Sequence information required for protein translocation from the cytoplasm

The Escherichia coli aspartate receptor: sequence specificity of a transmembrane helix studied by hydrophobic-biased random mutagenesis

The Escherichia coli aspartate receptor is a dimer with two transmembrane sequences per monomer that connect a periplasmic ligand binding domain to a cytoplasmic signaling domain. The method of 'hydrophobic-biased' random mutagenesis, that we describe here, was used to construct mutant aspartate receptors in which either the entire transmembrane sequence or seven residues near the center of the transmembrane sequence were replaced with hydrophobic and polar random residues. Some of these receptors responded to aspartate in an in vivo chemotaxis assay, while others did not. The acceptable substitutions included hydrophobic to polar residues, small to larger residues, and large to smaller residues. However, one mutant receptor that had only a few hydrophobic substitutions did not respond to aspartate. These results add to our understanding of sequence specificity in the transmembrane regions of proteins with more than one transmembrane sequence. This work also demonstrates a method of constructing families of mutant proteins containing random residues with chosen characteristics.

Antibody engineering: comparison of bacterial, yeast, insect and mammalian expression systems

Engineered antibody molecules, and their fragments, are being increasingly exploited as scientific and clinical tools. However, one factor that can limit the applicability of this technology is the ability to express large amounts of active protein. In this review we describe the relative advantages and disadvantages of bacterial, yeast, insect and mammalian expression systems, and discuss some of the problems that can be encountered when using them. There is no 'universal' expression system, that can guarantee high yields of recombinant product, as every antibody-based molecule will pose its own problems in terms of expression. As a result the choice of system will depend on many factors, including the molecular species being expressed, the precise sequence of the individual antibody and the preferences of the individual investigator. However, there are general rules with regards to the design of expression vectors and systems which will help the investigator to make informed choices as to which strategy might be appropriate for their application.

Interaction of alpha s2- and beta-casein signal peptides with DMPC and DMPG liposomes

Interactions of casein signal peptides (CSP) and derivatives were detected with dimyristoylphosphatidyl-glycerol and -choline liposomes. Fluorescence anisotrophy indicated that the peptides interact better with DMPG than DMPC, inserting at a limited depth in the bilayer. Stronger interaction was detected for derivatives of beta-CSP than of alpha s2-CSP. Tryptophan fluorescence (intrinsic, energy transfer, quenching) showed that the central hydrophobic core of CSP was buried in the bilayer whereas both ends remained outside, adopting a hairpin-like conformation. The secondary structure of the CSP was not affected by their interactions with phospholipids. beta-CSP derivatives show both lytic and fusogenic activities.

Efficient expression and secretion of Aspergillus niger RH5344 polygalacturonase in Saccharomyces cerevisiae

An Aspergillus niger endopolygalacturonase (EC 3.2.1.15) cDNA was expressed in the yeast Saccharomyces cerevisiae. Secretion of the protein into the growth medium was efficiently directed by the fungal leader sequence, and processing occurred at the same site as in Aspergillus. The expression level was significantly enhanced by using a "short" version of the yeast ADHI promoter. An additional increase in the yield of heterologous protein was due to a higher plasmid stability and a rise in plasmid copy number. This was achieved by deleting most of the bacterial sequences from the expression vector. The yeast-derived enzyme showed the same enzymatic and biochemical properties as the fungal polygalacturonase, such as substrate specificity, pH and temperature optima and pI value. The yeast-derived enzyme, however, showed a higher degree of glycosylation and exhibited a more pronounced temperature stability than the fungal enzyme.

Selection for transport competence of C-terminal polypeptides derived from Escherichia coli hemolysin: the shortest peptide capable of autonomous HlyB/HlyD-dependent secretion comprises the C-terminal 62 amino acids of HlyA

Escherichia coli hemolysin (HlyA) is secreted by a specific export machinery which recognizes a topogenic secretion signal located at the C-terminal end of HlyA. This signal sequence has been variously defined as comprising from 27 to about 300 amino acids at the C-terminus of HlyA. We have used here a combined genetic and immunological approach to select for C-terminal HlyA peptides that are still secretion-component. A deletion library of HlyA mutant proteins was generated in vitro by successive degradation of hylA from the 5' end with exonuclease III. Secretion competence was tested by immunoblotting of the supernatant of each clone with an antiserum raised against a C-terminal portion of hemolysin. It was found that the hemolysin secretion system has no apparent size limitation for HlyA proteins over a range from 1024 to 62 amino acids. The smallest autonomously secretable peptide isolated in this selection procedure consists of the C-terminal 62 amino acids of HlyA. This sequence is shared by all secretion-competent, truncated HlyA proteins, which suggests that secretion of the E. coli hemolysin is strictly post-translational. The capacity of the hemolysin secretion machinery was found to be unsaturated by the steady-state level of its natural HlyA substrate and large amounts of truncated HlyA derivatives could still be secreted in addition to full-length HlyA.

