Purification of the Escherichia coli secB gene product and demonstration of its activity in an in vitro protein translocation system

The Sec System: Protein Export in Escherichia coli

In Escherichia coli, proteins found in the periplasm or the outer membrane are exported from the cytoplasm by the general secretory, Sec, system before they acquire stably folded structure. This dynamic process involves intricate interactions among cytoplasmic and membrane proteins, both peripheral and integral, as well as lipids. In vivo, both ATP hydrolysis and proton motive force are required. Here, we review the Sec system from the inception of the field through early 2016, including biochemical, genetic, and structural data.

Folding in vitro and transport in vivo of pre-β-lactamase are SecB independent

The rate of folding of the precursor of β-lactamase is not influenced by the presence of SecB under conditions in which GroEL/ES retards the folding. Wild-type β-lactamase and several mutants in the signal or the mature protein, affecting either transport or enzyme kinetics and probably folding, were examined for total expression, total enzymatic activity, and transported β-lactamase (in vivo resistance) in secB^- and secB⁺ strains. We conclude that there is no indication of any relevant interaction between SecB and pre-β-lactamase in vitro, nor did the secB^- mutation affect the transport of wild-type β-lactamase or any of the mutants in vivo. Thus, putative Escherichia coli'folding modulators'must be of limited specificity.

Biophysical characterization of the influence of salt on tetrameric SecB

SecB is a tetrameric chaperone, with a monomeric molecular mass of 17 kDa, that is involved in protein translocation in Escherichia coli. It has been hypothesized that SecB undergoes a conformational change as a function of the salt concentration. To gain more insight into the salt-dependent behavior of SecB, we studied the protein in solution by dynamic light scattering, size exclusion chromatography, analytical ultracentrifugation, and small angle neutron scattering. The results clearly demonstrate the large influence of the salt concentration on the behavior of SecB. At high salt concentration, SecB is a non-spherical protein with a radius of gyration of 3.4 nm. At low salt concentration the hydrodynamic radius of the protein is apparently decreased, whereas the ratio of the frictional coefficients is increased. The protein solution behaves in a non-ideal way at low salt concentrations, as was shown by the analytical ultracentrifugation data and a pronounced interparticle effect observed by small angle neutron scattering. A possible explanation is a change in surface charge distribution dependent on the salt concentration in the solvent. We summarize our data in a model for the salt-dependent conformation of tetrameric SecB.

Crystals of acetylated SecB diffract to 2.3-A resolution

The molecular chaperone SecB is part of the protein translocation pathway in Escherichia coli. SecB was purified from an overproducing strain and crystallized, resulting in crystals diffracting to 2.3-A resolution. The analysis of electrospray ionization mass spectra of dissolved crystals of SecB indicated that we have crystallized an acetylated form of SecB. Sequence analysis suggests that the protein is fully acetylated at its N-terminus in vivo, indicating that potential deacetylation is artificially introduced by purification methods. The high degree of acetylation that we observed might account for the fact that the crystals obtained as described in this study diffract to higher resolution than those in previously reported trials.

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.

Overproduction of SecA suppresses the export defect caused by a mutation in the gene encoding the Escherichia coli export chaperone secB

SecB is a cytosolic protein required for rapid and efficient export of particular periplasmic and outer membrane proteins in Escherichia coli. SecB promotes export by stabilizing newly synthesized precursor proteins in a nonnative conformation and by targeting the precursors to the inner membrane. Biochemical studies suggest that SecB facilitates precursor targeting by binding to the SecA protein, a component of the membrane-embedded translocation apparatus. To gain more insight into the functional interaction of SecB and SecA, in vivo, mutations in the secA locus that compensate for the export defect caused by the secB missense mutation secBL75Q were isolated. Two suppressors were isolated, both of which led to the overproduction of wild-type SecA protein. In vivo studies demonstrated that the SecBL75Q mutant protein releases precursor proteins at a lower rate than does wild-type SecB. Increasing the level of SecA protein in the cell was found to reverse this slow-release defect, indicating that overproduction of SecA stimulates the turnover of SecBL75Q-precursor complexes. These findings lend additional support to the proposed pathway for precursor targeting in which SecB promotes targeting to the translocation apparatus by binding to the SecA protein.

