The antifolding activity of SecB promotes the export of the E. coli maltose-binding protein

Evidence is presented that the E. coli secB gene encodes a soluble protein that interacts with the mature region of the precursor maltose-binding protein (MBP), and promotes MBP export by preventing premature folding of the newly synthesized polypeptide into an export-incompetent form. The interaction of SecB with MBP was indicated by the finding that synthesis of various export-defective MBP species interfered with normal protein export by limiting SecB availability. The antifolding activity of SecB was demonstrated by the following: the defect in MBP export in SecB- cells was suppressed by mutational alterations affecting MBP folding; export of a mutant MBP that is accomplished in a strictly posttranslational mode was totally blocked in SecB- cells; and the rate of folding of wild-type MBP synthesized in vitro was found to be accelerated when SecB was absent and greatly retarded when excess SecB was present.     

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

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

Characterisation of preYvaY export reveals differences in the substrate specificities of Bacillus subtilis and Escherichia coli leader peptidases

Translocation, processing and secretion of YvaY, a Bacillus subtilis protein of unknown function, were characterised both in B. subtilis and in Escherichia coli. In its natural host B. subtilis, YvaY was transiently synthesised at the end of the exponential growth phase. It was efficiently secreted into the culture supernatant in spite of a calculated membrane spanning domain in the mature part of the protein. In E. coli, despite the high conservation of Sec-dependent transport components, processing of preYvaY was strongly impaired. To uncover which elements of E. coli and B. subtilis translocation systems are responsible for the observed substrate specificity, components of the B. subtilis Sec-system were co-expressed besides yvaY in E. coli. Expression of B. subtilis secA or secYEG genes did not affect processing, but expression of B. subtilis signal peptidase genes significantly enhanced processing of preYvaY in E. coli. While the major signal peptidases SipS or SipT had a strong stimulatory effect on preYvaY processing, the minor signal peptidases SipU, SipV or SipW had a far less stimulatory effect in E. coli. These results reveal that targeting and translocation of preYvaY is mediated by the E. coli Sec proteins but processing of preYvaY is not performed by E. coli signal peptidase LepB. Thus, differences in substrate specificities of E. coli LepB and the B. subtilis Sip proteins provide the bottleneck for export of YvaY in E. coli. Significant slower processing of preYvaY in absence of SecB indicated that SecB mediates targeting of the B. subtilis precursor.     

Regions of maltose-binding protein that influence SecB-dependent and SecA-dependent export in Escherichia coli.

In Escherichia coli, the efficient export of maltose-binding protein (MBP) is dependent on the chaperone SecB, whereas export of ribose-binding protein (RBP) is SecB independent. To localize the regions of MBP involved in interaction with SecB, hybrids between MBP and RBP in SecB mutant cells were constructed and analyzed. One hybrid consisted of the signal peptide and first third of the mature moiety of MBP, followed by the C-terminal two-thirds of RBP (MBP-RBP112). This hybrid was dependent upon SecB for its efficient export and exhibited a strong export defect in secA mutant cells. A hybrid between RBP and MBP with the same fusion point was also constructed (RBP-MBP116). The RBP-MBP116 hybrid remained SecB independent and only exhibited a partial export defect in secA mutant cells. In addition, MBP species with specific alterations in the early mature region were less dependent on SecB for their efficient export. The export of these altered MBP species was also less affected in secA mutant cells and in cells treated with sodium azide. These results present additional evidence for the targeting role of SecB.     

The folding properties of the Escherichia coli maltose-binding protein influence its interaction with SecB in vitro.

It has been proposed that the cytoplasmic SecB protein functions as a component of the Escherichia coli protein export machinery by serving as an antifolding factor that retards folding of the precursor maltose-binding protein (preMBP) into a translocation-incompetent form. In this study, it was found that SecB directly interacts with wild-type preMBP and various mutationally altered MBP species synthesized in vitro to form a SecB-MBP complex that can be precipitated with anti-SecB serum. The association of SecB with wild-type preMBP was relatively unstable; such a complex was formed only when SecB was present cotranslationally or after denaturation of previously synthesized preMBP and was detected with only low efficiency. In marked contrast, MBP species that were defective in the ability to assume the stable conformation of wild-type preMBP or that exhibited significantly slower folding kinetics formed much more stable complexes with SecB. In one case, we demonstrated that SecB did not need to be present cotranslationally for complex formation to occur. Formation of a complex between SecB and MBP was clearly not dependent on the MBP signal peptide. However, we were unable to detect complex formation between SecB and MBP lacking virtually the entire signal peptide but having a completely intact mature moiety. This MBP species folded at a rate considerably faster than that of wild-type preMBP. The propensity of this mutant protein to assume the native conformation of mature MBP apparently precludes a stable association with SecB, whereas an MBP species lacking a signal peptide but exhibiting altered folding properties did form a complex with SecB that could be precipitated with anti-SecB serum.     

