SecB Protein Stabilizes a Translocation-Competent State of Purified prePhoE Protein

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.

Determination of the intracellular concentration of the export chaperone SecB in Escherichia coli

SecB, a small tetrameric chaperone in Escherichia coli, plays a crucial role during protein export via the general secretory pathway by binding precursor polypeptides in a nonnative conformation and passing them to SecA, the ATPase of the translocon. The dissociation constants for the interactions are known; however to relate studies in vitro to export in a living cell requires knowledge of the concentrations of the proteins in the cell. Presently in the literature there is no report of a rigorous determination of the intracellular concentration of SecB. The values available vary over 60 fold and the details of the techniques used are not given. Here we use quantitative immunoblotting to determine the level of SecB expressed from the chromosome in E.coli grown in two commonly used media. In rich medium SecB was present at 1.6 ± 0.2 μM and in minimal medium at 2.5 ± 0.6 μM. These values allow studies of SecB carried out in vitro to be applied to the situation in the cell as SecB interacts with its binding partners to move precursor polypeptides through the export pathway.

Multitasking SecB chaperones in bacteria

Protein export in bacteria is facilitated by the canonical SecB chaperone, which binds to unfolded precursor proteins, maintains them in a translocation competent state and specifically cooperates with the translocase motor SecA to ensure their proper targeting to the Sec translocon at the cytoplasmic membrane. Besides its key contribution to the Sec pathway, SecB chaperone tasking is critical for the secretion of the Sec-independent heme-binding protein HasA and actively contributes to the cellular network of chaperones that control general proteostasis in Escherichia coli, as judged by the significant interplay found between SecB and the trigger factor, DnaK and GroEL chaperones. Although SecB is mainly a proteobacterial chaperone associated with the presence of an outer membrane and outer membrane proteins, secB-like genes are also found in Gram-positive bacteria as well as in certain phages and plasmids, thus suggesting alternative functions. In addition, a SecB-like protein is also present in the major human pathogen Mycobacterium tuberculosis where it specifically controls a stress-responsive toxin–antitoxin system. This review focuses on such very diverse chaperone functions of SecB, both in E. coli and in other unrelated bacteria.

YidC is involved in the biogenesis of the secreted autotransporter hemoglobin protease

Autotransporters (ATs) constitute an important family of virulence factors secreted by Gram-negative bacteria. Following their translocation across the inner membrane (IM), ATs temporarily reside in the periplasmic space after which they are secreted into the extracellular environment. Previous studies have shown that the AT hemoglobin protease (Hbp) of Escherichia coli requires a functional signal recognition particle pathway and Sec translocon for optimal targeting to and translocation across the IM. Here, we analyzed the mode of IM translocation of Hbp in more detail. Using site-directed photocross-linking, we found that the Hbp signal peptide is adjacent to YidC early during biogenesis. Notably, YidC is in part associated with the Sec translocon but has until now primarily been implicated in the biogenesis of IM proteins. In vivo, YidC appeared critical for the biogenesis of the ATs Hbp and EspC. For Hbp, depletion of YidC resulted in the formation of secretion-incompetent intermediates that were sensitive to degradation by the periplasmic protease DegP, indicating that YidC activity affects Hbp biogenesis at a late stage, after translocation across the IM. This is the first demonstration of a role for YidC in the biogenesis of an extracellular protein. We propose that YidC is required for maintenance of the translocation-competent state of certain ATs in the periplasm. The large periplasmic domain of YidC is not critical for this novel functionality as it can be deleted without affecting Hbp biogenesis.

Structure and function of the bacterial Sec translocon

Bacteria and archaea possess a protein complex in the plasma membrane that governs protein secretion and membrane protein insertion. Eukaryotes carry homologues in the endoplasmic reticulum (ER) where they direct the same reaction. A combination of experiments conducted on the systems found in all three domains of life has revealed a great deal about protein translocation. The channel provides a route for proteins to pass through the hydrophobic barrier of the membrane, assisted by various partner proteins which maintain an unfolded state of the substrate, target it to the channel and provide the energy and mechanical drive required for transport. In bacteria, the post-translational reaction utilizes an ATPase that couples the free energy of ATP binding and hydrolysis to move the substrate through the protein pore. This review will draw on genetic, biochemical and structural findings in an account of our current understanding of this mechanism.

