The general protein-export pathway is directly required for extracellular pullulanase secretion in Escherichia coli K12

The general secretory pathway of Klebsiella oxytoca: no evidence for relocalization or assembly of pilin-like PulG protein into a multiprotein complex

It has been proposed that the four type IV pilin-like proteins that are required for extracellular protein secretion by the general secretory pathway (GSP) might assemble into a trans-periplasm complex resembling a type IV pilus. To test this idea, we examined the subcellular distribution and oligomeric state of PulG, one of the type IV pilin-like proteins required for pullulanase secretion in Klebsiella oxytoca. Fractionation of Escherichia coli cells carrying a single copy of each pul gene showed that PulG protein was located in two distinct envelope fractions corresponding to the outer and cytoplasmic membranes. The protein was partially released by treating the membranes with Triton X-100 + EDTA or at high pH, but not by Triton X-100 alone or by 8 M urea, 6 M guanidine hydrochloride or 1 M NaCl. Like type IV pilins, non-sedimentable PulG that had been released from the membranes at high pH could be sedimented by centrifugation when the pH was lowered. Treatment of whole cells, sphaeroplasts or isolated membranes with a cleavable cross-linking agent produced mainly PulG homodimers. Previous studies showed that both PulO, which cleaves and N-methylates the PulG precursor, and PulE, a putative ATP-binding protein, share extensive sequence identity with proteins known to be required for type IV pilus processing and assembly. However, mutations which disrupted either pulE or pulO, or indeed the complete absence of all other components of the pullulanase secretion apparatus, had little or no effect on any of the properties of PulG protein described above. We conclude that there is no evidence that PulG protein assembles into a stable multiprotein complex or that processing of the PulG precursor causes a detectable change in its subcellular distribution.

The role of the lipoprotein sorting signal (aspartate +2) in pullulanase secretion

The analyses of hybrid proteins and of deletion and insertion mutations reveal that the only amino acid at the amino-proximal end of the cell surface lipoprotein pullulanase that is specifically required for its extracellular secretion is an aspartate at position +2, immediately after the fatty acylated amino-terminal cysteine. To see whether the requirement for this amino acid is related to its proposed role as a cytoplasmic membrane lipoprotein sorting signal, we used sucrose gradient floatation analysis to determine the subcellular location of pullulanase variants (with or without the aspartate residue) that accumulated in cells lacking the pullulanase-specific secretion genes. A non-secretable pullulanase variant with a serine at position +2 cofractionated mainly with the major peak of outer membrane porin. In contrast, most (55%) of a pullulanase variant with an aspartate at position +2 cofractionated with slightly lighter fractions that contained small proportions of both outer membrane porin and the cytoplasmic membrane marker NADH oxidase. Only 5% of this pullulanase variant cofractionated with the major NADH oxidase peak, while the rest (c. 40%) remained at the bottom of the gradient in fractions totally devoid of porin and NADH oxidase. When analysed by sedimentation through sucrose gradients, however, a large proportion of this variant was recovered from fractions near the top of the gradient that also contained the major NADH oxidase peak. When this peak fraction was applied to a floatation gradient the pullulanase activity remained at the bottom while the NADH oxidase floated to the top.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the Klebsiella pneumoniae pullulanase-encoding gene in Saccharomyces cerevisiae

A 3800-base pair (bp) DNA fragment encoding the mature pullulanase from Klebsiella pneumoniae was inserted between two different yeast expression-secretion cassettes and an yeast gene terminator. These cassettes were cloned into an yeast centromeric plasmid YCplacIII and transformed into laboratory strains of Saccharomyces cerevisiae. Transcription initiation signals were derived from the mating pheromone alpha-factor (MF alpha 1p) and alcohol dehydrogenase (ADC1p) gene promoters. Secretion of pullulanase was directed by the leader sequence of the yeast mating pheromone alpha-factor (MF alpha 1s). Transcription termination was effected by the yeast tryptophan synthase gene terminator (TRP5T). Southernblot analysis confirmed the presence of pulA in transformed yeasts and Northern-blot analysis revealed the presence of PUL1 mRNA. A pullulan agarose assay indicated the extracellular production of biologically active pullulanase by S. cerevisiae.

A novel secretion apparatus for the assembly of adhesive bacterial pili

The biogenesis of most types of bacterial pili requires two specialized proteins: a chaperone that caps the pilus subunits in the periplasm, and an outer membrane usher that receives the subunits and serves as an assembly platform. This secretion and assembly machinery is proposed to be a novel export apparatus found widely in Gram-negative pathogens.

