prl mutations in the Escherichia coli secG gene

Signatures of selection in core and accessory genomes indicate different ecological drivers of diversification among Bacillus cereus clades

Bacterial clades are often ecologically distinct, despite extensive horizontal gene transfer (HGT). How selection works on different parts of bacterial pan-genomes to drive and maintain the emergence of clades is unclear. Focusing on the three largest clades in the diverse and well-studied Bacillus cereus sensu lato group, we identified clade-specific core genes (present in all clade members) and then used clade-specific allelic diversity to identify genes under purifying and diversifying selection. Clade-specific accessory genes (present in a subset of strains within a clade) were characterized as being under selection using presence/absence in specific clades. Gene ontology analyses of genes under selection revealed that different gene functions were enriched in different clades. Furthermore, some gene functions were enriched only amongst clade-specific core or accessory genomes. Genes under purifying selection were often clade-specific, while genes under diversifying selection showed signs of frequent HGT. These patterns are consistent with different selection pressures acting on both the core and the accessory genomes of different clades and can lead to ecological divergence in both cases. Examining variation in allelic diversity allows us to uncover genes under clade-specific selection, allowing ready identification of strains and their ecological niche.

Suppressor Mutants: History and Today's Applications

For decades, biologist have exploited the near boundless advantages that molecular and genetic tools and analysis provide for our ability to understand biological systems. One of these genetic tools, suppressor analysis, has proven invaluable in furthering our understanding of biological processes and pathways and in discovering unknown interactions between genes and gene products. The power of suppressor analysis lies in its ability to discover genetic interactions in an unbiased manner, often leading to surprising discoveries. With advancements in technology, high-throughput approaches have aided in large-scale identification of suppressors and have helped provide insight into the core functional mechanisms through which suppressors act. In this review, we examine some of the fundamental discoveries that have been made possible through analysis of suppressor mutations. In addition, we cover the different types of suppressor mutants that can be isolated and the biological insights afforded by each type. Moreover, we provide considerations for the design of experiments to isolate suppressor mutants and for strategies to identify intergenic suppressor mutations. Finally, we provide guidance and example protocols for the isolation and mapping of suppressor mutants.

Pushing the Envelope: The Mysterious Journey Through the Bacterial Secretory Machinery, and Beyond

Gram-negative bacteria are contained by an envelope composed of inner and outer-membranes with the peptidoglycan (PG) layer between them. Protein translocation across the inner membrane for secretion, or insertion into the inner membrane is primarily conducted using the highly conserved, hourglass-shaped channel, SecYEG: the core-complex of the Sec translocon. This transport process is facilitated by interactions with ancillary subcomplex SecDF-YajC (secretion) and YidC (insertion) forming the holo-translocon (HTL). This review recaps the transport process across the inner-membrane and then further explores how delivery and folding into the periplasm or outer-membrane is achieved. It seems very unlikely that proteins are jettisoned into the periplasm and left to their own devices. Indeed, chaperones such as SurA, Skp, DegP are known to play a part in protein folding, quality control and, if necessary degradation. YfgM and PpiD, by their association at the periplasmic surface of the Sec machinery, most probably are also involved in some way. Yet, it is not entirely clear how outer-membrane proteins are smuggled past the proteases and across the PG to the barrel-assembly machinery (BAM) and their final destination. Moreover, how can this be achieved, as is thought, without the input of energy? Recently, we proposed that the Sec and BAM translocons interact with one another, and most likely other factors, to provide a conduit to the periplasm and the outer-membrane. As it happens, numerous other specialized proteins secretion systems also form trans-envelope structures for this very purpose. The direct interaction between components across the envelope raises the prospect of energy coupling from the inner membrane for active transport to the outer-membrane. Indeed, this kind of long-range energy coupling through large inter-membrane assemblies occurs for small molecule import (e.g., nutrient import by the Ton complex) and export (e.g., drug efflux by the AcrAB-TolC complex). This review will consider this hypothetical prospect in the context of outer-membrane protein biogenesis.

