Sequence information within the lamB genes in required for proper routing of the bacteriophage lambda receptor protein to the outer membrane of Escherichia coli K-12

Folding LacZ in the periplasm of Escherichia coli

Targeted, translational LacZ fusions provided the initial support for the signal sequence hypothesis in prokaryotes and allowed for selection of the mutations that identified the Sec translocon. Many of these selections relied on the fact that expression of targeted, translational lacZ fusions like malE-lacZ and lamB-lacZ42-1 causes lethal toxicity as folded LacZ jams the translocation pore. However, there is another class of targeted LacZ fusions that do not jam the translocon. These targeted, nonjamming fusions also show toxic phenotypes that may be useful for selecting mutations in genes involved in posttranslocational protein folding and targeting; however, they have not been investigated to the same extent as their jamming counterparts. In fact, it is still unclear whether LacZ can be fully translocated in these fusions. It may be that they simply partition into the inner membrane where they can no longer participate in folding or assembly. In the present study, we systematically characterize the nonjamming fusions and determine their ultimate localization. We report that LacZ can be fully translocated into the periplasm, where it is toxic. We show that this toxicity is likely due to LacZ misfolding and that, in the absence of the periplasmic disulfide bond catalyst DsbA, LacZ folds in the periplasm. Using the novel phenotype of periplasmic β-galactosidase activity, we show that the periplasmic chaperone FkpA contributes to LacZ folding in this nonnative compartment. We propose that targeted, nonjamming LacZ fusions may be used to further study folding and targeting in the periplasm of Escherichia coli.

Evidence for a coupling of synthesis and export of an outer membrane protein in Escherichia coli

We describe a lesion, lamB701-708, affecting the hydrophilic portion of the lambda receptor signal sequence. The C to A transversion of the sixth codon of the signal sequence changes a positively charged arginine to a neutral serine. The phenotype conferred by this alteration is unique among previously described signal sequence mutations. The results suggest an essential role for the charged amino acids of the hydrophilic segment in the initial interaction between a nascent secreted protein and a membrane export site. The results further suggest that synthesis of lambda receptor is coupled to its export.

Green fluorescent protein functions as a reporter for protein localization in Escherichia coli

The use of green fluorescent protein (GFP) as a reporter for protein localization in Escherichia coli was explored by creating gene fusions between malE (encoding maltose-binding protein [MBP]) and a variant of gfp optimized for fluorescence in bacteria (GFPuv). These constructs encode hybrid proteins composed of GFP fused to the carboxy-terminal end of MBP. Fluorescence was not detected when the hybrid protein was synthesized with the MBP signal sequence. In contrast, when the MBP signal sequence was deleted, fluorescence was observed. Cell fractionation studies showed that the fluorescent MBP-GFP hybrid protein was localized in the cytoplasm, whereas the nonfluorescent version was localized to the periplasmic space. Smaller MBP-GFP hybrid proteins, however, exhibited abnormal fractionation. Expression of the gene fusions in different sec mutants, as well as signal sequence processing assays, confirmed that the periplasmically localized hybrid proteins were exported by the sec-dependent pathway. The distinction between fluorescent and nonfluorescent colonies was exploited as a scorable phenotype to isolate malE signal sequence mutations. While expression of hybrid proteins comprised of full-length MBP did not result in overproduction lethality characteristic of some exported beta-galactosidase hybrid proteins, synthesis of shorter, exported hybrid proteins was toxic to the cells. Purification of MBP-GFP hybrid protein from the different cellular compartments indicated that GFP is improperly folded when localized outside of the cytoplasm. These results suggest that GFP could serve as a useful reporter for genetic analysis of bacterial protein export and of protein folding.

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.