The contribution of the cell wall to a transmembrane calcium gradient could play a key role in Bacillus subtilis protein secretion

A weak Ca(2+)-binding site (Ka = 0.8 x 10(3) M-1, at pH 7) was identified in the mature part of levansucrase. An amino acid substitution (Thr-236-->Ile) in this site alters simultaneously the affinity for calcium, the folding transition and the efficiency of the secretion process of levansucrase. Moreover, the ability of the Bacillus subtilis cell wall to concentrate calcium ions present in the culture medium was studied. We confirm the results of Beveridge and Murray who showed that the concentration factor is about 100 to 120 times. This property preserves a high concentration of Ca2+ (> 2 mM) on the external side of the cytoplasmic membrane, even in the absence of further Ca2+ supplementation in the growth medium. Such local conditions allow the spontaneous unfolding-folding transition of levansucrase en route for secretion. Since several exocellular proteins of B. subtilis are calcium-binding proteins, we propose that the high concentration of calcium ion in the microenvironment of the cell wall may play a key role in the ultimate step of their secretion process.

An outer membrane protein (OmpA) of Escherichia coli can be translocated across the cytoplasmic membrane of Bacillus subtilis

The translocation of secretory proteins derived from a Gram-positive (Staphylococcus hyicus prolipase) or a Gram-negative (Escherichia coli pre-OmpA protein) bacterium across the cytoplasmic membrane was studied in E. coli and Bacillus subtilis. In both microorganisms, the prolipase was found to be secreted across the plasma membrane when either the pre-prolipase signal peptide (38 amino acids in length) or the pre-OmpA signal peptide (21 amino acids in length) was used. Expression of the gene encoding the authentic pre-OmpA protein in B. subtilis resulted in the translocation of mature OmpA protein across the plasma membrane. Processing of the OmpA precursor in B. subtilis required the electrochemical potential and was sensitive to sodium azide, suggesting that the B. subtilis SecA homologue was involved in the translocation process. The mature OmpA protein, which was most likely present in an aggregated state, was fully accessible to proteases in protoplasted cells. Therefore, our results clearly demonstrate that an outer membrane protein can be secreted by B. subtilis, supporting the notion that the basic mechanism of protein translocation is highly conserved in Gram-positive and Gram-negative bacteria.

Genetic analysis of the gene cluster encoding nonfimbrial adhesin I from an Escherichia coli uropathogen

The chromosomally encoded nonfimbrial adhesion I (NFA-I) from Escherichia coli urinary tract isolate 827 (O83:K1:H4) mediates agglutination of human erythrocytes. Subclones were constructed from an NFA-I-expressing recombinant E. coli K-12 clone, derived from a genomic library of E. coli 827. Minicell analysis and nucleotide sequencing revealed that proteins of 30.5, 9, 80, 15, and 19 kDa encoded on a stretch of approximately 6 kb are involved in the expression of NFA-I. NFA-I exhibits a polymeric structure, which disintegrates with elevated temperature into a 19-kDa monomer but with some relatively stable dimers. By using gold-conjugated monoclonal antibodies directed against NFA-I in electron microscopy, the adhesin could be localized on the outer surface of the recombinant E. coli K-12 bacteria. The nucleotide sequence of the nfaA gene encoding the monomeric structural subunit of the adhesin was determined. An open reading frame of 184 amino acids encoding the NfaA precursor, which is processed to the mature protein, was found; it consisted of 156 amino acids with a calculated molecular weight of 16,000. Peptide sequencing of the NFA-I subunit protein confirmed that this open reading frame corresponds to the NfaA coding locus. Furthermore, the nucleotide sequence of the open reading frame termed NfaE, located at the proximal part of the DNA stretch responsible for NFA-I expression, was elaborated. NfaE consists of 247 amino acids, including a presumptive 29-amino-acid signal peptide, leading to a molecular weight of 24,000 for the mature protein. The nfaE sequence shares homology with the 27-kDa CS3 protein, which is involved in the assembly of CS3 fibrillae, and might encode the 30.5-kDa protein, detected in minicells.