Genetic Analysis of Protein Translocation

Cells in all domains of life must translocate newly synthesized proteins both across membranes and into membranes. In eukaryotes, proteins are translocated into the lumen of the ER or the ER membrane. In prokaryotes, proteins are translocated into the cytoplasmic membrane or through the membrane into the periplasm for Gram-negative bacteria or the extracellular space for Gram-positive bacteria. Much of what we know about protein translocation was learned through genetic selections and screens utilizing lacZ gene fusions in Escherichia coli. This review covers the basic principles of protein translocation and how they were discovered and developed. In particular, we discuss how lacZ gene fusions and the phenotypes conferred were exploited to identify the genes involved in protein translocation and provide insights into their mechanisms of action. These approaches, which allowed the elucidation of processes that are conserved throughout the domains of life, illustrate the power of seemingly simple experiments.

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.

Escherichia coli SecG is required for residual export mediated by mutant signal sequences and for SecY-SecE complex stability

Protein export to the bacterial periplasm is achieved by SecYEG, an inner membrane heterotrimer. SecY and SecE are encoded by essential genes, while SecG is not essential for growth under standard laboratory conditions. Using a quantitative and sensitive export assay, we show that SecG plays a critical role for the residual export mediated by mutant signal sequences; the magnitude of this effect is not proportional to the strength of the export defect. In contrast, export mediated by wild-type signal sequences is only barely retarded in the absence of SecG. When probed with mutant signal sequences, secG loss of function mutations display a phenotype opposite to that of prlA mutations in secY. The analysis of secG and prlA single and double mutant strains shows that the increased export conferred by several prlA alleles is enhanced in the absence of SecG. Several combinations of prlA alleles with a secG deletion cannot be easily constructed. This synthetic phenotype is conditional, indicating that cells can adapt to the presence of both alleles. The biochemical basis of this phenomenon is linked to the stability of the SecYE dimer in solubilized membranes. With prlA alleles that can be normally introduced in a secG deletion strain, SecG has only a limited effect on the stability of the SecYE dimer. With the other prlA alleles, the SecYE dimer can often be detected only in the presence of SecG. A possible role for the maintenance of SecG during evolution is proposed.

The Sec-dependent pathway

The Sec pathway for export of proteins across the cytoplasmic membrane to the bacterial periplasm and outer membrane was the first secretion pathway to be discovered in bacteria. A combination of bacterial genetics, development of an in vitro membrane vesicle system and the concurrent elaboration of the signal hypothesis from studies on eukaryotes led to the identification and characterization of two pathways leading to protein export through the SecYEG cytoplasmic membrane translocon. The Sec pathway is also required for assembly of proteins into the cytoplasmic membrane. Since the membrane translocon for Sec pathways is conserved across the three domains of life, the history of research progress in eukaryotes and bacteria was facilitated by the close interaction between those studying both classes of organisms.

The SecY translocation complex: convergence of genetics and structure

All organisms share a requirement for translocation of proteins across membranes. The major mechanism for this process is the universally conserved SecY/Sec61 pathway. Many years of extensive genetic and biochemical analyses identified the components of the SecY/Sec61 pathway, demonstrated that most exported proteins use this route for translocation, and led to understanding of many functions of the components. Recently, structural predictions based on genetic analyses in Escherichia coli were confirmed, in a striking and satisfying manner, by the solution of an X-ray crystal structure from an archaeal SecY complex. This review discusses the genetic background that led to those hypotheses and the convergence of genetic studies with structural data.

Deregulation of the SecYEG translocation channel upon removal of the plug domain

Previous studies have shown that the SecY plug is displaced from the center of the SecYEG channel during polypeptide translocation. The structural and functional consequences of the deletion of the plug are now examined. Both in vivo and in vitro observations indicate that the plug domain is not essential to the function of the translocon. In fact, deletion of the plug confers to the cell and to the membranes a Prl-like phenotype: reduced proton-motive force dependence of translocation, increased membrane insertion of SecA, diminished requirement for functional leader peptide, and weakened SecYEG subunit association. Although the plug domain does not seem essential, locking the plug in the center of the channel inactivates the translocon. Thus, the SecY plug is important to regulate the activity of the channel and to confer specificity to the translocation reaction. We propose that the plug contributes to the gating mechanism of the channel by maintaining the structure of the SecYEG complex in a compact closed state.