Catabolic repression of secB expression is positively controlled by cyclic AMP (cAMP) receptor protein-cAMP complexes at the transcriptional level

SecB, a protein export-specific chaperone, enhances the export of a subset of proteins across cytoplasmic membranes of Escherichia coli. Previous studies showed that the synthesis of SecB is repressed by the presence of glucose in the medium. The derepression of SecB requires the products of both the cya and crp genes, indicating that secB expression is under the control of catabolic repression. In this study, two secB-specific promoters were identified. In addition, 5' transcription initiation sites from these two promoters were determined by means of secB-lacZ fusions and primer extension. The distal P1 promoter appeared to be independent of carbon sources, whereas the proximal P2 promoter was shown to be subject to control by the cyclic AMP (cAMP) receptor protein (CRP)-cAMP complexes. Gel-mobility shift studies showed that this regulation results from direct interaction between the secB P2 promoter region and the CRP-cAMP complex. Moreover, the CRP binding site on the secB gene was determined by DNase I footprinting and further substantiated by mutational analysis. The identified secB CRP binding region is centered at the -61.5 region of the secB gene and differed from the putative binding sites predicted by computer analysis.

Differential translocation of protein precursors across SecY-deficient membranes of Escherichia coli: SecY is not obligatorily required for translocation of certain secretory proteins in vitro

SecY, a component of the protein translocation system in Escherichia coli, was depleted at a nonpermissive temperature in a strain which had a temperature-sensitive polar effect on the expression of its secY. Membrane vesicles prepared from these cells, when grown at the nonpermissive temperature, contained about 5% SecY and similarly low levels of SecG. As expected, translocation of alkaline phosphatase precursors across these SecY-deficient membranes was severely impaired and appeared to be directly related to the decrease of SecY amounts. However, despite such a dramatic reduction in SecY and SecG levels, these membranes exhibited 50 to 70% of the wild-type translocation activity, including the processing of the signal peptide, of OmpA precursor (proOmpA). This translocation activity in SecY-deficient membranes was still SecA and ATP dependent and was not unique to proOmpA, as lipoprotein and lambda receptor protein precursors were also transported efficiently. Membranes that were reconstituted from these SecY-depleted membranes contained undetectable amounts of SecY yet were also shown to possess substantial translocation activity for proOmpA. These results indicate that the requirement of SecY for translocation is not obligatory for all secretory proteins and may depend on the nature of precursors. Consequently, it is unlikely that SecY is the essential core channel through which all precursors traverse across membranes; rather, SecY probably contributes to efficiency and specificity.

Kinetic partitioning. Poising SecB to favor association with a rapidly folding ligand

Chaperones are a class of proteins that possess the remarkable ability to selectively bind polypeptides that are in a nonnative state. The selectivity of SecB, a molecular chaperone in Escherichia coli, for its ligands can be explained in part by a kinetic partitioning between folding of the polypeptide and association with SecB. It has clearly been established that kinetic partitioning can be poised to favor association with SecB by changing the rate constant for folding of the ligand. We now demonstrate that binding to SecB can be given a kinetic advantage over the pathway for folding by modulating the properties of the chaperone. By poising SecB to expose a hydrophobic patch, we were able to detect a complex between SecB and maltose-binding protein under conditions in which rapid folding of the polypeptide otherwise precludes formation of a kinetically stable complex. The data presented here are interpreted within the framework of a kinetic partitioning between binding to SecB and folding of the polypeptide. We propose that exposure of a hydrophobic patch on SecB increases the surface area for binding and thereby increases the rate constant for association. In this way association of SecB with the polypeptide ligand has a kinetic advantage over the pathway for folding.

