressed protein on the outer surface of PKo, a European strain of B. burgdorferi (B. Wilske, V. Preac-Mursic, S. Jauris, A. Hofmann, I. Pradel, E. Soutschek, E.Schwab, G. Will, and G. Wanner, Infect. Immun. 61:2182-2191, 1993). In addition, immunoelectron microscopy localized p23 to the outer membrane, confirming that p23 is the strain 2591 homolog of OspC. The North American strain B31, commonly used in serologic assays for Lyme disease, does not express OspC. Northern (RNA) blot analysis detected low levels of ospC mRNA in B31, and DNA sequencing of the ospC gene from B31 revealed a 54-bp deletion in the upstream regulatory region, possibly accounting for the low transcriptional activity of ospC. The ospC coding region from B31 was cloned and antibody-reactive OspC was expressed in Escherichia coli. An immunoglobulin M enzyme-linked immunosorbent assay using recombinant OspC as the target antigen shows promise for the serodiagnosis of early stages of Lyme disease.

Mutations in the alpha subunit of RNA polymerase that affect the regulation of porin gene transcription in Escherichia coli K-12

The two-component regulatory system consisting of OmpR and EnvZ controls the differential expression of major outer membrane porin proteins OmpF and OmpC of Escherichia coli K-12. We have isolated and characterized two mutations in rpoA, the gene encoding the alpha subunit of RNA polymerase, that decrease the expression of OmpF. These mutations have a number of properties that distinguish them from previously isolated rpoA mutations that affect porin expression. The rpoA203 mutation decreases the expression of porin genes ompF and ompC and also decreases the expression of the malE and phoA genes. In contrast, rpoA207 decreases the expression of ompF but does not affect ompC, malE, or phoA transcription. Our results suggest that mutations at various positions in the alpha subunit may affect the OmpR-dependent transcription of ompF and ompC differently and may be useful for analyzing the mechanism underlying their differential expression in response to medium osmolarity.

Proteus mirabilis urease: histidine 320 of UreC is essential for urea hydrolysis and nickel ion binding within the native enzyme

Proteus mirabilis urease, a nickel-containing enzyme, has been established as a critical virulence determinant in urinary tract infection. An amino acid sequence (residues 308 to 327: TVDEHLDMLMVCHHLDPSIP) within the large urease subunit, UreC, is highly conserved for every urease examined thus far and has been suggested to reside within the enzyme active site. Histidine residues have been postulated to play a role in catalysis by coordinating Ni2+ ions. To test this hypothesis, oligonucleotide-directed mutagenesis was used to change amino acid His-320 to Leu-320 within UreC. The base change (CAT for His-320 to CTT for Leu-320) was confirmed by DNA sequencing. The recombinant and mutant proteins were expressed at similar levels in Escherichia coli as detected by Western blotting (immunoblotting) of denaturing and nondenaturing gels. Specific activities of the enzymes were quantitated after partial purification. Strains expressing the mutant enzyme showed no detectable activity, whereas strains expressing the recombinant enzyme hydrolyzed urea at 149 mumol of NH3 per min per mg of protein. In addition, the mutant enzyme was able to incorporate only about one-half (58%) of the amount of 63Ni2+ incorporated by the active recombinant enzyme. While the mutation of His-320 to Leu-320 within UreC does not affect expression or assembly of urease polypeptide subunits UreA, UreB, and UreC His-320 of UreC is required for urea hydrolysis and proper incorporation of Ni2+ into apoenzyme.

The membrane binding C-terminus of protein A from Staphylococcus aureus affects its cellular localization and causes structural deformation when expressed in Escherichia coli

Protein A from Staphylococcus aureus is a powerful diagnostic reagent and has several uses in human disease therapy. Expression in non-pathogenic Escherichia coli containing recombinant plasmids coding for this protein has increased its availability, but can reduce the stability of the plasmid-bearing host. By employing immune electron microscopy, we have determined that E. coli containing stable plasmids coding for a truncated version of protein A, without the membrane binding site, secrete this protein through the cytoplasmic membrane and into the periplasmic space, where it accumulates. E. coli containing unstable plasmids, however, which code for the complete protein including the membrane-binding site, target the protein into the cytoplasmic membrane. This accumulation of protein A in the E. coli cytoplasmic membrane inhibits the formation of septa between dividing cells and results in aberrant elongated, multi-chromosomal forms.