In vivo studies of the role of SecA during protein export in Escherichia coli

SecA is found in Escherichia coli both tightly associated with the cytoplasmic membrane where it functions as a translocation ATPase during protein export and free in the cytosol (R. J. Cabelli, K. M. Dolan, L. Qian, and D. B. Oliver, J. Biol. Chem. 266:24420-24427, 1991; D. B. Oliver and J. Beckwith, Cell 30:311-319, 1982; W. Wickner, A. J. M. Driessen, and F.-U. Hartl, Annu. Rev. Biochem. 60:101-124, 1991). Here we show that SecA can be immunoprecipitated from the cytosol in complex with both fully elongated and nascent species of the precursor of maltose-binding protein, an exported, periplasmic protein. In addition, under conditions in which the distribution of SecA between the cytosolic and membrane-bound states changes from that normally observed, the distribution of precursor maltose-binding protein changes in parallel. These results support the idea that cytosolic SecA plays a role in export. With the aim of determining the roles of the multiple binding sites for ATP on SecA, we compared the export defect in a culture of E. coli expressing a temperature-sensitive allele of secA with the defect in a culture treated with sodium azide. The results indicate that the mutational change and treatment with sodium azide inhibit export by affecting different steps in the cycle of ATP binding and hydrolysis by SecA.

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.

Correct folding of alpha-lytic protease is required for its extracellular secretion from Escherichia coli

alpha-Lytic protease is a bacterial serine protease of the trypsin family that is synthesized as a 39-kD preproenzyme (Silen, J. L., C. N. McGrath, K. R. Smith, and D. A. Agard. 1988. Gene (Amst.). 69: 237-244). The 198-amino acid mature protease is secreted into the culture medium by the native host, Lysobacter enzymogenes (Whitaker, D. R. 1970. Methods Enzymol. 19:599-613). Expression experiments in Escherichia coli revealed that the 166-amino acid pro region is transiently required either in cis (Silen, J. L., D. Frank, A. Fujishige, R. Bone, and D. A. Agard. 1989. J. Bacteriol. 171:1320-1325) or in trans (Silen, J. L., and D. A. Agard. 1989. Nature (Lond.). 341:462-464) for the proper folding and extracellular accumulation of the enzyme. The maturation process is temperature sensitive in E. coli; unprocessed precursor accumulates in the cells at temperatures above 30 degrees C (Silen, J. L., D. Frank, A. Fujishige, R. Bone, and D. A. Agard. 1989. J. Bacteriol. 171:1320-1325). Here we show that full-length precursor produced at nonpermissive temperatures is tightly associated with the E. coli outer membrane. The active site mutant Ser 195----Ala (SA195), which is incapable of self-processing, also accumulates as a precursor in the outer membrane, even when expressed at permissive temperatures. When the protease domain is expressed in the absence of the pro region, the misfolded, inactive protease also cofractionates with the outer membrane. However, when the folding requirement for either wild-type or mutant protease domains is provided by expressing the pro region in trans, both are efficiently secreted into the extracellular medium. Attempts to separate folding and secretion functions by extensive deletion mutagenesis within the pro region were unsuccessful. Taken together, these results suggest that only properly folded and processed forms of alpha-lytic protease are efficiently transported to the medium.

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.

PrlA (SecY) and PrlG (SecE) interact directly and function sequentially during protein translocation in E. coli

Three strategies for genetic analysis show that two inner membrane components of the export machinery, PrlA (SecY) and PrlG (SecE), interact directly while catalyzing the translocation of secreted proteins across the cytoplasmic membrane of E. coli. The first, suppressor-directed inactivation (SDI), exploits the specific interaction between dominant prl suppressors of signal sequence mutations and mutant LacZ hybrid proteins. The second, Sec titration, extends SDI to allow the identification of various Sec proteins that are present in the translocation complex. The third uses the synthetic lethality of certain double-mutant strains to infer physical interactions between gene products. Biochemical data obtained with SDI strains allow the identification of two different secretory intermediates and indicate that PrlG functions before PrlA in the secretion pathway.

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.

Effect of the N-terminal hydrophobic sequence of hepatitis B virus surface antigen on the folding and assembly of hybrid beta-galactosidase in Escherichia coli

To investigate the mechanism of inclusion body formation and the effect of a hydrophobic sequence on the in vivo polypeptide folding, the aggregation caused by recombinant fusion beta-galactosidase in Escherichia coli was examined. Two plasmids were constructed: pTBG(H-) carried only the preS2 sequence of the hepatitis B virus surface antigen (HBsAg) in front of the beta-galactosidase gene (lacZ) while pTBG(H+) carried an additional sequence encoding the amino-terminal hydrophobic sequence of the S region of HBsAg between preS2 and lacZ. Unlike cells expressing the fusion protein not containing the hydrophobic sequence, E. coli JM109/pTBG(H+) exhibited temperature-sensitive production of beta-galactosidase. As the culture temperature increased the activity decreased dramatically. This decrease in activity was not due to a decrease in fusion polypeptide production, but rather the fusion polypeptides containing the hydrophobic sequence aggregated within the cells at high temperature. However once the fusion polypeptides folded into proper conformation at low temperature, they maintained the activity even at high temperature. The results indicate that aggregation is a consequence of incorrect folding and assembly of the polypeptides, and is not derived from the native structure. The aggregates of the pTBG(H+)-encoded fusion polypeptides did not revert to active form when the culture temperature was lowered.