Modeling the effects of prl mutations on the Escherichia coli SecY complex

The apparatus responsible for translocation of proteins across bacterial membranes is the conserved SecY complex, consisting of SecY, SecE, and SecG. Prior genetic analysis provided insight into the mechanisms of protein export, as well as the interactions between the component proteins. In particular, the prl suppressor alleles of secE and secY, which allow export of secretory proteins with defective signal sequences, have proven particularly useful. Here, we report the isolation of novel mutations in secE and secY, as well as the phenotypic effects of combinations of prl mutations. These new alleles, as well as previously characterized prl mutations, were analyzed in light of the recently published crystal structure of the archaeal SecY complex. Our results support and expand a model of Prl suppressor activity that proposes that all of the prlA and prlG alleles either destabilize the closed state of the channel or stabilize the open form. These mutants thus allow channel opening to occur without the triggering event of signal sequence binding that is required in a wild-type complex.

The protein-conducting channel SecYEG

In bacteria, the translocase mediates the translocation of proteins into or across the cytosolic membrane. It consists of a membrane embedded protein-conducting channel and a peripherally associated motor domain, the ATPase SecA. The channel is formed by SecYEG, a multimeric protein complex that assembles into oligomeric forms. The structure and subunit composition of this protein-conducting channel is evolutionary conserved and a similar system is found in the endoplasmic reticulum of eukaryotes and the cytoplasmic membrane of archaea. The ribosome and other membrane proteins can associate with the protein-conducting channel complex and affect its activity or functionality.

Versatility of inner membrane protein biogenesis in Escherichia coli

To further our understanding of inner membrane protein (IMP) biogenesis in Escherichia coli, we have accomplished the widest in vivo IMP assembly screen so far. The biogenesis of a set of model IMPs covering most IMP structures possible has been studied in a variety of signal recognition particle (SRP), Sec and YidC mutant strains. We show that the assembly of the complete set of model IMPs is assisted (i.e. requires the aid of proteinaceous factors), and that the requirements for assembly of the model IMPs into the inner membrane differ significantly from each other. This indicates that IMP assembly is much more versatile than previously thought.

Recognition of secretory proteins in Escherichia coli requires signals in addition to the signal sequence and slow folding

BACKGROUND

The Sec-dependent protein export apparatus of Escherichia coli is very efficient at correctly identifying proteins to be exported from the cytoplasm. Even bacterial strains that carry prl mutations, which allow export of signal sequence-defective precursors, accurately differentiate between cytoplasmic and mutant secretory proteins. It was proposed previously that the basis for this precise discrimination is the slow folding rate of secretory proteins, resulting in binding by the secretory chaperone, SecB, and subsequent targeting to translocase. Based on this proposal, we hypothesized that a cytoplasmic protein containing a mutation that slows its rate of folding would be recognized by SecB and therefore targeted to the Sec pathway. In a Prl suppressor strain the mutant protein would be exported to the periplasm due to loss of ability to reject non-secretory proteins from the pathway.

RESULTS

In the current work, we tested this hypothesis using a mutant form of lambda repressor that folds slowly. No export of the mutant protein was observed, even in a prl strain. We then examined binding of the mutant lambda repressor to SecB. We did not observe interaction by either of two assays, indicating that slow folding is not sufficient for SecB binding and targeting to translocase.

CONCLUSIONS

These results strongly suggest that to be targeted to the export pathway, secretory proteins contain signals in addition to the canonical signal sequence and the rate of folding.