Sequence-specific interactions of nascent Escherichia coli polypeptides with trigger factor and signal recognition particle

As nascent polypeptides exit the ribosomal tunnel they immediately associate with chaperones, folding catalysts, and targeting factors. These interactions are decisive for the future conformation and destination of the protein that is being synthesized. Using Escherichia coli as a model organism, we have systematically analyzed how the earliest contacts of nascent polypeptides with cytosolic factors depend on the nature and future destination of the emerging sequence using a photo cross-linking approach. Together, the data suggest that the chaperone trigger factor is adjacent to emerging sequences by default, consistent with both its placement near the nascent chain exit site and its cellular abundance. The signal recognition particle (SRP) effectively competes the contact with TF when a signal anchor (SA) sequence of a nascent inner membrane protein appears outside the ribosome. The SRP remains in contact with the SA and downstream sequences during further synthesis of approximately 30 amino acids. The contact with trigger factor is then restored unless another transmembrane segment reinitiates SRP binding. Importantly and in contrast to published data, the SRP appears perfectly capable of distinguishing SA sequences from signal sequences in secretory proteins at this early stage in biogenesis.

Early encounters of a nascent membrane protein: specificity and timing of contacts inside and outside the ribosome

An unbiased photo–cross-linking approach was used to probe the “molecular path” of a growing nascent Escherichia coli inner membrane protein (IMP) from the peptidyl transferase center to the surface of the ribosome. The nascent chain was initially in proximity to the ribosomal proteins L4 and L22 and subsequently contacted L23, which is indicative of progression through the ribosome via the main ribosomal tunnel. The signal recognition particle (SRP) started to interact with the nascent IMP and to target the ribosome–nascent chain complex to the Sec–YidC complex in the inner membrane when maximally half of the transmembrane domain (TM) was exposed from the ribosomal exit. The combined data suggest a flexible tunnel that may accommodate partially folded nascent proteins and parts of the SRP and SecY. Intraribosomal contacts of the nascent chain were not influenced by the presence of a functional TM in the ribosome.

SecB is a bona fide generalized chaperone in Escherichia coli

It is known that the DnaK and Trigger Factor (TF) chaperones cooperate in the folding of newly synthesized cytosolic proteins in Escherichia coli. We recently showed that despite a very narrow temperature range of growth and high levels of aggregated cytosolic proteins, E. coli can tolerate deletion of both chaperones, suggesting that other chaperones might be involved in this process. Here, we show that the secretion-dedicated chaperone SecB efficiently suppresses both the temperature sensitivity and the aggregation-prone phenotypes of a strain lacking both TF and DnaK. SecB suppression is independent of a productive interaction with the SecA subunit of the translocon. Furthermore, in vitro cross-linking experiments demonstrate that SecB can interact both co- and posttranslationally with short nascent chains of both secretory and cytosolic proteins. Finally, we show that such cotranslational substrate recognition by SecB is greatly suppressed in the presence of ribosome-bound TF, but not by DnaK. Taken together, our data demonstrate that SecB acts as a bona fide generalized chaperone.

Microbial molecular chaperones

Protein folding in the cell, long thought to be a spontaneous process, in fact often requires the assistance of molecular chaperones. This is thought to be largely because of the danger of incorrect folding and aggregation of proteins, which is a particular problem in the crowded environment of the cell. Molecular chaperones are involved in numerous processes in bacterial cells, including assisting the folding of newly synthesized proteins, both during and after translation; assisting in protein secretion, preventing aggregation of proteins on heat shock, and repairing proteins that have been damaged or misfolded by stresses such as a heat shock. Within the cell, a balance has to be found between refolding of proteins and their proteolytic degradation, and molecular chaperones play a key role in this. In this review, the evidence for the existence and role of the major cytoplasmic molecular chaperones will be discussed, mainly from the physiological point of view but also in relationship to their known structure, function and mechanism of action. The two major chaperone systems in bacterial cells (as typified by Escherichia coli) are the GroE and DnaK chaperones, and the contrasting roles and mechanisms of these chaperones will be presented. The GroE chaperone machine acts by providing a protected environment in which protein folding of individual protein molecules can proceed, whereas the DnaK chaperones act by binding and protecting exposed regions on unfolded or partially folded protein chains. DnaK chaperones interact with trigger factor in protein translation and with ClpB in reactivating proteins which have become aggregated after heat shock. The nature of the other cytoplasmic chaperones in the cell will also be reviewed, including those for which a clear function has not yet been determined, and those where an in vivo chaperone function has still to be proven, such as the small heat shock proteins IbpA and IbpB. The regulation of expression of the genes of the heat shock response will also be discussed, particularly in the light of the signals that are needed to induce the response. The major signals for induction of the heat shock response are elevated temperature and the presence of unfolded protein within the cell, but these are sensed and transduced differently by different bacteria. The best characterized example is the sigma 32 subunit of RNA polymerase from E. coli, which is both more efficiently translated and also transiently stabilized following heat shock. The DnaK chaperones modulate this effect. However, a more widely conserved system appears to be typified by the HrcA repressor in Bacillus subtilis, the activity of which is modulated by the GroE chaperone machine. Other examples of regulation of molecular chaperones will also be discussed. Finally, the likely future research directions for molecular chaperone biology in the post-genomic era will be briefly evaluated.