Mutagenesis of cellulase EGZ for studying the general protein secretory pathway in Erwinia chrysanthemi

Extracellular secretion of endoglucanase Z (EGZ) from Erwinia chrysanthemi is mediated by the so-called Out general secretion pathway and, presumably, involves recognition of EGZ-carried structural information by one or more of the Out proteins. Investigating the relationships between structure and secretability of EGZ was the purpose of the present work. EGZ is made of two independent domains, located at the N- and C-proximal sides, separated by a Ser/Thr-rich region, which are responsible for catalysis and cellulose-binding, respectively. The existence of a secretion region ('targeting signal') was investigated by studying the secretability of modified EGZ derivatives. These resulted from deletion or peptide insertion and were designed by using the domain organization cited above as a guide. Catalytic and/or cellulose-binding tests showed that all proteins exhibited at least a functional EGZ domain while immunoblot analyses confirmed that neither the insertions nor the deletions led to grossly misfolded proteins. In contrast, all of the proteins lost their secretability in E. chrysanthemi. This suggested that at least two secretion motifs existed, one lying within each functional domain. The role of the Ser/Thr-rich linker region was subsequently tested. Accordingly, two proteins containing a linker region whose length was increased by the addition of 8 and 18 additional residues and one protein lacking the linker region were studied. All three exhibited endoglucanase activity and cellulose-binding ability, confirming the independence of the domains within the context of EGZ/polysaccharide interaction. In contrast, none was secreted by E. chrysanthemi.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of lipopolysaccharide in the secretion of Escherichia coli alpha-haemolysin and Erwinia chrysanthemi proteases

The presence of the alpha-haemolysin secretion genes sensitizes Escherichia coli to vancomycin, a glycopeptide antibiotic that is normally excluded from the Gram-negative envelope (owing to its large size) (M(r) 1400). The selection of vancomycin mutants in strains carrying such genes was found to be a very powerful method for selecting non-haemolytic mutants. In this way, mutations in the known secretion genes, hlyB, hlyD and tolC, were obtained. However additional mutations mapped in genes rfaH and galU which are required for lipopolysaccharide (LPS) biosynthesis. Mutations in rfaH and galU strongly reduced alpha-haemolysin secretion as well as the secretion of Erwinia chrysanthemi proteases in E. coli without affecting their synthesis. These mutations markedly lowered the content of TolC protein, required for haemolysin secretion and also of the PrtF protein necessary for protease secretion. These results raise the possibility that LPS is involved in the correct incorporation of the TolC and PrtF proteins into the cell envelope.

Membrane traffic wardens and protein secretion in gram-negative bacteria

Recent progress in the genetic analysis of protein secretion in diverse Gram-negative bacteria has revealed three major, highly conserved but functionally independent pathways that involve accessory apparatus proteins. Protein secretion via the Type I pathway is signal sequence-independent with no free periplasmic intermediate. Secretion by the Type II pathway is signal sequence-dependent and via the periplasm. Recent results also suggest that a third (Type III) secretory pathway exists in which protein secretion is signal sequence-independent.

Translocation of a folded protein across the outer membrane in Escherichia coli

A mutation in the Escherichia coli dsbA gene (coding for a periplasmic disulfide oxidoreductase) reduces the rate of disulfide bond formation in the enzyme pullulanase and also reduces the rate at which the enzyme is secreted to the cell surface, as measured by protease accessibility. The enzyme did not become protease accessible when disulfide bond formation was completely prevented in the mutant strain by carboxymethylation. These results indicate that a disulfide bond may be required for, and certainly does not impede, the translocation of pullulanase across the outer membrane. Since it is unlikely that a disulfide bond could be formed and then reduced again in the periplasm, these results would appear to strengthen the argument that pullulanase polypeptides fold into or close to their final conformation before they are transported across the outer membrane. It is suggested that this might be a feature common to all proteins that are secreted by other Gram-negative bacteria by a pullulanase-like pathway.

Secretion across the bacterial outer membrane

Many bacteria secrete extracellular proteins such as hydrolytic enzymes or toxins. In Gram-negative bacteria, secreted proteins must cross the two membranes that constitute the cell envelope. Recent studies have identified several specific secretion systems that can be classified in three distinct pathways, and related systems have been discovered in a wide range of prokaryotic and eukaryotic cells.

Export of the outer membrane lipoprotein is defective in secD, secE, and secF mutants of Escherichia coli

The export of major outer membrane lipoprotein has been found to be affected in secD, secE, and secF mutants of Escherichia coli, which are defective in protein export in general. After a shift to the nonpermissive temperature, the kinetics of accumulation of prolipoprotein and pre-OmpA protein was indistinguishable from that of pre-OmpA protein accumulation in the secD and secF mutants but different in the secE mutant. The prolipoprotein accumulated in the secD, secE, and secF mutants at the nonpermissive temperature was not modified with glyceride. We conclude from these results and those of previous studies that the export of lipoprotein requires all common sec gene products except the SecB protein, i.e., the SecA, SecD, SecE, SecF, and SecY proteins.