Importance of electrostatic interactions in the rapid binding of polypeptides to GroEL

The question of how chaperones rapidly bind non-native proteins of very different sequence and function has been examined by determining the effect of ionic strength on the refolding of barnase on GroEL, and on the thermal denaturation of barnase in the presence of GroEL and SecB. Both chaperones bind the denatured state of barnase, so lowering the T(m) value. The refolding of barnase in the presence of GroEL is multiphasic, the slowest phase corresponding to the refolding of a singly bound molecule of barnase in the complex with GroEL. The fastest phase is related to the association of barnase and GroEL. At high ratios of GroEL to barnase and low ionic strength (less than 200 mM) this fast phase corresponds to the observed rate of binding. The rate of association of barnase and GroEL was found to be highly dependent on ionic strength, and at high ionic strength (greater than 600 mM) the majority of barnase molecules escaped binding and refolded free in solution. The data are consistent with an initial, transient, ionic interaction between barnase and GroEL, before hydrophobic binding occurs, allowing diffusion-controlled association and slow dissociation of unfolded polypeptide.

Carbon source-dependent synthesis of SecB, a cytosolic chaperone involved in protein translocation across Escherichia coli membranes

SecB is a cytosolic chaperone involved in protein translocation across cytoplasmic membranes in Escherichia coli. It has been shown to be required for efficient translocation of a subset of precursor proteins but is not essential for cell viability. This study investigated whether synthesis of SecB is growth rate dependent. Interestingly, the total amount of SecB synthesized in the cells was relatively small. Moreover, the levels of SecB were found to be carbon source dependent since more SecB was produced in cells grown in glycerol media than in cells grown in glucose media, regardless of the growth rate. This is in contrast to the other Sec proteins, whose synthesis is growth rate dependent and not related to glucose as a carbon source. In addition, cyclic AMP (cAMP) partially relieves the lower levels of SecB observed in glucose medium, a compensatory effect that depends on the presence of both cya and crp gene products. Thus, the glucose-dependent synthesis of SecB may be related to the cAMP-cAMP receptor protein complex-mediated activation.

Electrospray mass spectrometric investigation of the chaperone SecB

Electrospray ionization mass spectrometry was used to investigate the structure of the Escherichia coli chaperone protein SecB. It was determined that the N-terminal methionine of SecB has been removed and that more than half of all SecB monomers are additionally modified, most likely by acetylation of the N-terminus or a lysine. The use of gentle mass spectrometer interface conditions showed that the predominant, oligomeric form of SecB is a tetramer that is stable over a range of solution pH conditions and mass spectrometer interface heating (i.e., inlet capillary temperatures). At very high pH, SecB dimers are observed. SecB contains a region that is hypersensitive to cleavage by proteinase K and is thought to be involved in conformational changes that are crucial to the function of SecB. We identified the primary site of cleavage to be between Leu 141 and Gln 142. Fourteen amino acids are removed, but the truncated form remains a tetramer with stability similar to that of the intact form.

Crystallization of the chaperone protein SecB

The secretory protein SecB found in Escherichia coli is a molecular chaperone that binds to precursor forms of a number of proteins targeted for export to the periplasmic space. SecB maintains these proteins in a translocation-competent conformation facilitating the translocation process. The material has been cloned and expressed in E. coli. Crystals have been grown from polyethylene glycol 8000 by vapor diffusion using the hanging drop technique. These crystals are monoclinic, belonging to space group C2 with unit cell dimensions a = 56.0 A, b = 111.1 A, c = 134.7 A, and beta = 104 degrees. The crystals diffract to 8 A resolution on a Rigaku imaging plate detector. Dynamic light scattering experiments suggest that SecB exhibits aggregation behavior with a number of different precipitating agents. These results may explain resistance of SecB to forming ordered crystals.

Interaction of SecB with intermediates along the folding pathway of maltose-binding protein

SecB, a molecular chaperone involved in protein export in Escherichia coli, displays the remarkable ability to selectively bind many different polypeptide ligands whose only common feature is that of being nonnative. The selectivity is explained in part by a kinetic partitioning between the folding of a polypeptide and its association with SecB. SecB has no affinity for native, stably folded polypeptides but interacts tightly with polypeptides that are nonnative. In order to better understand the nature of the binding, we have examined the interaction of SecB with intermediates along the folding pathway of maltose-binding protein. Taking advantage of forms of maltose-binding protein that are altered in their folding properties, we show that the first intermediate in folding, represented by the collapsed state, binds to SecB, and that the polypeptide remains active as a ligand until it crosses the final energy barrier to attain the native state.