The roles of signal peptide and mature protein in RNase (barnase) export from Bacillus subtilis

Barnase, an extracellular RNAse from Bacillus amyloliquefaciens is secreted post-translationally from B. subtilis. The rate of secretion of barnase from B. subtilis was improved by replacement of the barnase signal peptide with a heterologous signal peptide. However, the barnase signal peptide exported Escherichia coli alkaline phosphatase faster than mature barnase. Heat shock of B. subtilis cells did not significantly alter the export of barnase using the barnase signal peptide. The slow rate of export of barnase from B. subtilis is due to both the signal peptide and the mature protein sequence rather than either alone.

Combinatorial mutagenesis of the lamB gene: residues 41 through 43, which are conserved in Escherichia coli outer membrane proteins, are informationally important in maltoporin structure and function

A new strategy for combinatorial mutagenesis was developed and applied to residues 40 through 60 of LamB protein (maltoporin), with the aim of identifying amino acids important for LamB structure and function. The strategy involved a template containing a stop codon in the target sequence and a pool of random degenerate oligonucleotides covering the region. In vitro mutagenesis followed by selection for function (Dex+, ability to utilize dextrins) corrected the nonsense mutation and simultaneously forced incorporation of a random mutation(s) within the region. The relative importance of each residue within the target was indicated by the frequency and nature of neutral and deleterious mutations recovered at each position. Residues 41 through 43 in LamB accepted few neutral substitutions, whereas residues 55 through 57 were highly flexible in this regard. Consistent with this finding was that the majority of defective mutants were altered at residues 41 to 43. Characterization of these mutants indicated that the nature of residues 41 to 43 influenced the amount of stable protein in the outer membrane. These results, as well as the conserved nature of this stretch of residues among outer membrane proteins, suggest that residues 41 to 43 of LamB play an important role in the process of outer membrane localization.

Characterization of the secretion efficiency of a plant signal peptide in Bacillus subtilis

The ability of the Bacillus subtilis secretion machinery to interact with a heterologous signal peptide was studied using a plant (wheat alpha-amylase) signal peptide. The plant signal peptide was capable of mediating secretion of Escherichia coli alkaline phosphatase and B. amyloliquefaciens levansucrase from B. subtilis. This secretion was dependent on the plant signal peptide, as deletion of five amino acids from the hydrophobic core resulted in a block of secretion. Attempts to improve the efficiency of the plant signal peptide in B. subtilis were made by increasing the length of the hydrophobic core from 10 to 16 residues by insertion of 2, 4, 5 or 6 amino acids. None of the alterations improved the secretion efficiency relative to the wild-type plant signal peptide.

Secretion of active truncated CD4 into Escherichia coli periplasm

A truncated molecule containing the first 183 amino acid residues of the HIV-1 receptor, CD4, was made by periplasmic secretion in Escherichia coli. The signal sequence from the E. coli proteins OmpA, PhoA, or OmpF was fused to the truncated CD4, under the control of either the trp or the lac promoter. The processed material secreted into the periplasm reacted with monoclonal antibodies and exhibited binding activity to the HIV-1 envelope protein gp120. Not all of the processed product was recovered in the periplasm by osmotic shock, suggesting that either the material aggregated in the periplasm or, during secretion, the molecule assumed some transient conformation that interfered with its translocation across the inner membrane. A mutation in prlA (a gene involved in secretion) increased the level of processing, suggesting that secretion of a heterologous protein in E. coli can be optimized by manipulating the host secretion apparatus.

Conformational and membrane-binding properties of a signal sequence are largely unaltered by its adjacent mature region

We have synthesized a peptide corresponding to the 25-residue signal sequence plus the first 28 residues of the Escherichia coli outer membrane protein LamB in order to explore the properties of a signal sequence in the presence of the N-terminal region of its passenger. In the last few years, there have been several observations of differing efficiencies of export when signal sequences are attached to different passenger proteins or when the first part of a passenger protein undergoes mutation. In the LamB case, gene fusions with lacZ have shown that the signal sequence plus the first 28 residues of mature LamB are necessary to direct beta-galactosidase into the export pathway [Rasmussen, B. A. & Silhavy, T. J. (1987) Genes Dev. 1, 185-196]. The origin of these observations and whether there is an influence of the mature region on the properties of the signal sequence have not been known. We find that the conformational and membrane-binding properties of the LamB signal sequence manifest in a 25-residue peptide are essentially unaltered in the context of the 53-residue peptide corresponding to this signal sequence plus the first 28 residues of the mature LamB protein. CD spectra show that the signal peptide and passenger domains are conformationally independent of each other in micelle or bilayer environments. Furthermore, the signal sequence leads to the spontaneous association of the 53-residue peptide with a lipid bilayer; alone, the mature domain does not interact with lipid bilayers. Fluorescence results show that the mode of interaction of the signal peptide with a bilayer is essentially unaltered by the presence of its mature region. This lack of influence of the mature domain on the behavior of the signal sequence is unexpected for juxtaposed polypeptides of comparable length and may be of physiological importance: N-terminal regions of secreted proteins may be selected to be passive, by comparison with their cognate signal sequences, which themselves must engage the export apparatus and actively interact with its components.