Effects of temperature on Escherichia coli overproducing beta-lactamase or human epidermal growth factor

The effects of temperature on strains of Escherichia coli which overproduce and excrete either beta-lactamase or human epidermal growth factor were investigated. E. coli RB791 cells containing plasmid pKN which has the tac promoter upstream of the gene for beta-lactamase were grown and induced with isopropyl-beta-D-thiogalactopyranoside in batch culture at 37, 30, 25, and 20 degrees C. The lower temperature greatly reduced the formation of periplasmic beta-lactamase inclusion bodies, increased significantly the total amount of beta-lactamase activity, and increased the purity of extracellular beta-lactamase from approximately 45 to 90%. Chemostat operation at 37 and 30 degrees C was difficult due to poor cell reproduction and beta-lactamase production. However, at 20 degrees C, continuous production and excretion of beta-lactamase were obtained for greater than 450 h (29 generations). When the same strain carried plasmid pCU encoding human epidermal growth factor, significant cell lysis was observed after induction at 31 and 37 degrees C, whereas little cell lysis was observed at 21 and 25 degrees C. Both total soluble and total human epidermal growth factor increased with decreasing temperature. These results indicate that some of the problems of instability of strains producing high levels of plasmid-encoded proteins can be mitigated by growth at lower temperatures. Further, lower temperatures can increase for at least some secreted proteins both total plasmid-encoded protein formed and the fraction that is soluble.

Membrane biogenesis in Escherichia coli: effects of a secA mutation

In Escherichia coli K-12, temperature-sensitive mutations in the secA gene have been shown to interfere with protein export. Here we show that the effect of a secA mutation is strongly pleiotropic on membrane biogenesis. Freeze-fracture experiments as well as cryosections of the cells revealed the appearance of intracytoplasmic membranes upon induction of the SecA phenotype. The permeability barrier of the outer membrane to detergents was lost. Two alterations in the outer membrane may be responsible for this effect, namely the reduced amounts of outer membrane proteins, or the reduction of the length of the core oligosaccharide of the lipopolysaccharide, which was observed in phage-sensitivity experiments and by SDS-polyacrylamide gel electrophoresis. Phospholipid analysis of the secA mutant, grown under restrictive conditions, revealed a lower content of the negatively charged phospholipid cardiolipin and of 18:1 fatty acid compared to those of the parental strain grown under identical conditions. These results are in line with the hypothesis that protein export and lipid metabolism are coupled.

PrlA is important for the translocation of exported proteins across the cytoplasmic membrane of Escherichia coli

Strains of Escherichia coli in which lacZ (specifies beta-galactosidase) is fused to genes that specify exported proteins such as LamB (lambda receptor) exhibit unusual phenotypes. In particular, such strains are killed by high-level expression of the LacZ hybrid protein. Previous results suggest that this overproduction phenotype is the consequence of a lethal jamming of the cellular protein export machinery and this hypothesis is supported by the observed accumulation of the precursor forms of many noncytoplasmic proteins within the moribund cell. Under conditions in which protein export is compromised, biochemical and immunocytochemical analyses indicate that these hybrid proteins can be found in transmembrane orientation. To identify the cellular component rendered rate-limiting by the LacZ hybrid protein under jamming conditions we have utilized signal sequence mutations, which block entry of the hybrid protein into the export pathway, and a dominant suppressor of these lesions, prlA4. Data obtained with a series of merodiploids heterozygous and homozygous for prlA+ and prlA4 show that PrlA is the component sequestered by hybrid jamming. Taken together, these results suggest that PrlA is a component of the export machinery that functions in the translocation of proteins across the cytoplasmic 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.