A novel class of secA alleles that exert a signal-sequence-dependent effect on protein export in Escherichia coli

The murine plasminogen activator inhibitor 2 (PAI2) signal sequence inefficiently promotes the export of E. coli alkaline phosphatase (AP). High-level expression of PAI2::AP chimeric proteins from the arabinose P(BAD) promoter is toxic and confers an Ara(S) phenotype. Most Ara(R) suppressors map to secA, as determined by sequencing 21 independent alleles. Mutations occur throughout the gene, including both nucleotide binding domains (NBDI and NBDII) and the putative signal sequence binding domain (SSBD). Using malE and phoA signal sequence mutants, we showed that the vast majority of these secA suppressors exhibit weak Sec phenotypes. Eight of these secA mutations were further characterized in detail. Phenotypically, these eight suppressors can be divided into three groups, each localized to one domain of SecA. Most mutations allow near-normal levels of wild-type preprotein export, but they enhance the secretion defect conferred by signal sequence mutations. Interestingly, one group exerts a selective effect on the export of PAI2::AP when compared to that of AP. In conclusion, this novel class of secA mutations, selected as suppressors of a toxic signal sequence, differs from the classical secA (prlD) mutations, selected as suppressors of defective signal sequences, although both types of mutations affect signal sequence recognition.

Critical regions of secM that control its translation and secretion and promote secretion-specific secA regulation

SecA is an essential ATP-driven motor protein that binds to presecretory or membrane proteins and the translocon and promotes the translocation or membrane integration of these proteins. secA is subject to a protein secretion-specific form of regulation, whereby its translation is elevated during secretion-limiting conditions. A novel mechanism that promotes this regulation involves translational pausing within the gene upstream of secA, secM. The secM translational pause prevents formation of an RNA helix that normally blocks secA translational initiation. The duration of this pause is controlled by the rate of secretion of nascent SecM, which in turn depends on its signal peptide and a functional translocon. We characterized the atypical secM signal peptide and found that mutations within the amino-terminal region specifically affect the secM translational pause and secA regulation, while mutations in the hydrophobic core region affect SecM secretion as well as translational pausing and secA regulation. In addition, mutational analysis of the 3' end of secM allowed us to identify a conserved region that is required to promote the translational pause that appears to be operative at the peptide level. Together, our results provide direct support for the secM translational pause model of secA regulation, and they pinpoint key sequences within secM that promote this important regulatory system.

Targeting, insertion, and localization of Escherichia coli YidC

YidC was recently shown to play an important role in the assembly of inner membrane proteins (IMPs) both in conjunction with and separate from the Sec-translocon. Little is known about the biogenesis and structural and functional properties of YidC, itself a polytopic IMP. Here we analyze the targeting and membrane integration of YidC using in vivo and in vitro approaches. The combined data indicate that YidC is targeted by the signal recognition particle and inserts at the SecAYEG-YidC translocon early during biogenesis, unlike its mitochondrial homologue Oxa1p. In addition, YidC is shown to be relatively abundant compared with other components involved in IMP assembly and is predominantly localized at the poles of the cell.

The F286Y mutation of PrlA4 tempers the signal sequence suppressor phenotype by reducing the SecA binding affinity

SecYEG forms the protein-conducting channel of the Escherichia coli translocase. It binds the peripheral ATPase SecA that drives the preprotein translocation reaction. PrlA4 is a double mutant of SecY that enables the translocation of preproteins with a defective or even missing signal sequence. The effect of the individual mutations, F286Y and I408N, was studied with SecYEG proteoliposomes. SecY(I408N) is responsible for the increased translocation of preproteins with a defective and normal signal sequence, and exhibits a stronger prl phenotype than PrlA4. This activity correlates with an elevated SecA-translocation ATPase and SecA binding affinity. SecY(F286Y) supports only a low SecA binding affinity, preprotein translocation and SecA translocation ATPase activity. These results suggest that the second site F286Y mutation reduces the strength of the I408N mutation of PrlA4 by lowering the SecA binding affinity.

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.