Escherichia coli signal peptides direct inefficient secretion of an outer membrane protein (OmpA) and periplasmic proteins (maltose-binding protein, ribose-binding protein, and alkaline phosphatase) in Bacillus subtilis.

Signal peptides of gram-positive exoproteins generally carry a higher net positive charge at their amino termini (N regions) and have longer hydrophobic cores (h regions) and carboxy termini (C regions) than do signal peptides of Escherichia coli envelope proteins. To determine if these differences are functionally significant, the ability of Bacillus subtilis to secrete four different E. coli envelope proteins was tested. A pulse-chase analysis demonstrated that the periplasmic maltose-binding protein (MBP), ribose-binding protein (RBP), alkaline phosphatase (PhoA), and outer membrane protein OmpA were only inefficiently secreted. Inefficient secretion could be ascribed largely to properties of the homologous signal peptides, since replacing them with the B. amyloliquefaciens alkaline protease signal peptide resulted in significant increases in both the rate and extent of export. The relative efficiency with which the native precursors were secreted (OmpA >> RBP > MBP > PhoA) was most closely correlated with the overall hydrophobicity of their h regions. This correlation was strengthened by the observation that the B. amyloliquefaciens levansucrase signal peptide, whose h region has an overall hydrophobicity similar to that of E. coli signal peptides, was able to direct secretion of only modest levels of MBP and OmpA. These results imply that there are differences between the secretion machineries of B. subtilis and E. coli and demonstrate that the outer membrane protein OmpA can be translocated across the cytoplasmic membrane of B. subtilis.     

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

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

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

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

A highly mobile C-terminal tail of the Escherichia coli protein export chaperone SecB.

The Escherichia coli export chaperone SecB binds nascent precursors of certain periplasmic and outer membrane proteins and prevents them from folding or aggregating in the cytoplasm. In this study, we demonstrate that the C-terminal 13 residues of SecB were highly mobile using (1)H NMR spectroscopy. A protein lacking the C-terminal 13 amino acids of wild-type SecB was found to retain the ability to bind unfolded maltose-binding protein (MBP) in vitro but to interfere with the normal kinetics of pre-MBP export when overexpressed in vivo. The defect in export was reversed by overproduction of the peripheral membrane ATPase SecA. Therefore, deletion of the mobile region of SecB may alter the interactions of SecB with SecA.     

Functional implementation of the posttranslational SecB-SecA protein-targeting pathway in Bacillus subtilis

Bacillus subtilis and its close relatives are widely used in industry for the Sec-dependent secretory production of proteins. Like other Gram-positive bacteria, B. subtilis does not possess SecB, a dedicated targeting chaperone that posttranslationally delivers exported proteins to the SecA component of the translocase. In the present study, we have implemented a functional SecB-dependent protein-targeting pathway into B. subtilis by coexpressing SecB from Escherichia coli together with a SecA hybrid protein in which the carboxyl-terminal 32 amino acids of the B. subtilis SecA were replaced by the corresponding part of SecA from E. coli. In vitro pulldown experiments showed that, in contrast to B. subtilis SecA, the hybrid SecA protein gained the ability to efficiently bind to E. coli SecB, suggesting that the structural details of the extreme C-terminal region of SecA constitute a crucial SecB binding specificity determinant. Using a poorly exported mutant maltose binding protein (MalE11) and alkaline phosphatase (PhoA) as model proteins, we could demonstrate that the secretion of both proteins by B. subtilis was significantly enhanced in the presence of the artificial protein targeting pathway. Mutations in SecB that do not influence its chaperone activity but prevent its interaction with SecA abolished the secretion stimulation of both proteins, demonstrating that the implemented pathway in fact critically depends on the SecB targeting function. From a biotechnological view, our results open up a new strategy for the improvement of Gram-positive bacterial host systems for the secretory production of heterologous proteins.     

Highly selective binding of nascent polypeptides by an Escherichia coli chaperone protein in vivo.

