Extracellular secretion of pullulanase is unaffected by minor sequence changes but is usually prevented by adding reporter proteins to its N- or C-terminal end

Towards the identification of type II secretion signals in a nonacylated variant of pullulanase from Klebsiella oxytoca

Pullulanase (PulA) from the gram-negative bacterium Klebsiella oxytoca is a 116-kDa surface-anchored lipoprotein of the isoamylase family that allows growth on branched maltodextrin polymers. PulA is specifically secreted via a type II secretion system. PelBsp-PulA, a nonacylated variant of PulA made by replacing the lipoprotein signal peptide (sp) with the signal peptide of pectate lyase PelB from Erwinia chrysanthemi, was efficiently secreted into the medium. Two 80-amino-acid regions of PulA, designated A and B, were previously shown to promote secretion of beta-lactamase (BlaM) and endoglucanase CelZ fused to the C terminus. We show that A and B fused to the PelB signal peptide can also promote secretion of BlaM and CelZ but not that of nuclease NucB or several other reporter proteins. However, the deletion of most of region A or all of region B, either individually or together, had only a minor effect on PelBsp-PulA secretion. Four independent linker insertions between amino acids 234 and 324 in PelBsp-PulA abolished secretion. This part of PulA, region C, could contain part of the PulA secretion signal or be important for its correct presentation. Deletion of region C abolished PelBsp-PulA secretion without dramatically affecting its stability. PelBsp-PulA-NucB chimeras were secreted only if the PulA-NucB fusion point was located downstream from region C. The data show that at least three regions of PulA contain information that influences its secretion, depending on their context, and that some reporter proteins might contribute to the secretion of chimeras of which they are a part.

Evaluation of bottlenecks in proinsulin secretion by Escherichia coli

This work evaluates three potential bottlenecks in recombinant human proinsulin secretion by Escherichia coli: protein stability, secretion capacity and the effect of molecular size on secretion efficiency. A maximum secretion level of 7.2 mg g(-1) dry cell weight was obtained in the periplasm of E. coli JM109(DE3) host cells. This value probably represents an upper limit in the transport capacity of E. coli cells secreting ZZ-proinsulin and similar proteins with the protein A signal peptide. A selective deletion study was performed in the fusion partner and no effect of the molecular size (17-24 kDa) was detected on secretion efficiency. The protective effect against proteolysis provided by the ZZ domain was thoroughly demonstrated in the periplasm of E. coli and it was also shown that a single Z domain is able to provide the same protection level without compromising the downstream processing. The use of this shorter fusion partner enables a 1.6-fold increase in the recovery of the target protein after cleavage of the affinity handle.

Depletion of apolipoprotein N-acyltransferase causes mislocalization of outer membrane lipoproteins in Escherichia coli

Lipoproteins in Gram-negative Enterobacteriaceae carry three fatty acids on the N-terminal cysteine residue, two as a diacylglyceride and one through an N-linkage following signal peptide cleavage. Most lipoproteins are anchored in the outer membrane, facing the periplasm, but some lipoproteins remain in the plasma membrane, depending on the amino acid at position +2, immediately after the fatty-acylated cysteine. In vitro, the last step in lipoprotein maturation, N-acylation of apolipoproteins by the plasma membrane apolipoprotein N-acyltransferase (Lnt), is necessary for efficient recognition of outer membrane lipoproteins by the Lol system, which transports them from the plasma to the outer membrane (Fukuda, A., Matsuyama, S.-I., Hara, T., Nakayama, J., Nagasawa, H., and Tokuda, H. (2002) J. Biol. Chem. 277, 43512-43518). To study the role of Lnt in vivo, we constructed a conditional lnt mutant of Escherichia coli. The apo-form of peptidoglycan-anchored major lipoprotein (Lpp) and two other outer membrane lipoproteins accumulated in the plasma membrane when lnt expression was reduced. We also found that Lnt is an essential protein in E. coli and that the lethality is partially because of the retention of apoLpp in the plasma membrane. Topology mapping of Lnt with beta-galactosidase and alkaline phosphatase fusions indicated the presence of six membrane-spanning segments. The lnt gene in a mutant of Salmonella enterica displaying thermosensitive Lnt activity (Gupta, S. D., Gan, K., Schmid, M. B., and Wu, H. C. (1993) J. Biol. Chem. 268, 16551-16556) was found to carry a mutation causing a single glutamate to lysine substitution at a highly conserved position in the last predicted periplasmic loop of the protein.