Expression of catalytically active recombinant Helicobacter pylori urease at wild-type levels in Escherichia coli

The genes encoding Helicobacter pylori urease, a nickel metalloenzyme, have been cloned and expressed in Escherichia coli. Enzymatic activity, however, has been very weak compared with that in clinical isolates of H. pylori. Conditions under which near wild-type urease activity was achieved were developed. E. coli. SE5000 containing recombinant H. pylori urease genes was grown in minimal medium containing no amino acids, NiCl2 was added to 0.75 microM, and structural genes ureA and ureB (pHP902) were overexpressed in trans to the complete urease gene cluster (pHP808). Under these conditions, E. coli SE5000 pHP808/pHP902) expressed a urease activity up to 87 mumol of urea per min per mg of protein (87 U/mg of protein), a level approaching that of wild-type H. pylori UMAB41 (100 U/mg of protein), from which the genes were cloned. Poor catalytic activity of recombinant clones grown in Luria broth or M9 medium containing 0.5% Casamino Acids was due to chelation of nickel ions by medium components, particularly histidine and cysteine. In cultures containing these amino acids, 63Ni2+ was prevented from being transported into cells and was not incorporated into urease protein. As a consequence, M9 minimal medium cultures containing histidine or cysteine produced only 0.05 and 0.9%, respectively, of active urease produced by control cultures containing no amino acids. We conclude that recombinant H. pylori urease is optimally expressed when Ni2+ transport is not inhibited and when sufficient synthesis of urease subunits UreA and UreB is provided.

Evaluating the fate of genetically modified microorganisms in the environment: are they inherently less fit?

Genetically modified microorganisms hold great promise for environmental applications. Nonetheless, some may have unintended adverse effects. Of particular concern for risk assessment is the simple fact that microorganisms are self-replicating entities, so that it may be impossible to control an adverse effect simply by discontinuing further releases of the organism. It has been suggested, however, that genetically modified microorganisms will be poor competitors and therefore unable to persist in the wild due to energetic inefficiency, disruption of genomic coadaptation, or domestication. Many studies support the hypothesis that genetically modified microorganisms are less fit than their progenitors, but there are a few noteworthy counter-examples in which genetic modifications unexpectedly enhance competitive fitness. Furthermore, subsequent evolution may eliminate the maladaptive effects of some genes, increasing the likelihood that a modified organism or its engineered genes will persist. Evaluating the likelihood that a genetically modified microorganism or its engineered genes will persist is a complex ecological and evolutionary problem. Therefore, an efficient regulatory framework would require such evaluations only when there are plausible scenarios for significant adverse environmental effects.

Protein secretion in Bacillus species

Bacilli secrete numerous proteins into the environment. Many of the secretory proteins, their export signals, and their processing steps during secretion have been characterized in detail. In contrast, the molecular mechanisms of protein secretion have been relatively poorly characterized. However, several components of the protein secretion machinery have been identified and cloned recently, which is likely to lead to rapid expansion of the knowledge of the protein secretion mechanism in Bacillus species. Comparison of the presently known export components of Bacillus species with those of Escherichia coli suggests that the mechanism of protein translocation across the cytoplasmic membrane is conserved among gram-negative and gram-positive bacteria differences are found in steps preceding and following the translocation process. Many of the secretory proteins of bacilli are produced industrially, but several problems have been encountered in the production of Bacillus heterologous secretory proteins. In the final section we discuss these problems and point out some possibilities to overcome them.

Expression of the pspA gene stimulates efficient protein export in Escherichia coli

Expression of several mutant forms of outer membrane protein PhoE of Escherichia coli, which are disturbed in normal biogenesis, resulted in high expression of a 26 kDa protein. This 26 kDa protein fractionated as a peripherally bound inner membrane protein. It appeared to be identical to a previously identified protein (PspA = phage shock protein A) of unknown function that is induced upon infection of E. coli with filamentous phages. PspA was not expressed upon synthesis of mutant PhoE proteins in a secB mutant, nor upon expression of a PhoE mutant that lacks the signal sequence, suggesting that entrance into the export pathway of prePhoE is essential for induction. PspA synthesis was also induced under other conditions that are known to block the export apparatus, i.e. in secA, secD and secF mutants when grown at their non-permissive temperature or upon induction of the synthesis of MalE-LacZ or LamB-LacZ hybrid proteins. The inducing conditions for PspA synthesis suggested a role for this protein in export. In vivo pulse-chase experiments showed that the translocation of (mutant) prePhoE and of the precursors of other exported proteins was retarded in a pspA mutant strain. Also, in in vitro translocation assays, a role for PspA in protein transport could be demonstrated.