Cloning, sequencing, and regulation of expression of an extracellular esterase gene from the plant pathogen Streptomyces scabies

The gene that encodes the extracellular esterase produced by Streptomyces scabies has been cloned and sequenced. The gene was identified by hybridization to a synthetic oligonucleotide that corresponds to the amino-terminal amino acid sequence determined for the secreted form of the esterase. Nucleotide sequence analysis predicted a 345-amino-acid open reading frame, a putative ribosome-binding site, and 39 amino acids at the amino terminus of the sequence that is not found in the secreted protein. This 39-amino-acid sequence has many of the characteristics common to known signal peptides. End mapping the esterase transcript revealed a single 5' end of the mRNA located 51 nucleotides upstream from the start point for translation. Northern (RNA) hybridization analysis of the esterase message by using the cloned esterase gene as a probe indicated that the esterase mRNA is about 1,440 nucleotides in length and was detected only when the cells were grown in the presence of zinc. These results suggest that the level of esterase mRNA detected in the cells is regulated by zinc.

SecB-independent export of Escherichia coli ribose-binding protein (RBP): some comparisons with export of maltose-binding protein (MBP) and studies with RBP-MBP hybrid proteins

The efficient export of the Escherichia coli maltose-binding protein (MBP) is known to be SecB dependent, whereas ribose-binding protein (RBP) export is SecB independent. When the MBP and RBP signal peptides were exchanged precisely at the signal peptidase processing sites, the resultant RBP-MBP and MBP-RBP hybrid proteins both were efficiently exported in SecB+ cells. However, only MBP-RBP was efficiently exported in SecB- cells; RBP-MBP exhibited a significant export defect, a finding that was consistent with previous proposals that SecB specifically interacts with the mature moiety of precursor MBP to promote export. The relatively slow, totally posttranslational export mode exhibited by certain mutant RBP and MBP-RBP species in SecB+ cells was not affected by the loss of SecB. In contrast, MBP and RBP-MBP species with similarly altered signal peptides were totally export defective in SecB- cells. Both export-defective MBP and RBP-MBP interfered with SecB-mediated protein export by depleting cells of functional SecB. In contrast, neither export-defective RBP nor MBP-RBP elicited such an interference effect. These and other data indicated that SecB is unable to interact with precursor RBP or that any interaction between these two proteins is considerably weaker than that of SecB with precursor MBP. In addition, no correlation could be established between a SecB requirement for export and PrlA-mediated suppression of signal peptide export defects. Finally, previous studies have established that wild-type MBP export can be accomplished cotranslationally, whereas wild-type RBP export is strictly a posttranslational process. In this study, cotranslational export was not detected for either MBP-RBP or RBP-MBP. This indicates that the export mode exhibited by a given precursor protein (cotranslational versus posttranslational) is determined by properties of both the signal peptide and the mature moiety.

The genetics of protein secretion in E. coli

Genetic studies have identified six genes whose products comprise the general protein secretion machinery of Escherichia coli. Insights from mutant analysis and the biochemical properties of the purified components allows the secretion pathway to be described in some detail. The picture emerging provides a useful paradigm for similar pathways in other organisms.

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 and sorting of the Escherichia coli outer membrane protein OmpA

Results of studies, mostly using the outer membrane, 325 residue protein OmpA, are reviewed which concern its translocation across the plasma membrane and incorporation into the outer membrane of Escherichia coli. For translocation, neither a unique export signal, acting in a positive fashion within the mature part of the precursor, nor a unique conformation of the precursor is required. Rather, the mature part of a secretory protein has to be export-compatible. Export-incompatibility can be caused by a stretch of 16 (but not 8 or 12) hydrophobic residues, too low a size of the polypeptide (smaller than 75 residue precursors), net positive charge at the N-terminus, or lack of a turn potential at the same site. It is not yet clear whether binding sites for chaperonins (SecB, trigger factor, GroEL) within OmpA are important in vivo. The mechanism of sorting of outer membrane proteins is not yet understood. The membrane part of OmpA, encompassing residues 1 to about 170, it thought to traverse the membrane eight times in antiparallel beta-sheet conformation. At least the structure of the last beta-strand (residues 160-170) is of crucial importance for membrane assembly. It must be amphiphilic or hydrophobic, these properties must extend over at least nine residues, and it must not contain a proline residue at or near its center. Membrane incorporation of OmpA involves a conformational change of the protein and it could be that the last beta-strand initiates folding and assembly in the outer membrane.