A single amino acid substitution in SecY stabilizes the interaction with SecA

The SecYEG complex constitutes a protein conducting channel across the bacterial cytoplasmic membrane. It binds the peripheral ATPase SecA to form the translocase. When isoleucine 278 in transmembrane segment 7 of the SecY subunit was replaced by a unique cysteine, SecYEG supported an increased preprotein translocation and SecA translocation ATPase activity, and allowed translocation of a preprotein with a defective signal sequence. SecY(I278C)EG binds SecA with a higher affinity than normal SecYEG, in particular in the presence of ATP. The increased translocation activity of SecY(I278C)EG was confirmed in a purified system consisting of SecYEG proteoliposomes, while immunoprecipitation in detergent solution reveal that translocase-preprotein complexes are more stable with SecY(I278C) than with normal SecY. These data imply an important role for SecY transmembrane segment 7 in SecA binding. As improved SecA binding to SecY was also observed with the prlA4 suppressor mutation, it may be a general mechanism underlying signal sequence suppression.

The PrlA and PrlG phenotypes are caused by a loosened association among the translocase SecYEG subunits

prlA mutations in the gene encoding the SecY subunit of the membrane domain of the Escherichia coli preprotein translocase confer many phenotypes: enhanced translocation rates, increased affinity for SecA, diminished requirement for functional leader sequences, reduced proton-motive force (PMF) dependence of preprotein translocation and facilitated translocation of preproteins with folded domains. We now report that both prlA and prlG mutations weaken the associations between the SecY, SecE and SecG subunits of the translocase. This loosened association increases the initiation of translocation by facilitating the insertion of SecA with its bound preprotein but reduces the stimulatory effect of the PMF during the initial step of translocation. Furthermore, the originally isolated prlA4 mutant, which possesses a particularly labile SecYEG complex, acquired a secondary mutation that restored the stability while conserving the flexibility of the complex. Combinations of certain prlA and prlG mutations, known to cause synthetic lethality in vivo, dramatically loosen subunit association and lead to complete disassembly of SecYEG. These findings underscore the importance of the loosened SecYEG association for the Prl phenotypes. We propose a model in which each of the PrlA and PrlG phenotypes derive from this enhanced SecYEG conformational flexibility.

Protein targeting to the bacterial cytoplasmic membrane

Proteins that perform their activity within the cytoplasmic membrane or outside this cell boundary must be targeted to the translocation site prior to their insertion and/or translocation. In bacteria, several targeting routes are known; the SecB- and the signal recognition particle-dependent pathways are the best characterized. Recently, evidence for the existence of a third major route, the twin-Arg pathway, was gathered. Proteins that use either one of these three different pathways possess special features that enable their specific interaction with the components of the targeting routes. Such targeting information is often contained in an N-terminal extension, the signal sequence, but can also be found within the mature domain of the targeted protein. Once the nascent chain starts to emerge from the ribosome, competition for the protein between different targeting factors begins. After recognition and binding, the targeting factor delivers the protein to the translocation sites at the cytoplasmic membrane. Only by means of a specific interaction between the targeting component and its receptor is the cargo released for further processing and translocation. This mechanism ensures the high-fidelity targeting of premembrane and membrane proteins to the translocation site.