The transmembrane topology of the sn-glycerol-3-phosphate permease of Escherichia coli analysed by phoA and lacZ protein fusions

The Escherichia coli glpT gene encodes a transport protein that mediates uptake of sn-glycerol-3-phosphate. This permease is a member of a class of bacterial organophosphate permeases which transport substrates by antiport with inorganic phosphate. The glpT gene product, probably an oligomer of a single polypeptide chain, is thought to span the cytoplasmic membrane several times, as predicted by the hydropathic profile. Protein fusions, in which varying lengths of the amino-terminal end of the permease is attached to alkaline phosphatase (phoA) and to beta-galactosidase (lacZ) were constructed. On the assumption that phoA fusions only exhibit high enzymatic activity when fused to extra-cytoplasmic regions of the target protein, whereas lacZ fusions will only be active when the beta-galactosidase portion is attached to cytoplasmic domains of the target protein, the activities of the fusions were used to test a two-dimensional model for the permease. The model proposes that GlpT contains 12 transmembrane segments divided by a larger cytoplasmic region. Despite some limitation caused by hot-spot sites of transpositions, the TnphoA approach was consistent with the model. In contrast, we feel that the enzymatic activity of lacZ fusions is only a limited parameter for studying the topology of a complex membrane protein.

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.

Analysis of expression and thermoregulation of the Yersinia pseudotuberculosis inv gene with hybrid proteins

A series of translational fusions between the Yersinia pseudotuberculosis inv locus and lacZ was constructed. Each Lac+ fusion strain expressed a hybrid protein containing invasin, the product of the inv locus, at its amino-terminal end. Analysis of these gene fusions allowed determination of the direction of translation of the inv gene. Previous studies of Y. pseudotuberculosis invasion have shown that entry into animal cells is temperature regulated. It is shown here that control of expression of the inv gene is also temperature regulated. phoA gene fusions to inv, when present in Y. pseudotuberculosis, were expressed at lower levels when bacteria were grown at 37 degrees C rather than at 28 degrees C. Similar fusions, in contrast, were regulated in a temperature-independent fashion in Escherichia coli, as was the wild-type inv gene. This implies that Y. pseudotuberculosis has chromosomally encoded trans-acting functions that normally thermoregulate expression of inv.

Phosphatidylglycerol is involved in protein translocation across Escherichia coli inner membranes

Newly synthesized proteins to be exported out of the cytoplasm of bacterial cells have to pass across the inner membrane. In Gram-negative bacteria ATP, a membrane potential, the products of the sec genes and leader peptidases (enzymes which cleave the N-terminal signal peptides of the precursor proteins) are required. The mechanism of translocation, however, remains elusive. Important additional roles for membrane lipids have been repeatedly suggested both on theoretical grounds and on the basis of experiments with model systems but no direct evidence had been obtained. We demonstrate here, using mutants of Escherichia coli defective in the synthesis of the major anionic membrane phospholipids, that phosphatidylglycerol is involved in the translocation of newly synthesized outer-membrane proteins across the inner membrane.

The fadL gene product of Escherichia coli is an outer membrane protein required for uptake of long-chain fatty acids and involved in sensitivity to bacteriophage T2

The fadL+ gene of Escherichia coli encodes an outer membrane protein (FadL) essential for the uptake of long-chain fatty acids (C12 to C18). The present study shows that in addition to being required for uptake of and growth on the long-chain fatty acid oleate (C18:1), FadL acts as a receptor of bacteriophage T2. Bacteriophage T2-resistant (T2r) strains lacked FadL and were unable to take up and grow on long-chain fatty acids. Upon transformation with the fadL+ clone pN103, T2r strains became sensitive to bacteriophage T2 (T2s), became able to take up long-chain fatty acids at wild-type levels, and contained FadL in the outer membrane.