Expression of heterologous peptides at two permissive sites of the MaIE protein: antigenicity and immunogenicity of foreign B-cell and T-cell epitopes

We previously determined a number of 'permissive' sites in the periplasmic maltose-binding protein (MalE) from Escherichia coli. These sites accept the insertion of heterologous peptides without major deleterious consequences for the activities, structure and cellular location of the protein. This study explores the versatility of two such permissive sites for the synthesis of foreign peptides, and examines the antigenicity and the immunogenicity of the inserts. One site is located after amino acid 133 (aa133) of MalE, and the other after aa303. Both sites tolerate inserts of up to at least 70 aa and accept sequences of different natures. Hydrophobic aa sequences are accepted, although strongly hydrophobic sequences, such as the Sendai virus F protein membrane anchor, affected export. We compared the antigenic and the immunogenic properties of peptides derived from the coat proteins of HBV and poliovirus which contain well defined B-cell epitopes. Specific monoclonal antibodies show that the antigenic properties of the inserted B-cell epitopes were different at the two sites. Despite these differences, the inserted peptides elicited strong and comparable antibody responses in mice against the corresponding synthetic peptides. In this case, and with these criteria, the molecular context of the peptides did not affect the immunogenicity of B-cell epitopes. We show for the first time that when a foreign peptide carrying a T-cell epitope was inserted in MalE, the hybrid proteins can elicit a T-cell response against the foreign peptide both in vivo and in vitro. Furthermore, the MalE hybrid was as efficient as free peptide in stimulating T-cell hybridomas in vitro. The MalE vectors provide a powerful genetic system to study how the position and the conformation of a peptide within a protein affect the B-cell and T-cell responses.

Nucleotide sequence of dcrA, a Desulfovibrio vulgaris Hildenborough chemoreceptor gene, and its expression in Escherichia coli

The amino acid sequence of DcrA (Mr = 73,000), deduced from the nucleotide sequence of the dcrA gene from the anaerobic, sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough, indicates a structure similar to the methyl-accepting chemotaxis proteins from Escherichia coli, including a periplasmic NH2-terminal domain (Mr = 20,700) separated from the cytoplasmic COOH-terminal domain (Mr = 50,300) by a hydrophobic, membrane-spanning sequence of 20 amino acid residues. The sequence homology of DcrA and these methyl-accepting chemotaxis proteins is limited to the COOH-terminal domain. Analysis of dcrA-lacZ fusions in E. coli by Western blotting (immunoblotting) and activity measurements indicated a low-level synthesis of a membrane-bound fusion protein of the expected size (Mr = approximately 137,000). Expression of the dcrA gene under the control of the Desulfovibrio cytochrome c3 gene promoter and ribosome binding site allowed the identification of both full-length DcrA and its NH2-terminal domain in E. coli maxicells.

Multicopy suppression: an approach to understanding intracellular functioning of the protein export system

Escherichia coli genes were cloned onto a multicopy plasmid and selected by the ability to restore growth and protein export defects caused by a temperature-sensitive secY or secA mutation. When secA51 was used as the primary mutation, only clones carrying groE, which specifies the chaperonin class of heat shock protein, were obtained. Selection using secY24 yielded three major classes of genes. The first class encodes another heat shock protein, HtpG; the most frequently obtained second class encodes a neutral histonelike protein, H-NS; and the third class, msyB, encodes a 124-residue protein of which 38 residues are acidic amino acids. Possible mechanisms of suppression as well as the significance and limitations of the multicopy suppression approach are discussed.

Molecular analysis and expression of a Borrelia burgdorferi gene encoding a 22 kDa protein (pC) in Escherichia coli

We describe the cloning and expression of the pc gene which encodes a major immunodominant protein of Borrelia burgdorferi, the causative agent of Lyme borreliosis. The pC protein was purified from lysates of B. burgdorferi strain PKo. After tryptic digestion of the pC protein the resulting oligopeptides were applied to a gas-phase sequenator. Thus partial amino acid sequences were obtained. The deduced oligonucleotides were used as hybridization probes. After Southern blotting a reactive band in the 3 kb range of PstI-digested genomic DNA was detected. The insertion of these fragments into pUC vectors finally resulted in pc-positive Escherichia coli clones. The gene (encoding a protein with 212 amino acids) was expressed in E. coli with varying deletions at the 5' end. A sequence comparison with other outer membrane proteins of B. burgdorferi indicates a processing of pC that is similar to that of lipoproteins.