An intramolecular disulphide bond reduces the efficacy of a lipoprotein plasma membrane sorting signal

To study lipoprotein sorting in Escherichia coli, we devised a novel screen in which sensitivity or resistance to bacteriophage T5 and colicin M reflects the membrane localization of the bacteriophage T5-encoded lipoprotein Llp, which inactivates the outer membrane (OM) T5 receptor (FhuA). When processed by lipoprotein signal peptidase, Llp has a serine at position +2, immediately after the fatty acylated N-terminal cysteine. As predicted by the '+2 lipoprotein sorting rule' that determines the localization of lipoproteins in the cell envelope, Llp is located in the OM. However, contrary to expectations, when serine +2 was replaced by aspartate, the canonical plasma membrane lipoprotein retention signal, Llp was still > or =40% targeted to the OM and protected cells against colicin M and phage T5. OM association of this Llp derivative was abolished when a peptide spacer was inserted between the aspartate and the rest of Llp or when the formation of an intramolecular disulphide bond in Llp was prevented by substituting one or other of the cysteines involved. Furthermore, analysis of a MalE-Llp hybrid protein with or without a lipid moiety demonstrated that fatty acylation of Llp is essential for its OM association and for protection against colicin M and bacteriophage T5. These data suggest (i) that phage-encoded Llp uses the endogenous E. coli Lol pathway for lipoprotein sorting to the OM and (ii) that the conformation of a lipoprotein can affect its sorting within the cell envelope.

The extracellular transport signal of the Vibrio cholerae endochitinase (ChiA) is a structural motif located between amino acids 75 and 555

ChiA, an 88-kDa endochitinase encoded by the chiA gene of the gram-negative enteropathogen Vibrio cholerae, is secreted via the eps-encoded main terminal branch of the general secretory pathway (GSP), a mechanism which also transports cholera toxin. To localize the extracellular transport signal of ChiA that initiates transport of the protein through the GSP, a chimera comprised of ChiA fused at the N terminus with the maltose-binding protein (MalE) of Escherichia coli and fused at the C terminus with a 13-amino-acid epitope tag (E-tag) was expressed in strain 569B(chiA::Kan(r)), a chiA-deficient but secretion-competent mutant of V. cholerae. Fractionation studies revealed that blockage of the natural N terminus and C terminus of ChiA did not prevent secretion of the MalE-ChiA-E-tag chimera. To locate the amino acid sequences which encoded the transport signal, a series of truncations of ChiA were engineered. Secretion of the mutant polypeptides was curtailed only when ChiA was deleted from the N terminus beyond amino acid position 75 or from the C terminus beyond amino acid 555. A mutant ChiA comprised of only those amino acids was secreted by wild-type V. cholerae but not by an epsD mutant, establishing that amino acids 75 to 555 independently harbored sufficient structural information to promote secretion by the GSP of V. cholerae. Cys77 and Cys537, two cysteines located just within the termini of ChiA(75-555), were not required for secretion, indicating that those residues were not essential for maintaining the functional activity of the ChiA extracellular transport signal.

A putative three-dimensional targeting motif of polygalacturonase (PehA), a protein secreted through the type II (GSP) pathway in Erwinia carotovora

Intramolecular information specifying protein secretion through the type II (GSP) pathway of Gram-negative bacteria was investigated. Two regions of the polygalacturonase (PehA) of Erwinia carotovora containing residues proposed to be included in a targeting motif were located, one close to the C-terminus between residues 342 and 369 and another between residues 84 and 135 in the large central loops. The regions were required together to promote secretion. Further residues in the middle of the protein were required for proper positioning of the regions, suggesting that they were both involved in interaction with the GSP. To our knowledge, this is the first time that a possible three-dimensional targeting motif has been defined. At least one of the motifs comprises a cluster on the surface of the protein. The two motifs are structurally dissimilar, suggesting that there are two distinct recognition regions in the GSP apparatus. Finally, we propose that the targeting motifs are of a complex conformational nature with some variability accommodated, as illustrated by the observation that many mutations exhibited no clear phenotype individually but, in combination, severely compromised secretion.