Cloning and characterization of a gene required for the secretion of extracellular enzymes across the outer membrane by Xanthomonas campestris pv. campestris

Nonpathogenic mutants of Xanthomonas campestris pv. campestris, generated from transposon mutagenesis, accumulated extracellular polygalacturonate lyase, alpha-amylase, and endoglucanase in the periplasm. The transposon Tn5 was introduced by a mobilizable, suicidal plasmid, pSUP2021 or pEYDG1. Genomic banks of wild-type X. campestris pv. campestris, constructed on the broad-host-range, mobilizable cosmid pLAFR1 or pLAFR3, were conjugated with one of the mutants, designated XC1708. Recombinant plasmids isolated by their ability to complement XC1708 can be classified into two categories. One, represented by pLASC3, can complement some mutants, whereas the other, represented by a single plasmid, pLAHH2, can complement all of the other mutants. Restriction mapping showed that the two recombinant plasmids shared an EcoRI fragment of 8.9 kb. Results from subcloning, deletion mapping, and mini-Mu insertional mutation of the 8.9-kb EcoRI fragment suggested that a 4.2-kb fragment was sufficient to complement the mutant XC1708. Sequence analysis of this 4.2-kb fragment revealed three consecutive open reading frames (ORFs), ORF1, ORF2, and ORF3. Hybridization experiments showed that Tn5 in the genome of XC1708 and other mutants complemented by pLASC3 was located in ORF3, which could code for a protein of 83.5 kDa. A signal peptidase II processing site was identified at the N terminus of the predicted amino acid sequence. Sequence homology of 51% was observed between the amino acid sequences predicted from ORF3 and the pulD gene of Klebsiella species.

An enzyme with type IV prepilin peptidase activity is required to process components of the general extracellular protein secretion pathway of Klebsiella oxytoca

The last gene (pulO) of the pulC-O pullulanase secretion gene operon of Klebsiella oxytoca codes for a protein that is 52% identical to the product of the pilD/xcpA gene required for extracellular protein secretion and type IV pilus biogenesis in Pseudomonas aeruginosa. The PilD/XcpA protein is known to remove the first six amino acids of the signal sequence of the type IV pilin precursor by cleaving after the glycine residue in the conserved sequence GF(M)XXXE (where X represents hydrophobic amino acids). This prepilin peptidase cleavage site is present in the products of four genes in the pulC-O operon (PulG, PulH, Pull and PulJ proteins). It is shown here that PulO processes the pulG gene product in vivo. Processing was maximal within 15 seconds, but experiments in which the expression of pulO was uncoupled from that of the other genes in the secretion operon suggest that processing can also occur post-translationally. The products of two pulG derivatives with internal inframe deletions were also processed by PulO, but the three PulG-PhoA hybrids, two PulJ-PhoA hybrids and the single PulH-PhoA hybrid tested did not appear to be processed. Sucrose gradient fraction experiments showed that both precursor and mature forms of PulG appear to be associated with low-density, outer membrane vesicles prepared by osmotic lysis of sphaeroplasts. Neither the xcpA gene nor the Bacillus subtilis gene comC, which is also homologous to pulO and codes for a protein with type IV prepilin peptidase activity, can correct the pullulanase secretion defect in an Escherichia coli strain carrying all of the genes required for secretion except pulO. Furthermore, neither XcpA nor ComC is able to process prePulG protein in vivo.

Protein secretion in bacteria

Most secretory proteins are synthesized as precursors with an amino-terminal signal peptide. Genetic identification of proteins essential for signal peptide dependent translocation to the Escherichia coli periplasm has led to the biochemical dissection of the secretion pathway. Additional mechanisms exist in Gram-negative bacteria for protein secretion to the extracellular environment.

Pullulanase secretion in Escherichia coli K-12 requires a cytoplasmic protein and a putative polytopic cytoplasmic membrane protein

The previously uncharacterized third and fourth genes (pulE and pulF) of the pullulanase secretion gene operon of Klebsiella oxytoca strain UNF5023 are, respectively, predicted to encode a 55 kDa polypeptide with a putative nucleotide-binding site, and a highly hydrophobic 44 kDa polypeptide that probably spans the cytoplasmic membrane several times. Expression of pulE in minicells or under the control of a strong bacteriophage T7 promoter resulted in the production of a c. 58 kDa cytoplasmic protein. A representative PulE-beta-galactosidase hybrid protein created by Tnlac mutagenesis was also found mainly in the cytoplasm. These results are in line with the predicted absence from PulE of a region of sufficient hydrophobicity to function as a signal sequence. The PulF polypeptide could not be detected either in minicells or when the gene was transcribed from the T7 promoter, but the acquirement of three pulF-lacZ gene fusions that encoded hybrid proteins with relatively high levels of beta-galactosidase activity indicates that this gene can be transcribed and translated. Gene disruption experiments indicated that both pulE and pulF are required for pullulanase secretion in Escherichia coli K-12. Both proteins exhibit considerable homology throughout their entire lengths with other proteins involved in protein secretion, pilin assembly, conjugation and transformation competence in a variety of bacteria. In addition, PulE protein has consensus sequences found in a wide variety of nucleotide-binding proteins. This study completes the initial characterization of the pullulanase secretion gene operon, which comprises 13 genes that are all essential for the transport of pullulanase across the outer membrane.