Cloning, mapping, and characterization of the Escherichia coli prc gene, which is involved in C-terminal processing of penicillin-binding protein 3

The prc gene, which is involved in cleavage of the C-terminal peptide from the precursor form of penicillin-binding protein 3 (PBP 3) of Escherichia coli, was cloned and mapped at 40.4 min on the chromosome. The gene product was identified as a protein of about 80 kDa in maxicell and in vitro systems. Fractionation of the maxicells producing the product suggested that the product was associated with the periplasmic side of the cytoplasmic membrane. This was consistent with the notion that the C-terminal processing of PBP 3 probably occurs outside the cytoplasmic membrane: the processing was found to be dependent on the secY and secA functions, indicating that the prc product or PBP 3 or both share the translocation machinery with other extracytoplasmic proteins. DNA sequencing analysis of the prc gene region identified an open reading frame, with two possible translational starts 6 bp apart from each other, that could code for a product with a calculated molecular weight of 76,667 or 76,432. The prc mutant was sensitive to thermal and osmotic stresses. Southern analysis of the chromosomal DNA of the mutant unexpectedly revealed that the mutation was a deletion of the entire prc gene and thus that the prc gene is conditionally dispensable. The mutation resulted in greatly reduced heat shock response at low osmolarity and in leakage of periplasmic proteins.

Characterization and sequence of the Escherichia coli stress-induced psp operon

We describe a new Escherichia coli operon, the phage shock protein (psp) operon, which is induced in response to heat, ethanol, osmotic shock and infection by filamentous bacteriophages. The operon includes at least four genes: pspA, B, C and E. PspA associates with the inner membrane and has the heptad repeats characteristic of proteins that can form coiled coils. The operon encodes a factor that activates psp expression, and deletion analyses indicate that this protein is PspC; PspC is predicted to possess a leucine zipper, a motif present in many eukaryotic transcription factors. The pspE gene is expressed in response to stress as part of the operon, but is also transcribed from its own promoter under normal conditions. In vitro studies suggest that PspA and C are modified in vivo. Expression of the psp genes does not require the heat shock sigma factor, sigma32. The increased duration of psp induction in a sigma32 mutant suggests that a product (or products) of the heat shock response down-regulates expression of the operon.

Use of alkaline phosphatase fusions to study protein secretion in Bacillus subtilis

We have constructed a vector designed to facilitate the study of protein secretion in Bacillus subtilis. This vector is based on a translational fusion between the expression elements and signal sequence of Bacillus amyloliquefaciens alkaline protease and the mature coding sequence for Escherichia coli alkaline phosphatase (phoA). We show that export of alkaline phosphatase from B. subtilis depends on a functional signal sequence and that alkaline phosphatase activity depends upon secretion. The vector design facilitates the insertion of heterologous coding sequences between the signal and phoA to generate three-part translational fusions. Such phoA fusions are easily analyzed by monitoring alkaline phosphatase activity on agar plates or in culture supernatants or by immunological detection. Exploitation of this methodology, which has proven to be extremely useful in the study of protein secretion in E. coli, has a variety of applications for studying protein secretion in B. subtilis.

Genetic analysis of the enterobactin gene cluster in Shigella flexneri

The genes for transport and synthesis of the phenolate siderophore enterobactin are present on the chromosomes of both Ent+ and Ent- clinical isolates of Shigella flexneri. To determine why Ent- S. flexneri isolates fail to express a functional enterobactin system, the structure and expression of enterobactin genes were examined. Several alterations may be responsible for the inability of S. flexneri to express enterobactin. (i) The mRNA levels produced from the entC and fepB genes were not derepressed in low-iron media. (ii) DNA sequence analysis of the entC-fepB intergenic region revealed an 83-bp noncontiguous deletion in the putative fepB leader sequence. The deleted sequences are in a region which would be capable of forming extensive stem-and-loop structures. (iii) An amber codon in the 5' portion of the entC gene was also detected. (iv) An IS1 element, previously mapped to the Ent- S. flexneri enterobactin gene cluster, was found to lie within a potential transcriptional termination sequence in the entF-fepE intergenic region. (v) A mutation responsible for the inactivation of the entF gene was mapped to the entF coding region by using entF hybrid gene fusions. (vi) A comparison of outer membrane profiles from an E. coli strain harboring the cloned fepA gene from either an Ent+ or Ent- Shigella isolate revealed that the Ent- FepA protein is present in the outer membrane but at greatly reduced levels than that of the Ent+ FepA protein. This observation, along with additional studies, suggests that the Ent- FepA may be defective in translation and/or translocation.