Export of hybrid proteins FhuA'-'LacZ and FhuA'-'PhoA to the cell envelope of Escherichia coli K-12

The fhuA gene of Escherichia coli K-12 encodes an outer membrane protein that acts as the ferrichrome-iron(III) receptor. To determine the export signals and sorting information within FhuA, gene fusions of fhuA'-'lacZ and fhuA'-'phoA were constructed. Although a FhuA'-'LacZ hybrid protein was detected in the Triton X-100-insoluble fraction of the cell envelope, direct immunoelectron microscopic observation showed that this protein remained in the cytoplasm. FhuA'-'PhoA hybrid proteins were all exported across the cytoplasmic membrane. Those hybrids containing up to 88 amino acids of FhuA (FhuA88) fused to PhoA were released along with other periplasmic proteins. Hybrids containing 180 or more amino acids of FhuA (FhuA180) fused to PhoA were associated with the outer membrane. It is proposed that some information inherent in the sequences between FhuA88 and FhuA180 confers stable association with the outer membrane.

Genetic analysis of the membrane insertion and topology of MalF, a cytoplasmic membrane protein of Escherichia coli

MalF is an essential cytoplasmic membrane protein of the maltose transport system of Escherichia coli. We have developed a general approach for analysis of the mechanism of integration of membrane proteins and their membrane topology by characterizing a series of fusions of beta-galactosidase to MalF. The properties of the fusion proteins indicate the following. (1) The first two presumed transmembrane segments of MalF are sufficient to anchor beta-galactosidase firmly to the inner membrane. (2) Hybrid proteins with beta-galactosidase fused to a presumed cytoplasmic domain of MalF have high beta-galactosidase specific activity; fusions to periplasmic domains have low activity. We propose therefore, that periplasmic and cytoplasmic domains of integral membrane proteins can be distinguished by the enzymatic properties of such hybrid proteins. In general, it appears that cleaved or non-cleaved signal sequences when attached to beta-galactosidase cause it to become embedded in the membrane, and this results in the inability of the hybrid proteins to assemble into active enzyme. Additional properties of these fusion proteins contribute to our understanding of the regulation of MalF synthesis. The MalF protein, synthesized as part of the malEFG operon of E. coli, is approximately 30-fold less abundant in the cell than MalE protein (the maltose-binding protein). Differential amounts of the fusion proteins indicate that a regulatory signal occurs within the malF gene that is responsible for the step-down in expression from the malE gene to the malF gene.

Subcellular localization of a PhoE-LacZ fusion protein in E. coli by protease accessibility experiments reveals an inner-membrane-spanning form of the protein

Protease accessibility experiments were employed to localize a PhoE-LacZ hybrid protein, encompassing a large N-terminal fragment of the outer membrane PhoE protein of E. coli, fused to beta-galactosidase, at the subcellular level. In previous studies, this protein was shown to co-fractionate with the outer membrane, whereas immunocytochemical methods suggested a cytoplasmic location. The present results confirm the latter localization. Moreover, it appears that a minor amount of hybrid protein spans the inner membrane, with the PhoE moiety in the periplasm and the beta-galactosidase moiety in the cytoplasm. These membrane-spanning proteins might be responsible for the lethal jamming of the export machinery, observed upon induction of synthesis of the protein.

secD, a new gene involved in protein export in Escherichia coli

New mutants of Escherichia coli altered in protein export were identified in phoA-lacZ and lamB-lacZ gene fusion strains by searching for mutants that showed an altered lactose phenotype. Several mutations mapped in a new gene, secD. These mutants were, in general, cold sensitive for growth, and the mutations led to an accumulation of precursor of exported proteins. The secD gene is closely linked to tsx on the E. coli chromosome, but separable from another gene proposed to be involved in export, ssaD, which maps nearby. A plasmid carrying secD+ was identified and used to show that the mutations are recessive. The secD gene may code for a component of the cellular export machinery.

The signal sequence suffices to direct export of outer membrane protein OmpA of Escherichia coli K-12

We studied whether information required for export is present within the mature form of the Escherichia coli 325-residue outer membrane protein OmpA. We had previously analyzed overlapping internal deletions in the ompA gene, and the results allowed us to conclude that if such information exists it must be present repeatedly within the membrane part of the protein encompassing amino acid residues 1 to 177 (R. Freudl, H. Schwarz, M. Klose, N. R. Movva, and U. Henning, EMBO J. 4:3593-3598, 1985). A deletion which removed the codons for amino acid residues 1 to 229 of the OmpA protein was constructed. In this construct the signal sequence was fused to the periplasmic part of the protein. The resulting protein, designated Pro-OmpA delta 1-229, was processed, and the mature 95-residue protein accumulated in the periplasm. Hence, information required for export does not exist within the OmpA protein.