Signal recognition particle (SRP), a ubiquitous initiator of protein translocation

In higher eukaryotes, most secretory and membrane proteins are synthesised by ribosomes which are attached to the membrane of the rough endoplasmic reticulum (RER). This allows the proteins to be translocated across that membrane already during their synthesis. The ribosomes are directed to the RER membrane by a cytoplasmic ribonucleoprotein particle, the signal recognition particle (SRP). SRP fulfills its task by virtue of three distinguishable activities: the binding of a signal sequence which, being part of the nascent polypeptide to be translocated, is exposed on the surface of a translating ribosome; the retardation of any further elongation; and the SRP-receptor-mediated binding of the complex of ribosome, nascent polypeptide and SRP to the RER membrane which results in the detachment of SRP from the signal sequence and the ribosome and the insertion of the nascent polypeptide into the membrane. Evidence is accumulating that SRP is not restricted to eukaryotes: SRP-related particles and SRP-receptor-related molecules are found ubiquitously and may function in protein translocation in every living organism. This review focuses on the mammalian SRP. A brief discussion of its overall structure is followed by a detailed description of the structures of its RNA and protein constituents and the requirements for their assembly into the particle. Homologues of SRP components from organisms other than mammals are mentioned to emphasize the components' conserved or less conserved features. Subsequently, the functions of each of the SRP constituents are discussed. This sets the stage for a presentation of a model for the mechanism by which SRP cyclically assembles and disassembles with translating ribosomes and the RER membrane. It may be expected that similar mechanisms are used by SRP homologues in organisms other than mammals. However, the mammalian SRP-mediated translocation mechanism may not be conserved in its entirety in organisms like Escherichia coli whose SRP lack components required for the function of the mammalian SRP. Possible translocation pathways involving the rudimentary SRP are discussed in view of the existence of alternative, chaperone-mediated translocation pathways with which they may intersect. The concluding two sections deal with open questions in two areas of SRP research. One formulates basic questions regarding the little-investigated biogenesis of SRP. The other gives an outlook over the insights into the mechanisms of each of the known activities of the SRP that are to be expected in the short and medium-term future.

Secretion of eukaryotic growth hormones in Escherichia coli is influenced by the sequence of the mature proteins

We report the construction of secretion plasmids expressing the fusion proteins, OmpA::pGH (pSpGH.01) and OmpA::hGH (phGH.01), and compare the secretion of mature porcine growth hormone (pGH) and human growth hormone (hGH) employing Escherichia coli. E. coli [phGH.01] secreted 10-15 micrograms hGH/ml/A600 cells into the periplasmic space, representing 30% of total periplasmic proteins. E. coli [pSpGH.01], however, secreted 30-fold less mature pGH. On the basis that both pSpGH.01 and phGH.01 are stably maintained in E. coli and in vitro transcription/translation data showed equivalent expression of OmpA::pGH and OmpA::hGH precursors, we attribute the higher secretion of hGH to the translocation-competent OmpA::hGH protein configuration. Two OmpA::GHF (growth hormone fusion) precursors, OmpA::GHF.02 and OmpA::GHF.03, both with hGH helix 3/helix 4 together instead of the pGH equivalent, secreted mature proteins as efficiently as OmpA::hGH. We propose that hGH helices 3 and 4 in these OmpA::GHF precursors play a major role in the folding of the precursor to a translocation-competent state, mimicking the translocation-competent nature of the OmpA::hGH precursor.

The PrsA lipoprotein is essential for protein secretion in Bacillus subtilis and sets a limit for high-level secretion

Mutations of the prsA gene of Bacillus subtilis have indicated that the gene is involved in protein secretion and it encodes a novel component of the cellular secretion machinery. We now demonstrate that the gene product is a membrane-associated lipoprotein, presumably bound to the outer face of the cytoplasmic membrane. Experiments to inactivate the prsA gene with insertions indicated that it is indispensable for viability. The cellular level of PrsA protein was shown to be decreased in prsA mutants with decreased level of exoproteins, consistent with an essential function in protein secretion. An increased amount of cellular PrsA protein was introduced by increasing the copy number of prsA in B. subtilis. This enhanced, from six- to twofold, the secretion of alpha-amylases and a protease in strains, which expressed high levels of these exoenzymes. This suggests that PrsA protein is the rate-limiting component of the secretion machinery, a finding that is of considerable biotechnological interest.