Alteration of a single tryptophan residue of the cellulose-binding domain blocks secretion of the Erwinia chrysanthemi Cel5 cellulase (ex-EGZ) via the type II system

Cel5 (formerly known as endoglucanase Z) of Erwinia chrysanthemi is secreted by the Out type II pathway. Previous studies have shown that the catalytic domain (CD), linker region (LR) and cellulose-binding domain (CBD) each contain information needed for secretion. The aim of this work was to further investigate the secretion-related information present in the CBD(Cel5). Firstly(, )deleting a surface-exposed flexible loop had no effect on secretion. This indicated that some structural freedom is tolerated by the type II system. Secondly, mutation of a single tryptophan residue, previously shown to be important for binding to cellulose, i.e. Trp43, was found also to impair secretion. This indicated that the flat cellulose-binding surface of CBD(Cel5 )contains secretion-related information. Thirdly, CBD(Cel5) was substituted by the CBD(EGG) of Alteromonas haloplanctis endoglucanase G, yielding a hybrid protein CD(Cel5)-LR(Cel5)-CBD(EGG) that exhibited 90 % identity with Cel5, including the Trp43 residue. The hybrid protein was not secreted. This indicated that the Trp43 residue is necessary but not sufficient for secretion. Here we propose a model in which the secretion of Cel5 involves a transient intramolecular interaction between the cellulose-binding surface of CBD(Cel5) and a region close to the entry into the active site in CD(Cel5). Once secreted, the protein may then open out to allow the cellulose-binding surface of CBD(Cel5 )to interact with the surface of the cellulose substrate. An implication of this model is that protein molecules fold to a specific secretion-competent conformation prior to secretion that is different from the folding state of the secreted species.

Multiple interactions between pullulanase secreton components involved in stabilization and cytoplasmic membrane association of PulE

We report attempts to analyze interactions between components of the pullulanase (Pul) secreton (type II secretion machinery) from Klebsiella oxytoca encoded by a multiple-copy-number plasmid in Escherichia coli. Three of the 15 Pul proteins (B, H, and N) were found to be dispensable for pullulanase secretion. The following evidence leads us to propose that PulE, PulL, and PulM form a subcomplex with which PulC and PulG interact. The integral cytoplasmic membrane protein PulL prevented proteolysis and/or aggregation of PulE and mediated its association with the cytoplasmic membrane. The cytoplasmic, N-terminal domain of PulL interacted directly with PulE, and both PulC and PulM were required to prevent proteolysis of PulL. PulM and PulL could be cross-linked as a heterodimer whose formation in a strain producing the secreton required PulG. However, PulL and PulM produced alone could also be cross-linked in a 52-kDa complex, indicating that the secreton exerts subtle effects on the interaction between PulE and PulL. Antibodies against PulM coimmunoprecipitated PulL, PulC, and PulE from detergent-solubilized cell extracts, confirming the existence of a complex containing these four proteins. Overproduction of PulG, which blocks secretion, drastically reduced the cellular levels of PulC, PulE, PulL, and PulM as well as PulD (secretin), which probably interacts with PulC. The Pul secreton components E, F, G, I, J, K, L, and M could all be replaced by the corresponding components of the Out secretons of Erwinia chrysanthemi and Erwinia carotovora, showing that they do not play a role in secretory protein recognition and secretion specificity.

Genetic dissection of the outer membrane secretin PulD: are there distinct domains for multimerization and secretion specificity?