In vivo selection and characterization of internal deletions in the lamB::lacZ gene fusion

Strain Pop3299 contains the lamB::lacZ42-12 gene fusion that encodes a hybrid protein that is efficiently exported to the cellular envelope of Escherichia coli. As a result of this efficient export, this strain is killed by the inducer maltose and unable to grow on minimal media supplemented with lactose. In an attempt to isolate mutants in which export of the hybrid protein is altered, we selected Lac+ mutants of strain Pop3299 on lactose tetrazolium media. Unlike mutants previously isolated on lactose minimal media, all the mutants we obtained carried large deletions within the lamB::lacZ gene fusion. Thus, it appears that the type of selection employed affects the type of mutations obtained. We have analyzed the nucleotide sequences of representative mutants, and demonstrate a correlation between the deletion size and the export-related maltose and lactose phenotypes. In addition, we demonstrate that the deletions do not appear to arise from regions of micro-homology.

The cytoplasmic carboxy terminus of M13 procoat is required for the membrane insertion of its central domain

The M13 coat protein spans the Escherichia coli plasma membrane with its amino-terminus facing the periplasm. It is made as a precursor--the procoat--with a typical leader peptide. Mutations which destroy the basic character of the carboxy-terminal domain of procoat, a domain which is oriented towards the cytoplasm, block membrane assembly, while insertion of three lysyl residues near the carboxy terminus partially restores assembly. Thus the information specifying membrane insertion of M13 procoat protein is found in its mature region as well as the leader and is not simply decoded in an amino to carboxy direction.

Effects of signal sequence mutations on the kinetics of alkaline phosphatase export to the periplasm in Escherichia coli

We isolated a collection of mutants defective in the export of alkaline phosphatase to the periplasm. Two classes of mutants were obtained: one class with lesions unlinked to the phoA gene and a second class harboring linked mutations. Among the former class, one mutant is cold sensitive for growth and may be defective in a component of the Escherichia coli secretory apparatus. Included in the latter class are 47 mutants which are characterized in detail in this report. To facilitate DNA sequence analysis of these mutants, we devised a convenient method that relies on homologous recombination in vivo to transfer phoA mutations from the bacterial chromosome directly onto the genome of a single-stranded M13 phage vector. DNA sequence analysis revealed that our collection of mutants comprises six unique mutations, all of which reside in the phoA signal sequence coding region and lend further support to the notion that the length of the hydrophobic core of the signal sequence is crucial for its function in protein export. Kinetic studies showed that in these mutants, the small fraction of alkaline phosphatase which succeeds in reaching a periplasmic location, despite a defective signal sequence, is translocated across the membrane in a slow, posttranslational fashion.

Periplasmic accumulation of truncated forms of outer-membrane PhoE protein of Escherichia coli K-12

In order to localize the information within PhoE protein of Escherichia coli K-12 required for export of the protein to the outer membrane, we have generated deletions throughout the phoE gene. Immunocytochemical labelling on ultrathin cryosections revealed that the polypeptides encoded by the mutant alleles are transported to, and accumulate in, the periplasm. These results show that, except for the signal sequence, there is no specific sequence within the PhoE protein that is essential for transport through the cytoplasmic membrane. The overall structure of the protein, rather than a particular sequence of amino acids, seems to be important for assembly into the outer membrane.

Cell surface exposure of the outer membrane protein OmpA of Escherichia coli K-12

The 325-residue OmpA protein is one of the major outer membrane proteins of Escherichia coli K-12. A model, in which this protein crosses the membrane eight times in an antiparallel beta-sheet conformation and in which regions around amino acids 25, 70, 110 and 154 are exposed at the cell surface, had been proposed. Linkers were inserted into the ompA gene with the result that OmpA proteins, carrying non-OmpA sequences between residues 153 and 154 or 160 and 162, were synthesized. Intact cells possessing these proteins were treated with proteases. Insertion of 15 residues between residues 153 and 154 made the protein sensitive to proteinase K and the sizes of the two cleavage products were those expected following proteolysis at the area of the insertion. Addition of at least 17 residues between residues 160 and 162 left the protein completely refractory to protease action. Thus, the former area is cell surface exposed while the latter area appears not to be. The insertions did not cause a decrease in the concentration of the hybrid proteins as compared to that of the OmpA protein, and in neither case was synthesis of the protein deleterious to cell growth. It is suggested that this method may serve to carry peptides of practical interest to the cell surface and that it can be used to probe surface-located regions of other membrane proteins.