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.

Identification of a chloroplast-encoded secA gene homologue in a chromophytic alga: possible role in chloroplast protein translocation

SecA is one of seven Sec proteins that comprise the prokaryotic protein translocation apparatus. A chloroplast-encoded secA gene has been identified from the unicellular chromophytic alga Pavlova lutherii. The gene predicts a protein that is related to the SecA proteins of Escherichia coli and Bacillus subtilis. The presence of secA, as well as the previously described secY and hsp70 genes, on the chloroplast genome of P. lutherii suggests that this eukaryotic organism utilises protein translocation mechanisms similar to those of bacterial cells.

Cloning and molecular characterization of the secY genes from Bacillus licheniformis and Staphylococcus carnosus: comparative analysis of nine members of the SecY family

SecY is a central component of the export machinery that mediates the translocation of secretory proteins across the plasma membrane of Escherichia coli. We have cloned and sequenced the secY genes from Bacillus licheniformis and Staphylococcus carnosus. The deduced amino acid sequences are highly homologous to those of other known SecY polypeptides, all having the potential to form 10 transmembrane segments. Comparative analysis of 9 SecY polypeptides, derived from different bacteria, revealed that 14 amino acid positions (2.7%) are identical in all SecY proteins and 89 (16.9%) show conservative changes. Clusters of conserved amino acid residues were found in 4 of the 10 transmembrane segments and 2 of the 6 cytoplasmic domains. It is suggested that the conserved regions might be involved in the translocation activity of SecY or might be required for the correct interaction of SecY with other components of the secretion apparatus.

In-vitro studies on the folding characteristics of the Escherichia coli precursor protein prePhoE. Evidence that SecB prevents the precursor from aggregating by forming a functional complex

We characterised the behaviour of the purified precursor protein prePhoE upon dilution from 8 M urea by CD, fluorescence spectroscopy and gel-filtration techniques. It is demonstrated that prePhoE rapidly adopts beta structure, folds and aggregates upon dilution to urea concentrations below 3 M. These processes are paralleled by a loss of translocation competence. Furthermore the interaction of prePhoE with SecB was investigated. SecB is shown to have a very high content of beta structure, therefore we propose that precursor recognition by SecB is mediated through beta-beta interaction. It is shown that SecB has little effect on the adoption of secondary structure and tertiary folding upon dilution of the precursor from urea. However, SecB prevents the precursor from aggregating by forming a functional and stable complex.

Electron microscopy of thin-sectioned three-dimensional crystals of SecA protein from Escherichia coli: structure in projection at 40 A resolution

SecA is a single-chain, membrane-associated polypeptide (102 kDa) which functions as an essential component of the protein export machinery of Escherichia coli. SecA has been crystallized from ammonium sulfate as small, three-dimensional bipyramidal crystals (0.1 x 0.1 x 0.05 mm). These crystals did not demonstrate detectable diffraction of X-rays from rotating anode sources. For study by electron microscopy, individual crystals were cross-linked in glutaraldehyde and OsO4 solutions, dehydrated, embedded in epoxy resin, and sectioned normal to crystallographic axial directions inferred from the external morphology of the crystals. Fourier transformation of processed images of untilted thin sections stained with uranyl acetate and lead citrate show reflections extending to 31 A resolution. Diffraction data and reconstructed images of the projected density of the unit cell contents indicate that the bipyramidal SecA crystals belong to orthorhombic space group C222(1) with unit cell dimensions a = 414 A, b = 381 A, and c = 243 A. Filtered images and density maps of mutually orthogonal projections of the unit cell contents are consistent with a three-dimensional model in which the asymmetric unit contains eight SecA monomers. The large unit cell dimensions and packing of protein monomers suggest that SecA is crystallizing as an oligomer of either dimers or tetramers.