Linker and deletion mutagenesis and gene fusions were used to probe the possible domain structure of the dodecameric outer membrane secretin PulD from the pullulanase secretion pathway of Klebsiella oxytoca. Insertions of 24 amino acids close to or within strongly predicted and highly conserved amphipathic beta strands in the C-terminal half of the polypeptide (the beta domain) abolished sodium dodecyl sulfate (SDS)-resistant multimer formation that is characteristic of this protein, whereas insertions elsewhere generally had less dramatic effects on multimer formation. However, the beta domain alone did not form SDS-resistant multimers unless part of the N-terminal region of the protein (the N domain) was produced in trans. All of the insertions except one, close to the C terminus of the protein, abolished function. The N domain alone was highly unstable and did not form SDS-resistant multimers even when the beta domain was present in trans. We conclude that the beta domain is a major determinant of multimer stability and that the N domain contributes to multimer formation. The entire or part of the N domain of PulD could be replaced by the corresponding region of the OutD secretin from the pectate lyase secretion pathway of Erwinia chrysanthemi without abolishing pullulanase secretion. This suggests that the N domain of PulD is not involved in substrate recognition, contrary to the role proposed for the N domain of OutD, which binds specifically to pectate lyase secreted by E. chrysanthemi (V. E. Shevchik, J. Robert-Badouy, and G. Condemine, EMBO J. 16:3007-3016, 1997).

Testing the '+2 rule' for lipoprotein sorting in the Escherichia coli cell envelope with a new genetic selection

We report a novel strategy for selecting mutations that mislocalize lipoproteins within the Escherichia coli cell envelope and describe the mutants obtained. A strain carrying a deletion of the chromosomal malE gene, coding for the periplasmic maltose-binding protein (MalE), cannot use maltose unless a wild-type copy of malE is present in trans. Replacement of the natural signal peptide of preMalE by the signal peptide and the first four amino acids of a cytoplasmic membrane-anchored lipoprotein resulted in N-terminal fatty acylation of MalE (lipoMalE) and anchoring to the periplasmic face of the cytoplasmic membrane, where it could still function. When the aspartate at position +2 of this protein was replaced by a serine, lipoMalE was sorted to the outer membrane, where it could not function. Chemical mutagenesis followed by selection for maltose-using mutants resulted in the identification of two classes of mutations. The single class I mutant carried a plasmid-borne mutation that replaced the serine at position +2 by phenylalanine. Systematic substitutions of the amino acid at position +2 revealed that, besides phenylalanine, tryptophan, tyrosine, glycine and proline could all replace classical cytoplasmic membrane lipoprotein sorting signal (aspartate +2). Analysis of known and putative lipoproteins encoded by the E. coli K-12 genome indicated that these amino acids are rarely found at position +2. In the class II mutants, a chromosomal mutation caused small and variable amounts of lipoMalE to remain associated with the cytoplasmic membrane. Similar amounts of another, endogenous outer membrane lipoprotein, NlpD, were also present in the cytoplasmic membrane in these mutants, indicating a minor, general defect in the sorting of outer membrane lipoproteins. Four representative class II mutants analysed were shown not to carry mutations in the lolA or lolB genes, known to be involved in the sorting of lipoproteins to the outer membrane.

Bacterial type II protein export and pilus biogenesis: more than just homologies?

Protein export by Gram-negative bacteria requires devoted machineries to allow for the passage of hydrolytic enzymes and toxins through the cell envelope. The Type II export machinery has a number of distinct characteristics, which include its role as an extension of Sec-dependent secretion, its ability to recognize and export fully folded substrates efficiently and, perhaps most significantly, the relationship between a subset of its gene products with the Type IV pilus-biogenesis apparatus. An important question is whether we can extrapolate our knowledge, albeit limited, of Type IV pilus biogenesis to understand the structure and function of the Type II export apparatus. This and other questions relating to the energetics of assembly and specificity of the apparatus are addressed in this article.