Structural and functional dissection of Sec62p, a membrane-bound component of the yeast endoplasmic reticulum protein import machinery

SEC62 is required for the import of secretory protein precursors into the endoplasmic reticulum (ER) of Saccharomyces cerevisiae. The DNA sequence of SEC62 predicts a 32-kDa polypeptide with two potential membrane-spanning segments. Two antisera directed against different portions of the SEC62 coding region specifically detected a 30-kDa polypeptide in cell extracts. A combination of subcellular fractionation, detergent and alkali extraction, and indirect immunofluorescence studies indicated that Sec62p is intimately associated with the ER membrane. Protease digestion of intact microsomes and analysis of the oligosaccharide content of a set of Sec62p-invertase hybrid proteins suggested that Sec62p spans the ER membrane twice, displaying hydrophilic amino- and carboxy-terminal domains towards the cytosol. Sec62p-invertase hybrid proteins that lack the Sec62p C terminus failed to complement the sec62-l mutation and dramatically inhibited the growth of sec62-l cells at a normally permissive temperature. The inhibitory action of toxic Sec62p-invertase hybrids was partially counteracted by the overexpression of Sec63p. Taken together, these data suggest that the C-terminal domain of Sec62p performs an essential function and that the N-terminal domain associates with other components of the translocation machinery, including Sec63p.

Genetic analysis of an MDR-like export system: the secretion of colicin V

The extracellular secretion of the antibacterial toxin colicin V is mediated via a signal sequence independent process which requires the products of two linked genes: cvaA and cvaB. The nucleotide sequence of cvaB reveals that its product is a member of a subfamily of proteins, involved in the export of diverse molecules, found in both eukaryotes and prokaryotes. This group of proteins, here referred to as the 'MDR-like' subfamily, is characterized by the presence of a hydrophobic region followed by a highly conserved ATP binding fold. By constructing fusions between the structural gene for colicin V, cvaC, and a gene for alkaline phosphatase, phoA, lacking its signal sequence, it was determined that 39 codons in the N-terminus of cvaC contained the structural information to allow CvaC-PhoA fusion proteins to be efficiently translocated across the plasma membrane of Escherichia coli in a CvaA/CvaB dependent fashion. This result is consistent with the location of point mutations in the cvaC gene which yielded export deficient colicin V. The presence of the export signal at the N-terminus of CvaC contrasts with the observed C-terminal location of the export signal for hemolysin, which also utilizes an MDR-like protein for its secretion. It was also found that the CvaA component of the colicin V export system shows amino acid sequence similarities with another component involved in hemolysin export, HlyD. The role of the second component in these systems and the possibility that other members of the MDR-like subfamily will also have corresponding second components are discussed. A third component used in both colicin V and hemolysin extracellular secretion is the E. coli host outer membrane protein, TolC.

Structural and functional dissection of Sec62p, a membrane-bound component of the yeast endoplasmic reticulum protein import machinery

SEC62 is required for the import of secretory protein precursors into the endoplasmic reticulum (ER) of Saccharomyces cerevisiae. The DNA sequence of SEC62 predicts a 32-kDa polypeptide with two potential membrane-spanning segments. Two antisera directed against different portions of the SEC62 coding region specifically detected a 30-kDa polypeptide in cell extracts. A combination of subcellular fractionation, detergent and alkali extraction, and indirect immunofluorescence studies indicated that Sec62p is intimately associated with the ER membrane. Protease digestion of intact microsomes and analysis of the oligosaccharide content of a set of Sec62p-invertase hybrid proteins suggested that Sec62p spans the ER membrane twice, displaying hydrophilic amino- and carboxy-terminal domains towards the cytosol. Sec62p-invertase hybrid proteins that lack the Sec62p C terminus failed to complement the sec62-l mutation and dramatically inhibited the growth of sec62-l cells at a normally permissive temperature. The inhibitory action of toxic Sec62p-invertase hybrids was partially counteracted by the overexpression of Sec63p. Taken together, these data suggest that the C-terminal domain of Sec62p performs an essential function and that the N-terminal domain associates with other components of the translocation machinery, including Sec63p.