The early interaction of the outer membrane protein phoe with the periplasmic chaperone Skp occurs at the cytoplasmic membrane

Spheroplasts were used to study the early interactions of newly synthesized outer membrane protein PhoE with periplasmic proteins employing a protein cross-linking approach. Newly translocated PhoE protein could be cross-linked to the periplasmic chaperone Skp at the periplasmic side of the inner membrane. To study the timing of this interaction, a PhoE-dihydrofolate reductase hybrid protein was constructed that formed translocation intermediates, which had the PhoE moiety present in the periplasm and the dihydrofolate reductase moiety tightly folded in the cytoplasm. The hybrid protein was found to cross-link to Skp, indicating that PhoE closely interacts with the chaperone when the protein is still in a transmembrane orientation in the translocase. Removal of N-terminal parts of PhoE protein affected Skp binding in a cumulative manner, consistent with the presence of two Skp-binding sites in that region. In contrast, deletion of C-terminal parts resulted in variable interactions with Skp, suggesting that interaction of Skp with the N-terminal region is influenced by parts of the C terminus of PhoE protein. Both the soluble as well as the membrane-associated Skp protein were found to interact with PhoE. The latter form is proposed to be involved in the initial interaction with the N-terminal regions of the outer membrane protein.

Depletion of phosphatidylethanolamine affects secretion of Escherichia coli alkaline phosphatase and its transcriptional expression

In this report we demonstrate that depletion of the major phospholipid phosphatidylethanolamine, a single non-bilayer forming phospholipid of Escherichia coli, significantly reduces the secretion efficiency of alkaline phosphatase in vivo. Secretion, however, is correlated with the content in membranes of cardiolipin, which in combination with selected divalent cations has a strong tendency to adopt a non-bilayer state indicating the possible involvement of lipid polymorphism in efficient protein secretion. Depletion of this zwitterionic phospholipid also inhibits expression of the protein controlled by the endogenous P(PHO) promoter but not the P(BAD) promoter, which is suggested to be due to the effect of unbalanced phospholipid composition on the orthophosphate signal transduction system (Pho regulon) through an effect on its membrane bound sensor.

Protein traffic in bacteria: multiple routes from the ribosome to and across the membrane

Bacteria use several routes to target their exported proteins to the plasma membrane. The majority are exported through pores formed by SecY and SecE. Two different molecular machineries are used to target proteins to the SecYE translocon. Translocated proteins, synthesized as precursors with cleavable signal sequences, require cytoplasmic chaperones, such as SecB, to remain competent for posttranslational transport. In concert with SecB, SecA targets the precursors to SecY and energizes their translocation by its ATPase activity. The latter function involves a partial insertion of SecA itself into the SecYE translocon, a process that is strongly assisted by a couple of membrane proteins, SecG, SecD, SecF, YajC, and the proton gradient across the membrane. Integral membrane proteins, however, are specifically recognized by a direct interaction between their noncleaved signal anchor sequences and the bacterial signal recognition particle (SRP) consisting of Ffh and 4.5S RNA. Recognition occurs during synthesis at the ribosome and leads to a cotranslational targeting to SecYE that is mediated by FtsY and the hydrolysis of GTP. No other Sec protein is required for integration unless the membrane protein also contains long translocated domains that engage the SecA machinery. Discrimination between SecA/SecB- and SRP-dependent targeting involves the specificity of SRP for hydrophobic signal anchor sequences and the exclusion of SRP from nascent chains of translocated proteins by trigger factor, a ribosome-associated chaperone. The SecYE pore accepts only unfolded proteins. In contrast, a class of redox factor-containing proteins leaves the cell only as completely folded proteins. They are distinguished by a twin arginine motif of their signal sequences that by an unknown mechanism targets them to specific pores. A few membrane proteins insert spontaneously into the bacterial plasma membrane without the need for targeting factors and SecYE. Insertion depends only on hydrophobic interactions between their transmembrane segments and the lipid bilayer and on the transmembrane potential. Finally, outer membrane proteins of Gram-negative bacteria after having crossed the plasma membrane are released into the periplasm, where they undergo distinct folding events until they insert as trimers into the outer membrane. These folding processes require distinct molecular chaperones of the periplasm, such as Skp, SurA, and PpiD.