Endochitinase is transported to the extracellular milieu by the eps-encoded general secretory pathway of Vibrio cholerae

The chiA gene of Vibrio cholerae encodes a polypeptide which degrades chitin, a homopolymer of N-acetylglucosamine (GlcNAc) found in cell walls of fungi and in the integuments of insects and crustaceans. chiA has a coding capacity corresponding to a polypeptide of 846 amino acids having a predicted molecular mass of 88.7 kDa. A 52-bp region with promoter activity was found immediately upstream of the chiA open reading frame. Insertional inactivation of the chromosomal copy of the gene confirmed that expression of chitinase activity by V. cholerae required chiA. Fluorescent analogues were used to demonstrate that the enzymatic activity of ChiA was specific for beta,1-4 glycosidic bonds located between GlcNAc monomers in chitin. Antibodies against ChiA were obtained by immunization of a rabbit with a MalE-ChiA hybrid protein. Polypeptides with antigenic similarity to ChiA were expressed by classical and El Tor biotypes of V. cholerae and by the closely related bacterium Aeromonas hydrophila. Immunoblotting experiments using the wild-type strain 569B and the secretion mutant M14 confirmed that ChiA is an extracellular protein which is secreted by the eps system. The eps system is also responsible for secreting cholera toxin, an oligomeric protein with no amino acid homology to ChiA. These results indicate that ChiA and cholera toxin have functionally similar extracellular transport signals that are essential for eps-dependent secretion.

TolAIII co-overexpression facilitates the recovery of periplasmic recombinant proteins into the growth medium of Escherichia coli

Overproduction of the third topological domain of the transmembrane protein TolA (TolAIII) in the periplasm of Escherichia coli confers a "leaky" phenotype to host cells by disrupting the integrity of the outer membrane and causing periplasmic proteins to leach into the growth medium. To examine the physiological consequences of TolAIII overexpression in more detail and assess the usefulness of this strategy for the release of periplasmic recombinant proteins into the extracellular fluid, we constructed a ColE1-compatible plasmid encoding a fusion between the ribose binding protein signal sequence and TolAIII under T7lac transcriptional control. About half of the total TolAIII synthesized in IPTG-induced cells aggregated in a precursor form in the cytoplasm. However, the majority of the mature protein was soluble and located in the extracellular fluid. TolAIII-overproducing cultures exhibited only slight growth defects upon entry into stationary phase but underwent extensive lysis when treated with 0.1% (w/v) SDS, and were unable to divide when supplemented with 0.02% SDS. The loss of outer membrane integrity resulted in long-term damage since cell viability was reduced by three orders of magnitude compared to control or uninduced cells. Overexpression of TolAIII did not significantly interfere with the translocation and processing of a plasmid-encoded fusion between the OmpA signal sequence and TEM-beta-lactamase but led to the release of most periplasmic proteins and 90% of the active enzyme into the extracellular fluid. Although the total levels of beta-lactamase accumulation in TolAIII-overproducing cultures was only 1.5- to 2-fold less than in control cells, the formation of periplasmic inclusions bodies was completely suppressed. A threshold concentration of TolAIII was necessary for efficient release of periplasmic proteins since the viability and detergent sensitivity of uninduced cells was comparable to that of control cultures and 80% of the beta-lactamase synthesized remained confined to the periplasm.

GSP-dependent protein secretion in gram-negative bacteria: the Xcp system of Pseudomonas aeruginosa

Bacteria have evolved several secretory pathways to release proteins into the extracellular medium. In Gram-negative bacteria, the exoproteins cross a cell envelope composed of two successive hydrophobic barriers, the cytoplasmic and outer membranes. In some cases, the protein is translocated in a single step across the cell envelope, directly from the cytoplasm to the extracellular medium. In other cases, outer membrane translocation involves an extension of the signal peptide-dependent pathway for translocation across the cytoplasmic membrane via the Sec machinery. By analogy with the so-called general export pathway (GEP), this latter route, including two separate steps across the inner and the outer membrane, was designated as the general secretory pathway (GSP) and is widely conserved among Gram-negative bacteria. In their great majority, exoproteins use the main terminal branch (MTB) of the GSP, namely the Xcp machinery in Pseudomonas aeruginosa, to reach the extracellular medium. In this review, we will use the P. aeruginosa Xcp system as a basis to discuss multiple aspects of the GSP mechanism, including machinery assembly, exoprotein recognition, energy requirement and pore formation for driving through the outer membrane.