Export and purification of a cytoplasmic dimeric protein by fusion to the maltose-binding protein of Escherichia coli

A hybrid between the maltose-binding protein (MalE) of Escherichia coli and the gene 5 protein (G5P) of phage M13 was constructed at the genetic level. MalE is a monomeric and periplasmic protein while G5P is dimeric and cytoplasmic. The hybrid (MalE-G5P) was synthesized in large amounts from a multicopy plasmid and efficiently exported into the periplasmic space of E. coli. The export was dependent on the integrity of the signal peptide. MalE-G5P was purified from a periplasmic extract by affinity chromatography on cross-linked amylose, with a yield larger than 50,000 molecules/E. coli cell. The hybrid specifically bound denatured but not double-stranded DNA cellulose, as native G5P. Sedimentation velocity and gel-filtration experiments showed that MalE-G5P exists as a dimer. Thus, it was possible to efficiently translocate through the membrane a normally cytoplasmic and dimeric protein, by fusion to MalE. Moreover, the passenger protein kept its activity, specificity and quaternary structure in the purified hybrid. MalE-G5P will enable the study of mutant G5P that no longer binds single-stranded DNA and therefore cannot be purified by DNA-cellulose chromatography.

Escherichia coli sec mutants accumulate a processed immature form of maltose-binding protein (MBP), a late-phase intermediate in MBP export

Protein translocation across the Escherichia coli cytoplasmic membrane may consist of several temporally or topographically distinct steps. Although early events in the translocation pathway have been characterized to some extent, the mechanisms responsible for the trans-bilayer movement of a polypeptide are only poorly understood. This article reports on our attempts to dissect the translocation pathway in vivo. A processed form of maltose-binding protein (MBP) was detected in the spheroplasts of secY and secA temperature-sensitive mutant cells that had been pulse-labeled at the permissive temperature (30 degrees C). This species of molecule was found to have an electrophoretic mobility identical to that of the mature MBP, but a considerable fraction of it was inaccessible to externally added protease. It had not attained the protease-resistant conformation characteristically observed for the exported mature protein. The radioactivity associated with this species decreased during chase and was presumably converted into the exported mature form, a process that required energy, probably the proton motive force, as demonstrated by its inhibition by an energy uncoupler. The spheroplast-associated processed form was more predominantly observed in the presence of a low concentration of chloramphenicol. A similar intermediate was also detected for beta-lactamase in wild-type cells. These results suggest that in a late phase of translocation, the bulk of the polypeptide chain can move through the membrane in the absence of the covalently attached leader peptide, and the secA-secY gene products are somehow involved in this process. We termed the processed intermediates processed immature forms.

Efficiency and diversity of protein localization by random signal sequences

Three randomly derived sequences that can substitute for the signal peptide of Saccharomyces cerevisiae invertase were tested for the efficiency with which they can translocate invertase or beta-galactosidase into the endoplasmic reticulum. The rate of translocation, as measured by glycosylation, was estimated in pulse-chase experiments to be less than 6 min. When fused to beta-galactosidase, these peptides, like the normal invertase signal sequence, direct the hybrid protein to a perinuclear region, consistent with localization to the endoplasmic reticulum. The diversity of function of random peptides was studied further by immunofluorescence localization of proteins fused to 28 random sequences: 4 directed the hybrid to the endoplasmic reticulum, 3 directed it to the mitochondria, and 1 directed it to the nucleus.

Efficiency and diversity of protein localization by random signal sequences

Three randomly derived sequences that can substitute for the signal peptide of Saccharomyces cerevisiae invertase were tested for the efficiency with which they can translocate invertase or beta-galactosidase into the endoplasmic reticulum. The rate of translocation, as measured by glycosylation, was estimated in pulse-chase experiments to be less than 6 min. When fused to beta-galactosidase, these peptides, like the normal invertase signal sequence, direct the hybrid protein to a perinuclear region, consistent with localization to the endoplasmic reticulum. The diversity of function of random peptides was studied further by immunofluorescence localization of proteins fused to 28 random sequences: 4 directed the hybrid to the endoplasmic reticulum, 3 directed it to the mitochondria, and 1 directed it to the nucleus.