The high affinity ATP binding site modulates the SecA-precursor interaction

SecA is the central component of the protein-translocation machinery of Escherichia coli. It is able to interact with the precursor protein, the chaperone SecB, the integral membrane protein complex SecYEG, acidic phospholipids and its own mRNA. We studied the interaction between prePhoE and SecA by using a site-specific photocrosslinking strategy. We found that SecA is able to interact with both the signal sequence and the mature domain of prePhoE. Furthermore, this interaction was dependent on the type of nucleotide bound. SecA in the ADP-bound conformation was unable to crosslink with the precursor, whereas the ATP-bound conformation was active in precursor crosslinking. The SecA-precursor interaction was maintained in the presence of E. coli phospholipids but was loosened by the presence of phosphatidylglycerol bilayers. Examining SecA ATP binding site mutants demonstrated that ATP hydrolysis at the N-terminal high affinity binding site is responsible for the changed interaction with the preprotein.

Translocase-bound SecA is largely shielded from the phospholipid acyl chains

Protein translocation in Escherichia coli is mediated by the SecA ATPase bound to the SecYEG membrane protein complex. SecA translocation ATPase activity as well as protein translocation is dependent on the presence of negatively charged lipids. By using a phospholipid with an acyl chain linked photoactivatable group, the lipid accessibility of SecA bound at the translocase was explored. SecA bound to lipid vesicles containing negatively charged lipids was found to be readily accessible for labeling by the photoactivatable phospholipid. The presence of an excess amount of SecYEG complex resulted in a remarkable reduction in the amount of lipid-accessible SecA irrespective of the nucleotide-bound form of SecA. These data demonstrate that the SecYEG-bound SecA is largely shielded from the phospholipid acyl chains and suggest the presence of two distinct pools of membrane-bound SecA that differ in the degree of lipid association.

Identification of a sequence motif that confers SecB dependence on a SecB-independent secretory protein in vivo

SecB is a cytosolic chaperone which facilitates the transport of a subset of proteins, including membrane proteins such as PhoE and LamB and some periplasmic proteins such as maltose-binding protein, in Escherichia coli. However, not all proteins require SecB for transport, and proteins such as ribose-binding protein are exported efficiently even in SecB-null strains. The characteristics which confer SecB dependence on some proteins but not others have not been defined. To determine the sequence characteristics that are responsible for the SecB requirement, we have inserted a systematic series of short, polymeric sequences into the SecB-independent protein alkaline phosphatase (PhoA). The extent to which these simple sequences convert alkaline phosphatase into a SecB-requiring protein was evaluated in vivo. Using this approach we have examined the roles of the polarity and charge of the sequence, as well as its location within the mature region, in conferring SecB dependence. We find that an insert with as few as 10 residues, of which 3 are basic, confers SecB dependence and that the mutant protein is efficiently exported in the presence of SecB. Remarkably, the basic motifs caused the protein to be translocated in a strict membrane potential-dependent fashion, indicating that the membrane potential is not a barrier to, but rather a requirement for, translocation of the motif. The alkaline phosphatase mutants most sensitive to the loss of SecB are those most sensitive to inhibition of SecA via azide treatment, consistent with the necessity for formation of a preprotein-SecB-SecA complex. Furthermore, the impact of the basic motif depends on location within the mature protein and parallels the accessibility of the location to the secretion apparatus.