Peptide binding by chaperone SecB: implications for recognition of nonnative structure

The molecular basis for recognition of nonnative proteins by the molecular chaperone SecB was investigated with an in vitro assay based on the protection of SecB from proteolysis when a ligand is bound. The SecB tetramer has multiple binding sites for positively charged peptides. When the peptide binding sites are occupied, the complex undergoes a conformational change to expose hydrophobic sites that bind the fluorescent probe 1-anilinonaphthalene-8-sulfonate. A model is proposed for interaction of nonnative polypeptides with both hydrophilic and hydrophobic sites on SecB.

Identification of a gene fragment which codes for the 364 amino-terminal amino acid residues of a SecA homologue from Bacillus subtilis: further evidence for the conservation of the protein export apparatus in gram-positive and gram-negative bacteria

A DNA fragment that codes for the 364 amino-terminal amino acid residues of a putative Bacillus subtilis SecA homologue has been cloned using the Escherichia coli secA gene as a probe. The deduced amino acid sequence showed 58% identity to the amino-terminus of the E. coli SecA protein. A DNA fragment which codes for 275 amino-terminal amino acid residues of the B. subtilis SecA homologue was expressed in E. coli and the corresponding gene product was shown to be recognized by anti-E. coli SecA antibodies. This polypeptide, although only about 30% the size of the E. coli SecA protein, also restored growth of E. coli MM52 (secAts) at the non-permissive temperature and the translocation defect of proOmpA in this mutant was relieved to a substantial extent.

Protein export in prokaryotes and eukaryotes. Theme with variations

Protein export in prokaryotes as well as in eukaryotes can be defined as protein transport across the plasma membrane. In both types of organisms there are various apparently ATP-dependent transport mechanisms which can be distinguished from one another and which show similarities when the prokaryotic mechanism is compared with the respective eukaryotic mechanism. First, one can distinguish between transport mechanisms which involve so-called signal or leader peptides and those which do not. The latter mechanisms seem to employ ATP-dependent transport systems which belong to the family of oligopeptide permeases and multiple drug resistance proteins. Second, in signal or leader peptide-dependent transport one can distinguish between transport mechanisms which involve ribonucleoparticles and those which employ molecular chaperones. Both mechanisms appear to converge at the level of ATP-dependent translocases.

A kinetic partitioning model of selective binding of nonnative proteins by the bacterial chaperone SecB

An in vitro assay for the interaction of SecB, a molecular chaperone from Escherichia coli, with polypeptide ligands was established based on the ability of SecB to block the refolding of denatured maltose-binding protein. Competition experiments show that SecB binds selectively to nonnative proteins with high affinity and without specificity for a particular sequence of amino acids. It is proposed that selectivity in binding is due to a kinetic partitioning of polypeptides between folding and association with SecB.

Molecular chaperones and protein translocation across the Escherichia coli inner membrane

Proteins that are able to translocate across biological membranes assume a loosely folded structure. In this review it is suggested that the loosely folded structure, referred to here as the 'pre-folded conformation', is a particular structure that interacts favourably with components of the export apparatus. Two soluble factors, SecB and GroEL, have been implicated in maintenance of the pre-folded conformation and have been termed 'molecular chaperones'. Results suggest that SecB may be a chaperone that is specialized for binding to exported protein precursors, while GroEL may be a general folding modulator that binds to many intracellular proteins.

Isolation and analysis of dominant secA mutations in Escherichia coli

The secA gene product is an autoregulated, membrane-associated ATPase which catalyzes protein export across the Escherichia coli plasma membrane. Previous genetic selective strategies have yielded secA mutations at a limited number of sites. In order to define additional regions of the SecA protein that are important in its biological function, we mutagenized a plasmid-encoded copy of the secA gene to create small internal deletions or duplications marked by an oligonucleotide linker. The mutagenized plasmids were screened in an E. coli strain that allowed the ready detection of dominant secA mutations by their ability to derepress a secA-lacZ protein fusion when protein export is compromised. Twelve new secA mutations were found to cluster into four regions corresponding to amino acid residues 196 to 252, 352 to 367, 626 to 653, and 783 to 808. Analysis of these alleles in wild-type and secA mutant strains indicated that three of them still maintained the essential functions of SecA, albeit at a reduced level, while the remainder abolished SecA translocation activity and caused dominant protein export defects accompanied by secA depression. Three secA alleles caused dominant, conditional-lethal, cold-sensitive phenotypes and resulted in some of the strongest defects in protein export characterized to date. The abundance of dominant secA mutations strongly favors certain biochemical models defining the function of SecA in protein translocation. These new dominant secA mutants should be useful in biochemical studies designed to elucidate SecA protein's functional sites and its precise role in catalyzing protein export across the plasma membrane.