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.

Escherichia coli expression and processing of human interleukin-1 beta fused to signal peptides

Escherichia coli expression, processing, and secretion of human interleukin-1 beta (IL-1 beta) fused to the signal peptide of E. coli OmpA or PhoA protein were studied. With fusion to either signal sequence, high-level expression was observed and the products accumulated to about 20% of total cell protein. In the fusion to OmpA leader sequence, more than 50% of the product has the OmpA signal peptide removed precisely. The majority of the processed material is not released by osmotic shock. On the other hand, very little of the material from the fusion to PhoA has the PhoA signal peptide removed. Use of the host with a mutation in prlA or prlF, variation of temperature for cell growth, and alteration of the amino acid residues around the cleavage site do not facilitate processing of the PhoA signal peptide. These results suggest that some component in the PhoA signal peptide, interacting with the Il-1 beta sequence, is interfering with the processing of the signal peptide.

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.

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.

Secretion of Bacillus subtilis levansucrase. Fe(III) could act as a cofactor in an efficient coupling of the folding and translocation processes

The refolding of levansucrase denatured by urea was studied as a possible model for the second step of the secretion pathway of this protein. The folding-unfolding transition was monitored by measuring intrinsic fluorescence and resistance to proteolysis. Both methods provided the same estimation for the unfolding free energy of levansucrase, delta GD, which was 30.1 +/- 1.7 kJ.mol-1 (7.2 +/- 0.4 kcal.mol-1) at pH 7 in 0.1 M-potassium phosphate buffer. The rate of refolding was greatly enhanced by Fe3+, whereas the Fe3+ chelator EDTA prevented correct refolding. Fe3+ allowed the protein to reach its folded form in medium in which the dielectric constant had been lowered by ethanol. The efficiency in vivo of the export of levansucrase bearing an amino acid modification which blocks the second step of the translocation pathway was greatly increased by high concentrations of Fe3+ in the culture medium. Assuming that the protein folding governs the second step of the secretion process of levansucrase, we discuss from an irreversible thermodynamic point of view the possible role of Fe3+ in the efficient coupling of the two events.

A comparative analysis of single- and multiple-residue substitutions in the alkaline phosphatase signal peptide

The alkaline phosphatase signal peptide participates in transport of the enzyme to the periplasmic space of Escherichia coli. The signal sequence, like that of other signal peptides, is composed of a polar amino-terminal segment, a central region rich in hydrophobic residues and a carboxy-terminal region recognized by signal peptidase. We have previously shown that an alkaline phosphatase signal peptide mutant containing a polyleucine core region functions efficiently in transport of the enzyme [D. A. Kendall, S. C. Bock, and E. T. Kaiser (1986) Nature 321, 706-708]. In this study, some of the amino acid changes involved in the polyleucine sequence are examined individually. A Phe to Leu substitution as the sole change results in impaired transport properties in contrast to when it is combined with three other amino acid changes in the polyleucine-containing sequence. A mutant with a Pro to Leu substitution in the hydrophobic core region is comparable to wild type while the same type of substitution (Pro to Leu) in the carboxy-terminal segment results in substantial accumulation of the mutant precursor. Finally, introduction of a basic residue into the hydrophobic segment (Leu to Arg substitution) results in a complete export block. These results exemplify the spectrum of properties produced by individual residue changes and suggest there is some interplay between hydrophobicity and conformation for signal peptide function.