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.

In vitro membrane integration of leader peptidase depends on the Sec machinery and anionic phospholipids and can occur post-translationally

A cell-free system based on a lysate and membrane vesicles from Escherichia coli is used to study characteristics of the membrane integration reaction of the polytopic membrane protein leader peptidase (Lep). Integration into inverted inner membrane vesicles was detected by partial protection against externally added protease. Integration is most efficient when coupled to translation but can also occur post-translationally and depends on the action of the proteinaceous Sec machinery and availability of anionic phospholipids. Lep is the first example of a membrane protein without cleavable signal sequence which requires anionic lipids for integration in vitro.

Mode of insertion of the signal sequence of a bacterial precursor protein into phospholipid bilayers as revealed by cysteine-based site-directed spectroscopy

The interactions between a bacterial precursor protein and phospholipids in bilayer-based model membrane systems is addressed in this study. The precursor-lipid interactions were assessed from the side of the lipid phase by fluorescence and electron spin resonance spectroscopy, using the precursor of the Escherichia coli outer membrane protein PhoE. The role of the signal sequence, as part of the precursor, in this interaction was investigated by using cysteine-based site-directed spectroscopy. For this purpose, purified cysteine-containing mutants of prePhoE, which were made by site-directed mutagenesis of the signal sequence part and of the mature part, and defined lipids were used. The location of the fluorescently labeled cysteine residues was established by resonance energy transfer and quenching experiments and those of the corresponding spin-labeled cysteine residues by paramagnetic relaxation enhancement. It was demonstrated that precursor-phospholipid interactions exist in model membrane systems and also that these interactions were dependent on the presence of anionic phospholipids and resulted in a deep insertion of (parts of) the precursor into the lipid bilayer. Furthermore, the results with the cysteine mutations in the signal sequence of the precursor indicate that both termini of the signal sequence are located near or at the membrane surface, with only the fluorescence of the labeled cysteines in the signal sequence part being protected against aqueous quenchers. The results demonstrate that, when part of the intact precursor, the signal sequence experiences similar lipid-protein interactions as do isolated signal peptides. They also indicate that the signal sequence inserts entirely as a looped structure into the membrane. In addition, the data also indicate that the mature part of the precursor has an affinity for the membrane.

Influence of the signal sequence and chaperone SecB on the interaction between precursor protein prePhoE and phospholipids

To investigate in a direct way the interaction between a precursor protein and phospholipids, monolayer studies were performed using the purified precursor of Escherichia coli outer-membrane protein PhoE. It was demonstrated that prePhoE can insert efficiently into monolayers of dioleoylglycerophosphoglycerol (Ole2GroPGro) and dioleoylglycerophosphoethanolamine (Ole2GroPEtn), this insertion was mainly driven by hydrophobic forces. Compared with previous results obtained with PhoE signal peptide, the full-length precursor protein does not show the specific interaction with acidic lipids. PrePhoE inserted into a Ole2GroPGro monolayer occupies an area of 28 +/- 3 [corrected] nm2/molecule, which is approximately 10-fold larger than the area occupied by the PhoE signal peptide. The purified mature PhoE protein has a lower capacity to insert into Ole2GroPGro and Ole2GroPEtn monolayers and is, in contrast to prePhoE, fully accessible to proteinase K after interacting with a Ole2GroPGro monolayer. The results demonstrate that in the context of the precursor protein both the signal sequence and mature domain of prePhoE insert into lipid monolayers. It was found that PhoE, like prePhoE, can form in vitro a complex with the cytosolic chaperone SecB. Complexation with SecB increases the insertion of (pre)PhoE into acidic lipid monolayers. The high lipid affinity of prePhoE was also demonstrated by vesicle-binding experiments which showed that SecB dissociates from the SecB-prePhoE complex upon binding of the precursor to the bilayer. The implications of these findings for preprotein translocation are discussed and in addition some extrapolations to the insertion of PhoE into the outer membrane are made.

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.