Roles of SecG in ATP- and SecA-dependent protein translocation

SecA, the translocation ATPase in Escherichia coli, undergoes cycles of conformational changes (insertion/deinsertion) in response to ATP and a preprotein. The membrane-embedded portion of protein translocase, SecYEG, has crucial roles in the SecA-driven preprotein translocation reaction. We previously identified a secY mutation (secY205) that did not allow an ATP- and preprotein-dependent (productive) insertion of SecA as well as secA mutations that suppressed the secY205 translocation defect. One of the suppressor mutations, secA36, also suppressed the cold-sensitive phenotype of the secG deletion mutant. In vitro experiments at 20 degreesC showed that inverted membrane vesicles lacking SecG were almost inactive in combination with the wild-type SecA protein in translocation of proOmpA as well as in the accompanying ATP hydrolysis. In contrast, the SecA36 mutant protein was found to be able to execute the translocation activity fully at this temperature, even in the absence of SecG. A SecG requirement and its alleviation by the SecA36 alteration also were shown for the SecA insertion reaction. The finding that the SecA36 protein no longer requires assistance from SecG in its insertion and in its catalysis of protein translocation agrees with the idea that SecG normally assists in the functioning of SecA. In agreement with this notion, when the intrinsic SecA function was compromised by a lowered ATP concentration, SecG became essential even at 37 degreesC and even for the SecA36 protein. We propose that in the normal translocase, SecG cooperates with SecA to facilitate efficient movement of preprotein in each catalytic cycle of SecA.

Translocation, folding, and stability of the HflKC complex with signal anchor topogenic sequences

HflK and HflC are plasma membrane proteins of Escherichia coli, each having a large C-terminal domain exposed to the periplasmic space and an N-terminally located transmembrane segment, which should act as a signal anchor sequence for their biogenesis. They form a complex, HflKC. We studied in vivo processes of biogenesis of this pair of membrane proteins. Translocation of the C-terminal domains across the membrane, as assessed by their accessibility to externally added protease, was completed within 1 min after the synthesis in wild-type cells as well as in the secB mutant cells or in the FtsY-depleted cells. In contrast, translocation of these domains was retarded markedly when sodium azide was added to inhibit SecA ATPase and blocked almost completely in secY- or secD-defective mutant cells. Thus, although targeting of these membrane proteins depends neither on the SecB chaperone nor on the SRP pathway, their translocation occurs exclusively via the Sec translocase complex. Translocated HflK molecules were then folded into a partially protease-resistant conformation, taking a few minutes, and this folding was induced upon association with HflC. Singly expressed HflK and HflC were unstable in vivo and periplasmic proteases DegP and Prc were involved in the degradation of the HflK subunit. We characterized several hflA alleles isolated in early studies; they alter the HflK or the HflC sequence and destabilize the HflKC complex.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

PrlA4 prevents the rejection of signal sequence defective preproteins by stabilizing the SecA-SecY interaction during the initiation of translocation

In Escherichia coli, precursor proteins are translocated across the cytoplasmic membrane by translocase. This multisubunit enzyme consists of a preprotein-binding and ATPase domain, SecA, and the SecYEG complex as the integral membrane domain. PrlA4 is a mutant of SecY that enables the translocation of preproteins with a defective, or missing, signal sequence. Inner membranes of the prlA4 strain efficiently translocate Delta8proOmpA, a proOmpA derivative with a non-functional signal sequence. Owing to the signal sequence mutation, Delta8proOmpA binds to the translocase with a lowered affinity and the recognition is not restored by the prlA4 SecY. At the ATP-dependent initiation of translocation, the binding affinity of SecA for SecYEG is lowered causing the premature loss of bound preproteins from the translocase. The prlA4 membranes, however, bind SecA with a much higher affinity than the wild-type, and during initiation, the SecA and preprotein remain bound at the translocation site allowing an improved efficiency of translocation. It is concluded that the prlA4 strain prevents the rejection of defective preproteins from the export pathway by stabilizing SecA at the SecYEG complex.

The Sec system

Proteins designated to be secreted by Escherichia coli are synthesized with an amino-terminal signal peptide and associate as nascent chains with the export-specific chaperone SecB. Translocation occurs at a multisubunit membrane-bound enzyme termed translocase, which consists of a peripheral preprotein-binding site and an ATPase domain termed SecA, a core heterotrimeric integral membrane protein complex with SecY, SecE and SecG as subunits, and an accessory integral membrane protein complex containing SecD and SecF. Major new insights have been gained into the cascade of preprotein targeting events and the enzymatic mechanism or preprotein translocation. It has become clear that preproteins are translocated in a stepwise fashion involving large nucleotide-induced conformational changes of the molecular motor SecA that propels the translocation reaction.