Evidence for partial export of Vitreoscilla hemoglobin into the periplasmic space in Escherichia coli. Implications for protein function

The Vitreoscilla hemoglobin protein has been implicated in earlier studies to serve a globin-like function under oxygen-limited growth conditions. Evidence is presented using fractionation as well as proteinase K accessibility techniques to prove that a considerable amount of this protein is localized in the periplasmic space of the cell. Genetic evidence points towards the existence of information within the N-terminal domain of the protein that plays a role in the process of protein export. However, this sequence is not cleaved in the process of translocation. Analysis of the primary structure of this region reveals several unusual features. Instead of positively charged residues at its amino terminus, it has a negative charge. The overall hydrophobicity of the central region of this sequence is significantly lower than in typical leader peptides due to the presence of a charged residue. In keeping with the likelihood that such an export signal may not be very efficient, a substantial fraction of the total cellular hemoglobin can also be detected in the cytoplasm. Heme is incorporated in both cytoplasmic and periplasmic globin as indicated by the ability of protein from both fractions to bind carbon monoxide. The secretion of this protein into the periplasm raises questions concerning the physiological significance of its localization. Dimensional analysis of a model based on the facilitated diffusion hypothesis, which was initially proposed to account for the effects of eukaryotic globins on oxygen transport, suggests that periplasmic globin can support an additional oxygen flux to the respiratory apparatus that may be physiologically significant.

Mutations that improve export of maltose-binding protein in SecB- cells of Escherichia coli

It previously has been proposed that the Escherichia coli SecB protein promotes the export of the maltose-binding protein (MBP) from the cytoplasm by preventing the folding of the precursor MBP (preMBP) into a translocation-incompetent conformation. The export of wild-type MBP is only partially blocked in SecB- cells. In contrast, the export of MBP16-1, an MBP species with a defective signal peptide, is totally dependent on SecB; hence, SecB- cells that synthesize MBP16-1 are unable to utilize maltose as a sole carbon source. The selection of Mal+ revertants primarily yielded mutants with alterations in the MBP16-1 signal peptide that permitted SecB-independent MBP export to the periplasm to various extents. Although each of these alterations increased the overall hydrophobicity of the signal peptide, it was not possible to strictly equate changes in hydrophobicity with the degree of SecB-independent export. Somewhat unexpectedly, two mutants were obtained in which MBP export in SecB- cells was markedly superior to that of the wild-type MBP. Although wild-type MBP is not cotranslationally translocated in SecB- cells, the two mutant proteins designated MBP172 and MBP173 exhibited significant cotranslational export in the absence of SecB. Thus, the role of SecB was partially supplanted by a signal peptide that promoted more rapid movement of MBP through the export pathway. When preMBP included the MBP172 signal peptide as well as an alteration in the mature moiety that slows folding, the SecB requirement for maximal MBP export efficiency was almost totally eliminated. These results provide additional strong support for the proposed antifolding role of SecB in MBP export.

Analysis of mutational alterations in the hydrophilic segment of the maltose-binding protein signal peptide

Oligonucleotide-directed mutagenesis was employed to investigate the role of the hydrophilic segment of the Escherichia coli maltose-binding protein (MBP) signal peptide in the protein export process. The three basic residues residing at the amino terminus of the signal peptide were systematically substituted with neutral or acidic residues, decreasing the net charge in a stepwise fashion from +3 to -3. It was found that a net positive charge was not absolutely required for MBP export to the periplasm. However, export was most rapid and efficient when the signal peptide retained at least a single basic residue and a net charge of +1. The nature of the adjacent hydrophobic core helped to determine the effect of charge changes in the hydrophilic segment on MBP export, which suggested that these two regions of the signal peptide do not have totally distinct functions. Although the stepwise decrease in net charge of the signal peptide also resulted in a progressive decrease in the level of MBP synthesis, the data do not readily support a model in which MBP synthesis and export are obligately coupled events. The export defect resulting from alterations in the hydrophilic segment was partially suppressed in strains harboring certain prl alleles but not in strains harboring prlA alleles that are highly efficient suppressors of signal sequence mutations that alter the hydrophobic core.

Nostoc commune UTEX 584 gene expressing indole phosphate hydrolase activity in Escherichia coli

A gene encoding an enzyme capable of hydrolyzing indole phosphate was isolated from a recombinant gene library of Nostoc commune UTEX 584 DNA in lambda gt10. The gene (designated iph) is located on a 2.9-kilobase EcoRI restriction fragment and is present in a single copy in the genome of N. commune UTEX 584. The iph gene was expressed when the purified 2.9-kilobase DNA fragment, free of any vector sequences, was added to a cell-free coupled transcription-translation system. A polypeptide with an Mr of 74,000 was synthesized when the iph gene or different iph-vector DNA templates were expressed in vitro. When carried by different multicopy plasmids and phagemids (pMP005, pBH6, pB8) the cyanobacterial iph gene conferred an Iph+ phenotype upon various strains of Escherichia coli, including a phoA mutant. Hydrolysis of 5-bromo-4-chloro-3-indolyl phosphate was detected in recombinant E. coli strains grown in phosphate-rich medium, and the activity persisted in assay buffers that contained phosphate. In contrast, indole phosphate hydrolase activity only developed in cells of N. commune UTEX 584, when they were partially depleted of phosphorus, and the activity associated with these cells was suppressed partially by the addition of phosphate to assay buffers. Indole phosphate hydrolase activity was detected in periplasmic extracts from E. coli (Iph+) transformants.