The C terminus of SecA is involved in both lipid binding and SecB binding

Using C-terminal deletion mutations in secA, we localized the previously proposed (Breukink, E., Keller, R.C. A., and de Kruijff, B. (1993), FEBS Lett. 331, 19-24) second lipid binding site on SecA. Since removal of these residues completely abolished the property of SecA to cause aggregation of negatively charged phosphatidyl-glycerol vesicles, we conclude that the C-terminal 70 amino acid residues of SecA are involved in lipid-binding. The C-terminal 70 amino acid residues of SecA are important for efficient in vitro translocation of the SecB-dependent precursor of PhoE across inverted inner membrane vesicles. Moreover, in vivo studies showed that this region is essential for growth. SecB and a SecB-precursor complex were shown to inhibit the SecA-mediated lipid vesicle aggregation, suggesting that the overall acidic SecB protein binds at or near the second lipid binding site on SecA. This together with the observation that the SecA mutant protein lacking the C-terminal 70 residues had a strongly reduced ability to mediate binding of SecB-precursor complexes to inverted inner membrane vesicles demonstrates that the C terminus of SecA is also involved in SecB binding.

Evidence for a loop-like insertion mechanism of pro-Omp A into the inner membrane of Escherichia coli

We have studied the insertion of pro-OmpA into the Escherichia coli membrane in vivo using various mutants that have either alterations in the amino-terminal parts of the signal peptide or in the mature region that flanks the signal peptide. A pro-OmpA mutant with an amino terminal extension of 142 residues derived from ribulokinase (AraB) was analysed for its membrane insertion. The AraB portion, which includes a cluster of seven charged residues close to the signal sequence, did not interfere with the Sec components and allowed efficient export of OmpA. During translocation the AraB portion remained in the cytoplasm. Further mutants of OmpA were constructed in the carboxy-terminal region flanking the signal sequence. Pro-OmpA does not translocate across the membrane when a charge cluster, comprised of Lys-Arg-Arg-Glu-Arg, is introduced after positions 5, 11 or 15 of the mature region, but is translocated when the cluster is introduced after position 22. This defines a region of about 20 residues in the mature part of pro-OmpA that is crucial for membrane insertion. These results suggest that in the case of the Sec-dependent pro-OmpA, as with the Sec-independent M13 procoat, the precursor assumes a loop-like structure involving the signal peptide and the early part of the mature region, leaving the amino terminus of the signal peptide at the cytoplasmic face.

Ability of MBP or RBP signal peptides to influence folding and in vitro translocation of wild-type and hybrid precursors

Maltose-binding protein (MBP), whose export in E. coli is dependent upon the chaperone SecB, and ribose-binding protein (RBP), whose export is SecB-independent, have been used to generate hybrid secretory proteins. Here, in vitro techniques were used to analyze MBP, RBP, RBP-MBP (RBP signal and MBP mature), and MBP-RBP (MBP signal and RBP mature). In protease-protection experiments, RBP folded considerably faster than MBP, RBP-MBP, or MBP-RBP. Only the folding properties of proteins containing the MBP mature moiety were influenced by SecB. In post-translational translocation assays, MBP exhibited the highest translocation efficiency. The hybrids RBP-MBP and MBP-RBP showed intermediate levels, and RBP translocation was not detected in these assays. These experiments demonstrate the influence of the signal peptide in determining folding properties and translocation efficiency of precursor secretory 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.

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.

Elevated cytosolic concentrations of SecA compensate for a protein translocation defect in Escherichia coli cells with reduced levels of negatively charged phospholipids

Cellular extracts from cells with reduced synthesis of negatively charged phospholipids were found to support in vitro translocation of the precursor of the outer membrane protein PhoE with increased efficiency. Analysis of these extracts revealed that they contain increased levels of SecA. SecA depletion resulted in a loss of the translocation stimulatory activity, which could be restored by re-addition of purified SecA. We conclude that elevated cytosolic levels of SecA counteract the reduction of translocation efficiency due to low levels of negatively charged phospholipids in the inner membrane.

Biogenesis of outer membrane protein PhoE of Escherichia coli. Evidence for multiple SecB-binding sites in the mature portion of the PhoE protein

Efficient in vivo translocation of the precursor of Escherichia coli outer membrane protein PhoE across the inner membrane is shown to depend on SecB protein. A set of mutants, carrying internal deletions in the phoE gene, was used to locate a possible SecB-binding site and/or a site that makes the protein dependent on SecB for export. Except for two small mutant PhoE proteins, the in vivo and in vitro translocation of all mutant proteins was more efficient in the presence of SecB. The interaction of SecB protein with wild-type and mutant PhoE proteins, synthesized in vitro, was further studied in co-immunoprecipitation experiments with anti-SecB protein serum. The efficiencies of co-immunoprecipitation of precursor and mature PhoE were very similar, indicating the absence of a SecB-binding site in the signal sequence. Moreover, all mutant proteins with deletions in the mature moiety of the PhoE protein were co-immunoprecipitated in these assays, albeit mostly with reduced efficiency. Taken together, these results indicate the existence of multiple SecB-binding sites in the mature portion of the PhoE protein.