Determinants for protein translocation across mammalian endoplasmic reticulum. Membrane insertion of truncated and full-length prelysozyme molecules

The translocation of fragments of prelysozyme lacking varying portions of the COOH terminus of the protein is studied in comparison to full-length prelysozyme using transcription-coupled capping of RNA and subsequent translation in a wheat germ cell-free system. The fragments are generated by restricting cloned lysozyme cDNA at selected sites. We found that fragments of 102 and 74 amino acid residues could still be translocated by mammalian endoplasmic reticulum. Addition of signal-recognition particles (SRP) to the cell-free system blocked the nascent chain synthesis. The SRP-depleted membrane by itself could neither process nor translocate the prepolypeptide chain. The presence of both components was essential for processing and translocation as well as the release of the nascent chain arrest induced by SRP. However, when the size of the fragment was limited to 51 amino acids, the SRP-induced arrest, the translocation and processing failed to take place. These results define minimum length and structural requirements for translocation of the nascent chain across mammalian endoplasmic reticulum.

Protein fusions of beta-galactosidase to the ferrichrome-iron receptor of Escherichia coli K-12

The fusion-generating phage lambda plac Mu1 was used to produce fusions of lacZ to fhuA, the gene encoding the ferrichrome-iron receptor (FhuA protein) in the outer membrane of Escherichia coli K-12. Fusions to the fhuA gene in a delta (lac) strain were selected by their resistance to bacteriophage phi 80 vir. Ten independent (fhuA'-'lacZ) fusions were all Lac+ and were resistant to the lethal agents which require the FhuA protein as receptor, i.e., phi 80 vir, T5, T1, UC-1, and colicin M; none could utilize ferrichrome as the sole iron source. Specialized transducing phages were obtained by illegitimate excision from the chromosome of each of the fusion-bearing strains, and EcoRI fragments which encoded the fusions were subcloned into the high-copy plasmid pMLB524. Physical mapping of the fusion-containing plasmids confirmed the presence of three restriction sites which were also located on the chromosomal DNA of sequences near the fhuA gene. The direction of transcription of the fhuA gene was deduced from the direction of transcription of the (fhuA'-'lacZ) gene fusion. Identification of the chimeric proteins was made by both radiolabeling cells and immunoprecipitating the LacZ-containing proteins with antibody to beta-galactosidase and by preparing whole cell extracts from Lac+ cells containing the cloned gene fusions. Two sizes of (FhuA'-'LacZ) proteins were detected, 121 kDa and 124 kDa. The DNA sequences at the unique fusion joints were determined. The sequence information allowed us to identify three distinct fusion joints which were grouped as follows, type I fusions, 5'-ACT GCT CAG CCA A-3'; type IIa fusions, 5'-GCG GTT GAA CCG A-3'; and type IIb fusions: 5'-ACC GCT GCA CCT G-3'. To orient these fhuA fusion joints, the complete nucleotide sequence of the fhuA gene was determined from a 2,902-base-pair fragment of DNA. A single open reading frame was found which translated into a 747-amino acid polypeptide. The signal sequence of 33 amino acids was followed by a mature protein with a molecular weight of 78,992. Alignment of the amino acid sequence of the FhuA protein with the amino acid sequences presented for two other tonB-dependent receptor proteins in the outer membrane of E. coli showed an area of local homology at the amino terminus of all three proteins.

Protein translocation across and integration into membranes

This review concentrates mainly on the translocation of proteins across the endoplasmic reticulum membrane and cytoplasmic membrane in bacteria. It will start with a short historical review and will pinpoint the crucial questions in the field. Special emphasis will be given to the present knowledge on the molecular details of the first steps, i.e., on the function of the signal recognition particle and its receptor. The knowledge on the signal peptidase and the ribosome receptor(s) will also be summarized. The various models for the translocation of proteins across and the integration of proteins into membranes will be critically discussed. In particular, the function of signal, stop-transfer, and insertion sequences will be dealt with and molecular differences discussed. The cotranslational mode of membrane transfer will be compared with the post-translational transport found for mitochondria and chloroplasts. This review will conclude with open questions and an outlook.