The mature portion of Escherichia coli maltose-binding protein (MBP) determines the dependence of MBP on SecB for export

The product of the secB gene is required for export of a subset of secreted proteins to the outer membrane and periplasm of Escherichia coli. Precursor maltose-binding protein (MBP) accumulates in the cytoplasm of secB-carrying mutants, but export of alkaline phosphatase is only minimally affected by secB mutations. When export of MBP-alkaline phosphatase hybrid proteins was analyzed in wild-type and secB-carrying mutant strains, the first third of mature MBP was sufficient to render export of the hybrid proteins dependent on SecB. Substitution of a signal sequence from a SecB-independent protein had no effect on SecB-dependent export. These findings show that the first third of mature MBP is capable of conferring export incompetence on an otherwise competent protein.

Assembly of the OmpF porin of Escherichia coli B. Immunological and kinetic studies of the integration pathway

The different conformations of the outer membrane protein OmpF of Escherichia coli B were studied with immunological probes. The antigenic determinants recognized by one monoclonal (MoF3) and two polyclonal antibodies were investigated under various conditions of solubilization which modify the association of OmpF with other membrane components, such as lipopolysaccharide. Several polymeric forms of the protein could be detected after extraction at 37 degrees C or 56 degrees C. The monoclonal antibody, which is specific to an exposed region of native OmpF, recognized various trimeric forms in an immunoprecipitation assay. Under the same conditions, the binding of polyclonal antibodies apparently induced strong conformational rearrangements, since the pattern of trimeric forms detected was greatly modified. The conversion of newly synthesized monomers of OmpF to the various trimer forms was investigated using these antibodies. The trimerization occurred rapidly but the appearance of the native conformation of OmpF was delayed. Some additional step was required to expose the MoF3-specific antigenic site at the surface of the trimeric form. These results are discussed in relation to the structure of OmpF and its association with lipopolysaccharide in the outer membrane.

On the role of the mature part of an Escherichia coli outer membrane protein (OmpA) in translocation across the plasma membrane

The 325-residue OmpA protein, which is synthesized as a precursor with a 21-residue signal sequence, is a polypeptide of the outer membrane of Escherichia coli K-12. The signal peptide is able to direct translocation across the plasma membrane of virtually any fragment of this protein. It had, therefore, been concluded that information required for this translocation does not exist within the mature part of the protein. This view has been criticized and it was suggested that our data showed that both the signal sequence and residues within the first 44 amino acid residues of the mature protein contributed to an optimal translocation mechanism. It is shown that, at least as far as is detectable, this is not so. The apparent rates of processing of various pro-OmpA constructs were measured. It was found that these rates did not depend on the presence of amino acid residues 4 through 45 but on the size of the polypeptides; the processing rate decreased with decreasing size. A possible explanation for this phenomenon is offered. While the results do not exclude the possibility that a defined area of the mature protein is involved in optimizing translocation, there is so far no evidence for it.

Protein translocation across membranes

Many newly synthesized proteins must be translocated across a membrane to reach their final destinations. Translocation requires a signal on the protein itself, a loose conformation of the protein, energy, and receptor-like components in the cytosol and on the target membrane.

Secretion of interleukin-1 beta and Escherichia coli galactokinase by Streptomyces lividans

The functionality of the Streptomyces lividans beta-galactosidase signal peptide to direct heterologous protein export was examined. The signal peptide plus eight amino acids of mature protein were sufficient to export not only a naturally exported protein, interleukin-1 beta, but also a naturally occurring cytoplasmic protein, Escherichia coli galactokinase. Interestingly, cells which expressed yet exported galactokinase were phenotypically Gal-. The potential use of the exported galactokinase system for the isolation and characterization of mutations within signal peptides and the export machinery of the host is discussed.