SecB-binding does not maintain the translocation-competent state of prePhoE

The role of SecB protein in the export of the precursor of outer membrane protein PhoE and mutant forms of this precursor was studied in vitro. When synthesized in the absence of SecB, translocation-competent prePhoE was observed post-translationally, but addition of SecB was required for efficient translocation into inner membrane vesicles. The translocation competency of in vitro synthesized prePhoE diminished with a similar half-life during incubations in the presence or absence of SecB. The loss of translocation competency of prePhoE, synthesized in the presence of SecB, was not due to dissociation of prePhoE-SecB complexes as could be demonstrated in co-immunoprecipitation experiments with anti-SecB serum. Apparently, SecB does not maintain the translocation-competent conformation of prePhoE, but is mainly required for efficient targeting of this precursor to the export apparatus.

SecA insertion into phospholipids is stimulated by negatively charged lipids and inhibited by ATP: a monolayer study

SecA-lipid interactions are believed to be important for the translocation of precursor proteins across the inner membrane of Escherichia coli [Lill, R., Dowhan, W., & Wickner, W. (1990) Cell 60, 271-280]. SecA insertion into the phospholipid bilayer could a role in this process. We investigated this possibility by studying the interactions between SecA and different phospholipids using the monolayer technique. It was established that SecA is surface-active and can insert into lipid monolayers. This insertion was greatly enhanced by the negatively charged lipids DOPG and Escherichia coli cardiolipin. Insertion of SecA into these negatively charged lipids could be detected up to initial surface pressures of 34 mN/m for DOPG and 36 mN/m for Escherichia coli cardiolipin, implying a possible role for negatively charged lipids in the insertion of SecA in biological membranes. High salt concentrations did not inhibit the SecA insertion into DOPG monolayers, suggesting not only an electrostatic but also a hydrophobic interaction of SecA with the lipid monolayer. ATP decreased both the insertion (factor 2) and binding (factor 3) of SecA to DOPG monolayers. ADP and phosphate gave a decrease in the SecA insertion to the same extent as ATP, but the binding of SecA was only slightly reduced. AMP-PNP and ATP-gamma-S did not have large effects on the insertion or on the binding of SecA to DOPG monolayers. The physiological significance of these results in protein translocation is discussed.

Energy requirements for protein translocation across the Escherichia coli inner membrane

Both ATP and an electrochemical potential play roles in translocating proteins across the inner membrane of Escherichia coli. Recent discoveries have dissected the overall transmembrane movement into separate subreactions with different energy requirements, identified a translocation ATPase, and reconstituted both energy-requiring steps of the reaction from purified components. A more refined understanding of the energetics of this fundamental process is beginning to provide answers about the basic issues of how proteins move across the hydrophobic membrane barrier.

Stereochemical analysis of interaction of signal peptide with phospholipids at the initiation of protein translocation across the membrane

Stereochemical analysis of signal peptide interaction with E. coli membrane phospholipids revealed the structural complementarity of N-terminus of signal peptide alpha-helix and acid phospholipids. The formation of their complex leads to neutralization of charges and decrease in hydrophilicity of both components, and promotes insertion of peptide and phospholipid into the membrane, not separately but as a complex. Interaction of acid phospholipids with the E. coli alkaline phosphatase (AP) signal peptide was thoroughly analyzed, and it was shown that in this case a complex of signal peptide alpha-helix with phosphatidylglycerol is inserted into the membrane with the lowest energy expense. On the basis of the results of stereochemical analysis and the available experimental data, a molecular mechanism of protein translocation initiation across the membrane has been proposed, in which the key events are the formation of the complex "signal peptide alpha-helix-acid phospholipid", the coupled insertion of hydrophobic peptide-lipid complex into a nonpolar membrane interior and translocation across the membranes.

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.

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.