Chaperone proteins bind to newly synthesized polypeptides and assist in various assembly reactions. The Escherichia coli chaperone protein SecB binds precursors of exported proteins and assists in export. In vitro, SecB can bind to many unfolded proteins. In this report, we demonstrate that SecB binding in vivo is highly selective; the major polypeptides that are bound by SecB are nascent precursors of the exported proteins maltose-binding protein (MBP), LamB, OmpF, and OmpA. These results support the hypothesis that the primary physiological function of SecB is to stimulate protein export. By interacting with nascent polypeptides, SecB probably stimulates their cotranslational association with the membrane-bound protein translocation apparatus.     

Cloning of the debranching-enzyme gene from Thermoanaerobium brockii into Escherichia coli and Bacillus subtilis.

The gene for an enzyme with single or dual specificity on complex carbohydrates has been transferred from its native host (Thermoanaerobium brockii), a thermophilic anaerobe, into Escherichia coli and Bacillus subtilis. Most of the gene coding region is in a 2.2-kilobase PstI fragment that is common to the E. coli and B. subtilis chimeric vectors pCPC902 and pCPC903, respectively. Although the T. brockii debranching enzyme secreted from B. subtilis was unglycosylated and had less thermostability, more enzyme was secreted from B. subtilis (0.80 to 1.0 U/ml) than from T. brockii (0.23 U/ml). E. coli did not export any measurable enzyme. From the fermentation broth of B. subtilis containing pCPC903, three active species of the debranching enzyme were separated; two species are possibly protease digestion products of the larger protein (105,000 molecular weight). Whereas the enzyme can cleave all of the alpha-1----6 glucosidic linkages (and none of the alpha-1----4 bonds) in pullulan, it hydrolyzed mostly alpha-1----4 and very few of the alpha-1----6 linkages in starch. Upon hydrolysis of pullulan by the enzyme, only maltotriose was produced, while starch was digested to various-sized oligomers.     

Functional specificity of the replication fork-arrest complexes of Bacillus subtilis and Escherichia coli: significant specificity for Tus-Ter functioning in E. coli

The Escherichia coli replication terminator TerB was inserted in its two alternate orientations into a Bacillus subtilis fork-arrest assay plasmid. After transferring these new plasmids into B. subtilis, which could overproduce the E. coli terminator protein Tus, it was shown that the E. coli Tus-TerB complex could cause polar replication fork arrest, albeit at a very low level, in B. subtilis. A new B. subtilis-E. coli shuttle plasmid was designed to allow the insertion of either the Terl (B. subtilis) or TerB (E. coli) terminator at the same site and in the active orientation in relation to the approaching replication fork generated in either organism. Fork-arrest assays for both terminator-containing plasmids replicating in both organisms which also produced saturating levels of either the B. subtilis terminator protein (RTP) or Tus were performed. The efficiency of the Tus-TerB complex in causing fork arrest was much higher in E. coli than in B. subtilis. The efficiency of the B. subtilis RTP-Terl complex was higher in B. subtilis than in E. coli, but the effect was significantly less. Evidently a specificity feature in E. coli operates to enhance appreciably the fork-arrest efficiency of a Tus-Ter complex. The specificity effect is of less significance for an RTP-Ter complex functioning in B. subtilis.     

Molecular cloning and expression of Bacillus licheniformis beta-lactamase gene in Escherichia coli and Bacillus subtilis.

The chromosomal beta-lactamase (penicillinase, penP) gene from Bacillus licheniformis 749/C has been cloned in Escherichia coli. The locations of the target sites for various restriction enzymes on the 4.2-kilobase EcoRI fragment were determined. By matching the restriction mapping data with the potential nucleotide sequences of the penP gene deduced from known protein sequence, we established the exact position of the penP gene on the fragment. A bifunctional plasmid vector carrying the penP gene, plasmid pOG2165, was constructed which directs the synthesis of the heterologous beta-lactamase in both E. coli and Bacillus subtilis hosts. The protein synthesized in E. coli and B. subtilis is similar in size to the processed beta-lactamase made in B. licheniformis. Furthermore, the beta-lactamase made in B. subtilis is efficiently secreted by the host into the culture medium, indicating that B. subtilis is capable of carrying out the post-translational proteolytic cleavage(s) to convert the membrane-bound precursor enzyme into the soluble extracellular form.     

The SecB chaperone is involved in the secretion of the Serratia marcescens HasA protein through an ABC transporter.