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

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

The purified E. coli integral membrane protein SecY/E is sufficient for reconstitution of SecA-dependent precursor protein translocation

We have previously reconstituted the soluble phase of precursor protein translocation in vitro using purified proteins (the precursor proOmpA, the chaperone SecB, and the ATPase SecA) in addition to isolated inner membrane vesicles. We now report the isolation of the SecY/E protein, the integral membrane protein component of the E. coli preprotein translocase. The SecY/E protein, reconstituted into proteoliposomes, acts together with SecA protein to support translocation of proOmpA, the precursor form of outer membrane protein A. This translocation requires ATP and is strongly stimulated by the protonmotive force. The initial rates and the extents of translocation into either native membrane vesicles or proteoliposomes with pure SecY/E are comparable. The SecY/E protein consists of SecY, SecE, and an additional polypeptide. Antiserum against SecY immunoprecipitates all three components of the SecY/E protein.

SecB protein: a cytosolic export factor that associates with nascent exported proteins

Soluble factors participate in protein translocation across a variety of biological membranes. The Escherichia coli soluble protein SecB (the product of the secB gene) is involved in the export of periplasmic and outer membrane proteins. The isolation of secB mutations permitted the demonstration that SecB is required for rapid and efficient export of certain proteins. Consistent with the results of these genetic studies, purified SecB has been shown to stimulate protein translocation across E. coli inner membrane vesicles in vitro. This article presents a review of these past studies of SecB, speculation on the role of SecB in protein translocation, and a comparison of SecB and other factors, trigger factor and GroEL.

SecA protein: autoregulated initiator of secretory precursor protein translocation across the E. coli plasma membrane

Several classes of secA mutants have been isolated which reveal the essential role of this gene product for E. coli cell envelope protein secretion. SecA-dependent, in vitro protein translocation systems have been utilized to show that SecA is an essential, plasma membrane-associated, protein translocation factor, and that SecA's ATPase activity appears to play an essential but as yet undefined role in this process. Cell fractionation studies suggested that SecA protein is in a dynamic state within the cell, occurring in soluble, peripheral, and integral membraneous states. These data have been used to argue that SecA is likely to promote the initial insertion of secretory precursor proteins into the plasma membrane in a manner dependent on ATP hydrolysis. The protein secretion capability of the cell has been shown to translationally regulate secA expression with SecA protein serving as an autogenous repressor, although the exact mechanism and purpose of this regulation need to be defined further.

Protein translocation in vitro: biochemical characterization of genetically defined translocation components

Recent years have seen the convergence of both genetic and biochemical approaches in the study of protein translocation in E. coli. The powerful combination of these approaches is exemplified in the use of an in vitro protein synthesis-protein translocation system to analyze the role of genetically defined components of the protein translocation machinery. We describe in this review recent results focusing on the function of the secA, secB, and secY gene products and the demonstration of their requirement for in vitro protein translocation. The SecA protein was recently shown to possess ATPase activity and was proposed to be a component of the translocation ATPase. We present a speculative working model whereby the translocator complex is composed of the integral membrane proteins SecY, SecD, SecE, and SecF, forming an aqueous channel in the cytoplasmic membrane, and the tightly associated peripheral membrane protein SecA functioning as the catalytic subunit of the translocator or "protein-ATPase."

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.

No specific recognition of leader peptide by SecB, a chaperone involved in protein export

Most proteins destined for export from Escherichia coli are made as precursors containing amino-terminal leader sequences that are essential for export and that are removed during the process. The initial step in export of a subset of proteins, which includes maltose-binding protein, is binding of the precursor by the molecular chaperone SecB. This work shows directly that SecB binds with high affinity to unfolded maltose-binding protein but does not specifically recognize and bind the leader. Rather, the leader modulates folding to expose elements in the remainder of the polypeptide that are recognized by SecB.