The requirement for DsbA in pullulanase secretion is independent of disulphide bond formation in the enzyme

Results from previous studies have suggested that an intramolecular disulphide bond in the exoprotein pullulanase is needed for its recognition and transport across the outer membrane. This interpretation of the data is shown here to be incorrect: pullulanase devoid of all potential disulphide bonds is secreted with apparently the same efficiency as the wild-type protein. Furthermore, the periplasmic disulphide bond, oxidoreductase DsbA, previously shown to catalyse the formation of a disulphide bond in pullulanase and to decrease its transit time in the periplasm, is shown here to be required for the rapid secretion of pullulanase devoid of disulphide bonds. Several possible explanations for the role of DsbA in pullulanase secretion are discussed.

Recent progress and future directions in studies of the main terminal branch of the general secretory pathway in Gram-negative bacteria--a review

The main terminal branch (MTB) of the general secretory pathway is used by a wide variety of Gram- bacteria to transport exoproteins from the periplasm to the outside milieu. Recent work has led to the identification of the function of two of its 14 (or more) components: an enzyme with type-IV prepilin peptidase activity and a chaperone-like protein required for the insertion of another of the MTB components into the outer membrane. Despite these important discoveries, little tangible progress has been made towards identifying MTB components that determine secretion specificity (presumably by binding to cognate exoproteins) or which form the putative channel through which exoproteins are transported across the outer membrane. However, the idea that the single integral outer membrane component of the MTB could line the wall of this channel, and the intriguing possibility that other components of the MTB form a rudimentary type-IV pilus-like structure that might span the periplasm both deserve more careful examination. Although Escherichia coli K-12 does not normally secrete exoproteins, its chromosome contains an apparently complete set of genes coding for MTB components. At least two of these genes code for functional proteins, but the operon in which twelve of the genes are located does not appear to be expressed. We are currently searching for conditions which allow these genes to be expressed with the eventual aim of identifying the protein(s) that E. coli K-12 can secrete.

Pullulanase: model protein substrate for the general secretory pathway of gram-negative bacteria

Pullulanase of Klebsiella oxytoca is one of a wide variety of extracellular proteins that are secreted by Gram-negative bacteria by the complex main terminal branch (MTB) of the general secretory pathway. The roles of some of the 14 components of the MTB are now becoming clear. In this review it is proposed that most of these proteins form a complex, the secretion, that spans the cell envelope to control the opening and closing of channel in the outer membrane. Progress toward the goal of testing this model is reviewed.

Identification of two regions of Klebsiella oxytoca pullulanase that together are capable of promoting beta-lactamase secretion by the general secretory pathway

Pullulanase (PulA) is a 116 kDa amylolytic lipoprotein secreted by the Gram-negative bacterium Klebsiella oxytoca via the general secretory pathway. A deletion strategy was used in an attempt to determine the nature and the location of the secretion signal(s) in PulA presumed to be necessary for its specific secretion. The starting material was a gene fusion coding for an efficiently secreted PulA-beta-lactamase hybrid protein. Successive series of exonuclease III-generated deletions were used to remove internal segments of PulA from this hybrid. A simple plate test allowed the identification of truncated hybrids that retained beta-lactamase activity and that were secreted. Two non-adjacent regions, A and B (78 and 80 amino acids, respectively), were together necessary and sufficient to promote beta-lactamase translocation across the outer membrane. Secretion of PulA itself was markedly reduced when either of these regions was deleted, and was completely abolished when both regions were eliminated.

Secretion of recombinant proteins by Gram-negative bacteria

During the past few years, significant progress has been made towards our understanding of the molecular mechanisms governing the translocation of proteins through bacterial cell membranes. Successful attempts in promoting the secretion of recombinant proteins by employing this knowledge and by empirical efforts have been registered. However, a further in-depth understanding of membrane-translocation mechanisms is required before predictable manipulations of secretion systems can be made to secrete native recombinant proteins that are not naturally targeted to the extracellular compartment.