The secretion pathways of the heme-binding protein HasA from Serratia marcescens and of the metalloproteases A, B, C and G from Erwinia chrysanthemi have been reconstituted in Escherichia coli. They are secreted in a single step from the cytoplasm across both membranes of the Gram-negative envelope, after recognition of their specific C-terminal secretion signal by their cognate ABC transporter. We report strong evidence that both HasA and the metalloproteases bind the SecB chaperone involved in the export of several envelope proteins via the Sec pathway. We also show that the secretion of the HasA protein is strongly dependent upon SecB in the reconstituted system, whereas that of the proteases is not. HasA secretion in the original host is strongly inhibited by a protein known to interfere with E.coli SecB function. We propose that the proteins secreted by the ABC pathway may have to be unfolded for efficient secretion.     

Genetic analysis of pathway specificity during posttranslational protein translocation across the Escherichia coli plasma membrane

In Escherichia coli, the SecB/SecA branch of the Sec pathway and the twin-arginine translocation (Tat) pathway represent two alternative possibilities for posttranslational translocation of proteins across the cytoplasmic membrane. Maintenance of pathway specificity was analyzed using a model precursor consisting of the mature part of the SecB-dependent maltose-binding protein (MalE) fused to the signal peptide of the Tat-dependent TorA protein. The TorA signal peptide selectively and specifically directed MalE into the Tat pathway. The characterization of a spontaneous TorA signal peptide mutant (TorA*), in which the two arginine residues in the c-region had been replaced by one leucine residue, showed that the TorA*-MalE mutant precursor had acquired the ability for efficiently using the SecB/SecA pathway. Despite the lack of the "Sec avoidance signal," the mutant precursor was still capable of using the Tat pathway, provided that the kinetically favored Sec pathway was blocked. These results show that the h-region of the TorA signal peptide is, in principle, sufficiently hydrophobic for Sec-dependent protein translocation, and therefore, the positively charged amino acid residues in the c-region represent a major determinant for Tat pathway specificity. Tat-dependent export of TorA-MalE was significantly slower in the presence of SecB than in its absence, showing that SecB can bind to this precursor despite the presence of the Sec avoidance signal in the c-region of the TorA signal peptide, strongly suggesting that the function of the Sec avoidance signal is not the prevention of SecB binding; rather, it must be exerted at a later step in the Sec pathway.     

Interaction of the Bacillus subtilis DnaA-like protein with the Escherichia coli DnaA protein.

Plasmids carrying the intact Bacillus subtilis dnaA-like gene and two reciprocal hybrids between the B. subtilis and Escherichia coli dnaA genes were constructed. None of the plasmids could transform wild-type E. coli cells unless the cells contained surplus E. coli DnaA protein (DnaAEc). A dnaA (Ts) strain integratively suppressed by the plasmid R1 origin could be transformed by plasmids carrying either the B. subtilis gene (dnaABs) or a hybrid gene containing the amino terminus of the E. coli gene and the carboxyl terminus of the B. subtilis gene (dnaAEc/Bs). In cells with surplus E. coli DnaA protein, expression of the E. coli dnaA gene was derepressed by the B. subtilis DnaA protein and by the hybrid DnaAEc/Bs protein, whereas it was strongly repressed by the reciprocal hybrid protein DnaABs/Ec. The plasmids carrying the different dnaA genes probably all interfere with initiation of chromosome replication in E. coli by decreasing the E. coli DnaA protein concentration to a limiting level. The DnaABs and the DnaAEc/Bs proteins effect this decrease possibly by forming inactive oligomeric proteins, while the DnaABs/Ec protein may decrease dnaAEc gene expression.     

Protein traffic for secretion and related machinery of Bacillus subtilis

Gram-positive sporulating Bacillus subtilis secretes high levels of protein. Its complete genome sequence, published in 1997, encodes 4,106 proteins. Bioinformatic searches have predicted that about half of all B. subtilis proteins are related to the cell membrane through export to the extracellular medium, insertion, and attachment. Key features of the B. subtilis protein secretion machinery are the absence of an Escherichia coli SecB homolog and the presence of an SRP (signal recognition particle) that is structurally rather similar to human SRP. In addition, B. subtilis contains five type I signal peptidases (SipS, T, U, V, and W). Our in vitro assay system indicated that co-operation between the SRP-protein targeting system to the cell membrane and the Sec protein translocation machinery across the cytoplasmic membrane constitutes the major protein secretion pathway in B. subtilis. Furthermore, the function of the SRP-Sec pathway in protein localization to the cell membrane and spore was analyzed.