The sec and prl genes of Escherichia coli

Two general approaches have been used to define genetically the genes that encode components of the cellular protein export machinery. One of these strategies identifies mutations that confer a conditional-lethal, pleiotropic export defect (sec, secretion). The other identifies dominant suppressors of signal sequence mutations (prl, protein localization). Subsequent characterization reveals that in at least three cases, prlA/secY, prlD/secA, and prlG/secE, both types of mutations are found within the same structural gene. This convergence is satisfying and provides compelling evidence for direct involvement of these gene products in the export process.

Export of the periplasmic maltose-binding protein of Escherichia coli

The export of the maltose-binding protein (MBP), the malE gene product, to the periplasm of Escherichia coli cells has been extensively investigated. The isolation of strains synthesizing MalE-LacZ hybrid proteins led to a novel genetic selection for mutants that accumulate export-defective precursor MBP (preMBP) in the cytoplasm. The export defects were subsequently shown to result from alterations in the MBP signal peptide. Analysis of these and a variety of mutants obtained in other ways has provided considerable insight into the requirements for an optimally functional MBP signal peptide. This structure has been shown to have multiple roles in the export process, including promoting entry of preMBP into the export pathway and initiating MBP translocation across the cytoplasmic membrane. The latter has been shown to be a late event relative to synthesis and can occur entirely posttranslationally, even many minutes after the completion of synthesis. Translocation requires that the MBP polypeptide exist in an export-competent conformation that most likely represents an unfolded state that is not inhibitory to membrane transit. The signal peptide contributes to the export competence of preMBP by slowing the rate at which the attached mature moiety folds. In addition, preMBP folding is thought to be further retarded by the binding of a cytoplasmic protein, SecB, to the mature moiety of nascent preMBP. In cells lacking this antifolding factor, MBP export represents a race between delivery of newly synthesized, export-competent preMBP to the translocation machinery in the cytoplasmic membrane and folding of preMBP into an export-incompetent conformation. SecB is one of three E. coli proteins classified as "molecular chaperones" by their ability to stabilize precursor proteins for membrane translocation.

Effects of production of abnormal proteins on the rate of killing of Escherichia coli by streptomycin

The role of abnormal membrane proteins in modulating the rate of killing by streptomycin was investigated. Davis et al. (B.D. Davis, L. Chen, and P.T. Tai, Proc. Natl. Acad. Sci. USA 83:6164-6168, 1986) have proposed that misread membrane proteins created by the action of streptomycin on translating ribosomes cause the formation of nonspecific membrane channels which allow increased uptake of the antibiotic and contribute to its bactericidal action. Pretreatment of Escherichia coli with a low concentration of puromycin enhanced the rate of killing by streptomycin. The effect of the pretreatment with puromycin was transient, since approximately normal rates of killing by streptomycin were restored after 30 min of incubation in antibiotic-free medium. This time period correlates with the time required to degrade labile polypeptides in puromycin-treated cells. The induction of a specific abnormal malE-lacZ fusion protein, which is capable of disrupting the normal membrane protein secretion process, also increased the rate of killing by streptomycin. Induction of malF-phoA fusion proteins, which have no significant effects on membrane integrity, did not alter susceptibility to streptomycin. These observations suggest that certain abnormal membrane proteins can contribute to the bactericidal action of streptomycin.

Export and processing analysis of a fusion between the extracellular heat-stable enterotoxin and the periplasmic B subunit of the heat-labile enterotoxin in Escherichia coli

As an initial approach in the study of the mechanism of secretion of the extracellular heat-stable enterotoxin of Escherichia coli (STA), and in order to use this polypeptide as an extracellular carrier we previously constructed a fusion between the complete STA toxin (pre-pro-STA) and the mature B subunit of the periplasmic heat-labile enterotoxin (LTB); the resulting STA-LTB hybrid was not secreted to the extracellular environment, and cells expressing the hybrid lysed at temperatures above 35 degrees C. In this work we have established that the hybrid is initially detected as pre-pro-STA-LTB and converted to pro-STA-LTB, which lacks the 19 amino acids that share the properties of a signal peptide; the sequenced 17 amino-terminal residues of pro-STA-LTB defined the processing site of pre-pro-STA-LTB at pro-3phe-2ala-1 decreases gln+1. This process was sensitive to an energy uncoupler (CCCP) and was correlated with translocation of pro-STA-LTB across the inner membrane. Additionally, we are able to show that although pre-pro-STA-LTB is processed at 37 degrees C and 29 degrees C, it is more efficiently processed at the latter temperature. At 37 degrees C, pro-STA-LTB was poorly released into the periplasm, resulting in accumulation of this protein, pre-pro-STA-LTB, and pre-beta-lactamase in the inner membrane, and in cell lysis. In contrast, at 29 degrees C pro-STA-LTB was localized in the periplasm and in the inner membrane, and pre-pro-STA-LTB and pre-beta-lactamase did not accumulate; however, translocation of periplasmic pro-STA-LTB across the outer membrane still did not occur, and a second processing step that would eliminate the pro segment from pro-STA-LTB was never observed. Thus, the fusion of pre-pro-STA and LTB resulted in a polypeptide that, while incompatible with secretion to the extracellular medium, is exported to the periplasm in a temperature-conditional fashion. This latter observation is consistent with an STA secretion pathway whereby pre-pro-STA is first processed to periplasmic pro-STA by the removal of a 19-amino-acid signal peptide.