Expression and properties of prochymosin derivatives containing extensions of various length in the pro-part

A series of hybrid prochymosin derivatives containing portions of the simian virus 40 small-t antigen in the pro-part was constructed. Portions comprising 93, 63, 47, 12, and 1 amino acid (aa) from the N terminus of the small-t antigen were separately fused via eight polylinker-encoded amino acids to a prochymosin product commencing with the 5th aa of the pro-part. All the DNAs coding for the hybrid proteins were put under pL-promoter control in the expression constructs. Expression revealed that only fusion of the 47-aa or 12-aa stretch of the small-t antigen to prochymosin gave stable protein products and that only the latter one allowed the hybrid prochymosin to be activated to chymosin. The products containing 93 aa and 63 aa of small-t antigen were unstable and degraded. Complete removal of the small-t antigen portion led to mRNA instability, probably owing to inefficient initiation of translation.

The expression of mutant pilins in Pseudomonas aeruginosa: fifth position glutamate affects pilin methylation

The expression within Pseudomonas aeruginosa PAO1 of three mutant pilin genes from P. aeruginosa PAK was studied to determine their effects on pilin stability, translocation into the membrane, leader peptide removal, and methylation of the mature N-terminal phenylalanine. The results revealed that a deletion of 4 or 8 amino acids within the immediate N-terminus of pilin had deleterious effects upon leader peptide cleavage. In addition, while the 4-amino-acid deletion did not affect pilin partitioning into the membrane, the 8-amino-acid deletion decreased the amount of pilin found within the membrane fraction. Of considerable interest was the finding that the mutation within the mature pilin of the glutamate at position 5 to a lysine did not prevent leader peptide removal but did inhibit the methylation of the N-terminal phenylalanine.

Role of domain II of Pseudomonas exotoxin in the secretion of proteins into the periplasm and medium by Escherichia coli

Pseudomonas exotoxin (PE) is composed of structural domains I, II, and III; when interacting with mammalian cells the function of domain I is cell recognition, the function of domain II is membrane translocation, and domain III functions in ADP ribosylation. PE is secreted by Pseudomonas aeruginosa into its growth medium. The domain responsible for secretion has been examined by expressing modified PE genes in Escherichia coli under the control of a T7 promoter. Without a signal sequence, PE accumulates within the cell, but PE is secreted into the periplasm when part or all of domain I is removed. PE appears in the periplasm and medium when domain I and part of domain II are removed. Domain II alone is secreted into the periplasm, whereas domain III alone remains within the cell. Addition of an OmpA signal sequence results in secretion of mature PE into the periplasm and secretion of domains II-III into the medium. A protein composed of transforming growth factor alpha fused to the amino terminus of domains II-III is secreted into the periplasm without a signal sequence and into the medium with a signal sequence. A protein composed of domain(s) II or II-III fused to the amino terminus of alkaline phosphatase is secreted into the periplasm and the medium with or without a signal sequence. We conclude that domain II contains important information for protein secretion.

Genetic analysis of sequences in maltoporin that contribute to binding domains and pore structure

Maltoporin (LamB protein) is a maltodextrin transport protein in the outer membrane of Escherichia coli with binding sites for bacteriophage lambda and maltosaccharides. Binding of starch by bacteria was found to inhibit swarming of Escherichia coli in soft agar plates; the inhibition was dependent on the maltodextrin affinity of maltoporin. On the basis of this observation, chemotactic cell-sorting techniques were developed for the isolation and analysis of mutants with an altered starch-binding phenotype. Fifteen lamB mutations generated by hydroxylamine and linker mutagenesis, as well as spontaneous mutations, were analyzed. The effects of the mutations on starch and lambda-binding, as well as transport specificity, were assayed. Mutations that affect residues near 8 to 18, 74 to 82, and 118 to 121 were found to affect starch binding and maltodextrin-selective functions strongly, confirming and extending previous results with substitutions at these regions. Substitutions and insertions in two previously undefined regions in the protein, in or near residues 194 and 360, also resulted in defects in maltodextrin-specific functions and indicate that C-terminal parts of the protein also contribute to the discontinuous binding and pore domains. There was a detectable transport defect in all binding-affected mutants, and one mutation caused near-total pore blocking towards both maltose and nonmaltoside. The highly discontinuous phage lambda-binding site was affected by mutations near residues 9 and 10 and 194, as well as previously established regions near residues 18, 148 to 165, 245 to 259, and 380 to 400. The significance of these mutations is discussed in the context of a model of the functional topology of maltoporin. The additional role of regions near residues 10 and 120 in maltoporin assembly, as well as starch binding, was suggested by the temperature-sensitive biogenesis of maltoporin in strains with one- or two-codon insertion at these sites.