Neisseria gonorrhoeae prepilin export studied in Escherichia coli

The pilE gene of Neisseria gonorrhoeae MS11 and a series of pilE-phoA gene fusions were expressed in Escherichia coli. The PhoA hybrid proteins were shown to be located in the membrane fraction of the cells, and the prepilin product of the pilE gene was shown to be located exclusively in the cytoplasmic membrane. Analysis of the prepilin-PhoA hybrids showed that the first 20 residues of prepilin can function as an efficient export (signal) sequence. This segment of prepilin includes an unbroken sequence of 8 hydrophobic or neutral residues that form the N-terminal half of a 16-residue hydrophobic region of prepilin. Neither prepilin nor the prepilin-PhoA hybrids were processed by E. coli leader peptidase despite the presence of two consensus cleavage sites for this enzyme just after this hydrophobic region. Comparisons of the specific molecular activities of the four prepilin-PhoA hybrids and analysis of their susceptibility to proteolysis by trypsin and proteinase K in spheroplasts allow us to propose two models for the topology of prepilin in the E. coli cytoplasmic membrane. The bulk of the evidence supports the simplest of the two models, in which prepilin is anchored in the membrane solely by the N-terminal hydrophobic domain, with the extreme N terminus facing the cytoplasm and the longer C terminus facing the periplasm.

Extracellular secretion of pectate lyase by the Erwinia chrysanthemi out pathway is dependent upon Sec-mediated export across the inner membrane

The plant pathogenic enterobacterium Erwinia chrysanthemi EC16 secretes several extracellular, plant cell wall-degrading enzymes, including pectate lyase isozyme PelE. Secretion kinetics of 35S-labeled PelE indicated that the precursor of PelE was rapidly processed by the removal of the amino-terminal signal peptide and that the resulting mature PelE remained cell bound for less than 60 s before being secreted to the bacterial medium. PelE-PhoA (alkaline phosphatase) hybrid proteins generated in vivo by TnphoA insertions were mostly localized in the periplasm of E. chrysanthemi, and one hybrid protein was observed to be associated with the outer membrane of E. chrysanthemi in an out gene-dependent manner. A gene fusion resulting in the substitution of the beta-lactamase signal peptide for the first six amino acids of the PelE signal peptide did not prevent processing or secretion of PelE in E. chrysanthemi. When pelE was overexpressed, mature PelE protein accumulated in the periplasm rather than the cytoplasm in cells of E. chrysanthemi and Escherichia coli MC4100 (pCPP2006), which harbors a functional cluster of E. chrysanthemi out genes. Removal of the signal peptide from pre-PelE was SecA dependent in E. coli MM52 even in the presence of the out gene cluster. These data indicate that the extracellular secretion of pectic enzymes by E. chrysanthemi is an extension of the Sec-dependent pathway for general export of proteins across the bacterial inner membrane.

Two distinct steps in pullulanase secretion by Escherichia coli K12

Two distinct steps in the secretion of the extracellular, cell-surface-anchored lipoprotein pullulanase by Escherichia coli were uncoupled by allowing export of the enzyme to the cytoplasmic membrane via the signal peptide/sec-gene-dependent general export pathway, and then inducing the pulC-O operon of genes required for translocation to the cell surface. The secretion intermediate cofractionated mainly with intermediate-density vesicles when cells were gently lysed and the resulting vesicles were separated by isopycnic sucrose density centrifugation. Cytoplasmic forms of pullulanase (which are not exported because they lack a functional signal peptide) are more sensitive to heat inactivation, denaturation by sodium dodecyl sulphate and carboxymethylation than the intermediate and cell-surface forms. The latter are distinguished only by the fact that the secretion intermediate is less susceptible to proteinase K and trypsin, and is partially inaccessible to substrate or in an inactive conformation in sphaeroplasts. These and other results indicate that the secretion intermediate can acquire considerable higher-ordered structure, including disulphide bridges, before it is transported to the cell surface; this seems to rule out the possibility that it is threaded through this membrane as a locally unfolded polypeptide.