Lethal mutations in the structural gene of an outer membrane protein (OmpA) of Escherichia coli K12

The gene ompA encodes a major outer membrane protein of Escherichia coli. Localized mutagenesis of the part of the gene corresponding to the 21-residue signal sequence and the first 45 residues of the protein resulted in alterations which caused cell lysis when expressed. DNA sequence analyses revealed that in one mutant type the last CO2H-terminal residue of the signal sequence, alanine, was replaced by valine. The proteolytic removal of the signal peptide was much delayed and most of the unprocessed precursor protein was fractioned with the outer membrane. However, this precursor was completely soluble in sodium lauryl sarcosinate which does not solubilize the OmpA protein or fragments thereof present in the outer membrane. Synthesis of the mutant protein did not inhibit processing of the OmpA or OmpF proteins. In the other mutant type, multiple mutational alterations had occurred leading to four amino acid substitutions in the signal sequence and two affecting the first two residues of the mature protein. A reduced rate of processing could not be clearly demonstrated. Membrane fractionation suggested that small amounts of this precursor were associated with the plasma membrane but synthesis of this mutant protein also did not inhibit processing of the wild-type OmpA or OmpF proteins. Several lines of evidence left no doubt that the mature mutant protein is stably incorporated into the outer membrane. It is suggested that the presence, in the outer membrane, of the mutant precursor protein in the former case, or of the mutant protein in the latter case perturbs the membrane architecture enough to cause cell death.

Maltose-binding protein does not modulate the activity of maltoporin as a general porin in Escherichia coli

Maltoporin (lambda receptor) is part of the maltose transport system in Escherichia coli and is necessary for the facilitated diffusion of maltose and maltodextrins across the outer membrane. Maltoporin also allows the diffusion of nonmaltodextrin substrates, albeit with less efficiency. The preference of maltoporin for maltodextrins in vivo is thought to be the result of an interaction of maltoporin with the maltose-binding protein, the malE gene product. In a recent report Heuzenroeder and Reeves (J. Bacteriol. 144:431-435, 1980) suggested that this interaction establishes a gating mechanism which inhibits the diffusion of nonmaltodextrin substrates, such as lactose. To reinvestigate this important conclusion, we constructed ompR malTc strains carrying either the malE+ gene, the nonpolar malE444 deletion, or the malE254 allele, which specifies an interaction-deficient maltose-binding protein. Lactose uptake was measured at different concentrations below the Km of this transport system and under conditions where transport was limited by the diffusion through maltoporin. We found no difference in the kinetics of lactose uptake irrespective of the malE allele. We conclude that the maltose-binding protein does not modulate the activity of maltoporin as a general outer membrane porin.

Fine structure of a membrane anchor domain

We describe a detailed deletion analysis of the anchoring domain of a model membrane protein. Removal of the 23 contiguous uncharged amino acids from the carboxy terminus of the bacteriophage fl gene III protein (pIII) converts it from an integral membrane protein to a secreted periplasmic form. Deletions that remove six or fewer residues of the hydrophobic core result in no diminution of the protein's capacity to anchor in the membrane. Longer deletions into this hydrophobic domain gradually destablize the protein-membrane association. pIII derivatives with over half of the hydrophobic core deleted retain substantial residual anchor function. The basic residues, arginine and lysine, which provide a carboxy-terminal boundary for this domain, can be deleted without loss of anchoring capacity.

Purification and properties of the myo-inositol-binding protein from a Pseudomonas sp

A myo-inositol-binding protein was isolated from a Pseudomonas sp. soil isolate and was purified to homogeneity. Its molecular weight is 30,000, and it has a single binding site. The amino acid analysis showed that the protein contains three tryptophan residues and no cysteine. Tryptophan residues seem to be involved in the binding of the ligand, as shown by the modification of the fluorescence spectra and by the fact that oxidation of tryptophan residues with N-bromosuccinimide abolished the binding of myo-inositol. Sequence analysis of the N-terminal segment of 37 amino acids showed that 13 are conserved when compared with the galactose-binding protein of Escherichia coli.