Phage shock protein, a stress protein of Escherichia coli

Filamentous phage infection induces the synthesis of large amounts of an Escherichia coli protein, phage shock protein (Psp), the product of a previously undescribed gene. This induction is due to the phage gene IV protein, pIV, an integral membrane protein. The uninduced level of Psp is undetectable, but when induced by prolonged synthesis of pIV, it can become one of the most abundant proteins in the cell. Psp is also synthesized transiently in response to several stresses (heat, ethanol, and osmotic shock). High-level synthesis occurs only after extreme treatment. Unlike the members of the heat shock regulon, Psp induction does not require the heat shock sigma factor, sigma 32; some stimuli that elicit sigma 32-dependent heat shock proteins do not induce Psp synthesis. The level of Psp induction after extreme stress is even higher in sigma 32 mutant cells, which are unable to mount a normal heat shock response, suggesting that these parallel stress responses are interrelated.

The role of the mature part of secretory proteins in translocation across the plasma membrane and in regulation of their synthesis in Escherichia coli

Presently available data are reviewed which concern the role of the mature parts of secretory precursor proteins in translocation across the plasma membrane of Escherichia coli. The following conclusions can be drawn; i) signals, acting in a positive fashion and required for translocation do not appear to exist in the mature polypeptides; ii) a number of features have been identified which either affect the efficiency of translocation or cause export incompatibility. These are: alpha) protein folding prior to translocation; beta) restrictions regarding the structure of N-terminus; gamma) presence of lipophilic anchors; delta) too low a size of the precursor. Efficiency of translocation is also enhanced by binding of chaperonins (SecB, trigger factor, GroEL) to precursors. Binding sites for chaperonins appear to exist within the mature parts of the precursors but the nature of these sites has remained rather mysterious. Mutant periplasmic proteins with a block in release from the plasma membrane have been described, the mechanism of this block is not known. The mature parts of secretory proteins can also be involved in the regulation of their synthesis. It appears that exported proteins are already recognized as such before they are channelled into the export pathway and that their synthesis can be feed-back inhibited at the translational level.

On remaining cytoplasmic

The published literature contains a number of examples of normally non-cytoplasmic proteins whose transport out of the cytoplasm is not completely abolished by drastic alterations to their routing signals (signal sequences, etc). Furthermore, there are numerous examples of cytoplasmic proteins that can be routed to and across plasma or organelle membranes by fusing them to routing signals. These 2 sets of observations lead to a re-evaluation of the reliability and accuracy of protein routing and to consideration of the consequences of the errors which might occur.

Genetic analysis of the switch that controls porin gene expression in Escherichia coli K-12

The two-component regulatory system, OmpR and EnvZ, in Escherichia coli controls the differential expression of ompF and ompC in response to medium osmolarity. Previous studies suggest that EnvZ functions as a membrane sensor relaying information to the DNA-binding protein, OmpR, which in turn activates expression of the appropriate promoter. A strategy has been devised to isolate and characterize a collection of missense mutations in ompR that alter, but do not abolish protein function. Mutants were isolated using strains that contain the ompR and envZ genes in separate chromosomal locations yet maintain the production of both regulatory proteins at physiological levels. Such an arrangement facilitates ompR diploid analysis and tests of epistasis with known envZ mutations. The data obtained indicate that OmpR works in both a positive and negative fashion to control the transcription of ompF and this result forms the basis of a model for porin regulation that explains the switch from OmpF to OmpC production in response to increasing medium osmolarity.