Product of a new gene, syd, functionally interacts with SecY when overproduced in Escherichia coli

Translation arrest cancellation of VemP, a secretion monitor in Vibrio, is regulated by multiple cis and trans factors, including SecY

VemP is a secretory protein in the Vibrio species that monitors cellular protein-transport activity through its translation arrest, allowing expression of the downstream secD2-secF2 genes in the same operon, which encode components of the protein translocation machinery. When cellular protein-transport function is fully active, secD2/F2 expression remains repressed as VemP translation arrest is canceled immediately. The VemP arrest cancellation occurs on the SecY/E/G translocon in a late stage in the translocation process and requires both trans factors, SecD/F and PpiD/YfgM, and a cis element, Arg-85 in VemP; however, the detailed molecular mechanism remains elusive. This study aimed to elucidate how VemP passing through SecY specifically monitors SecD/F function. Genetic and biochemical studies showed that SecY is involved in the VemP arrest cancellation and that the arrested VemP is stably associated with a specific site in the protein-conducting pore of SecY. VemP-Bla reporter analyses revealed that a short hydrophobic segment adjacent to Arg-85 plays a critical role in the regulated arrest cancellation with its hydrophobicity correlating with the stability of the VemP arrest. We identified Gln-65 and Pro-67 in VemP as novel elements important for the regulation. We propose a model for the regulation of the VemP arrest cancellation by multiple cis elements and trans factors with different roles.

Cryo-EM structure of the entire FtsH-HflKC AAA protease complex

The membrane-bound AAA protease FtsH is the key player controlling protein quality in bacteria. Two single-pass membrane proteins, HflK and HflC, interact with FtsH to modulate its proteolytic activity. Here, we present structure of the entire FtsH-HflKC complex, comprising 12 copies of both HflK and HflC, all of which interact reciprocally to form a cage, as well as four FtsH hexamers with periplasmic domains and transmembrane helices enclosed inside the cage and cytoplasmic domains situated at the base of the cage. FtsH K61/D62/S63 in the β2-β3 loop in the periplasmic domain directly interact with HflK, contributing to complex formation. Pull-down and in vivo enzymatic activity assays validate the importance of the interacting interface for FtsH-HflKC complex formation. Structural comparison with the substrate-bound human m-AAA protease AFG3L2 offers implications for the HflKC cage in modulating substrate access to FtsH. Together, our findings provide a better understanding of FtsH-type AAA protease holoenzyme assembly and regulation.

The Dynamic SecYEG Translocon

The spatial and temporal coordination of protein transport is an essential cornerstone of the bacterial adaptation to different environmental conditions. By adjusting the protein composition of extra-cytosolic compartments, like the inner and outer membranes or the periplasmic space, protein transport mechanisms help shaping protein homeostasis in response to various metabolic cues. The universally conserved SecYEG translocon acts at the center of bacterial protein transport and mediates the translocation of newly synthesized proteins into and across the cytoplasmic membrane. The ability of the SecYEG translocon to transport an enormous variety of different substrates is in part determined by its ability to interact with multiple targeting factors, chaperones and accessory proteins. These interactions are crucial for the assisted passage of newly synthesized proteins from the cytosol into the different bacterial compartments. In this review, we summarize the current knowledge about SecYEG-mediated protein transport, primarily in the model organism Escherichia coli, and describe the dynamic interaction of the SecYEG translocon with its multiple partner proteins. We furthermore highlight how protein transport is regulated and explore recent developments in using the SecYEG translocon as an antimicrobial target.

Reversible autoinhibitory regulation of Escherichia coli metallopeptidase BepA for selective β-barrel protein degradation

Escherichia coli periplasmic zinc-metallopeptidase BepA normally functions by promoting maturation of LptD, a β-barrel outer-membrane protein involved in biogenesis of lipopolysaccharides, but degrades it when its membrane assembly is hampered. These processes should be properly regulated to ensure normal biogenesis of LptD. The underlying mechanism of regulation, however, remains to be elucidated. A recently solved BepA structure has revealed unique features: In particular, the active site is buried in the protease domain and conceivably inaccessible for substrate degradation. Additionally, the His-246 residue in the loop region containing helix α9 (α9/H246 loop), which has potential flexibility and covers the active site, coordinates the zinc ion as the fourth ligand to exclude a catalytic water molecule, thereby suggesting that the crystal structure of BepA represents a latent form. To examine the roles of the α9/H246 loop in the regulation of BepA activity, we constructed BepA mutants with a His-246 mutation or a deletion of the α9/H246 loop and analyzed their activities in vivo and in vitro. These mutants exhibited an elevated protease activity and, unlike the wild-type BepA, degraded LptD that is in the normal assembly pathway. In contrast, tethering of the α9/H246 loop repressed the LptD degradation, which suggests that the flexibility of this loop is important to the exhibition of protease activity. Based on these results, we propose that the α9/H246 loop undergoes a reversible structural change that enables His-246-mediated switching (histidine switch) of its protease activity, which is important for regulated degradation of stalled/misassembled LptD.

Identification and characterization of a translation arrest motif in VemP by systematic mutational analysis

VemP ( Vibrio protein export monitoring polypeptide) is a secretory protein comprising 159 amino acid residues, which functions as a secretion monitor in Vibrio and regulates expression of the downstream V.secDF2 genes. When VemP export is compromised, its translation specifically undergoes elongation arrest at the position where the Gln¹⁵⁶ codon of vemP encounters the P-site in the translating ribosome, resulting in up-regulation of V.SecDF2 production. Although our previous study suggests that many residues in a highly conserved C-terminal 20-residue region of VemP contribute to its elongation arrest, the exact role of each residue remains unclear. Here, we constructed a reporter system to easily and exactly monitor the in vivo arrest efficiency of VemP. Using this reporter system, we systematically performed a mutational analysis of the 20 residues (His¹³⁸-Phe¹⁵⁷) to identify and characterize the arrest motif. Our results show that 15 residues in the conserved region participate in elongation arrest and that multiple interactions between important residues in VemP and in the interior of the exit tunnel contribute to the elongation arrest of VemP. The arrangement of these important residues induced by specific secondary structures in the ribosomal tunnel is critical for the arrest. Pro scanning analysis of the preceding segment (Met¹²⁰-Phe¹³⁷) revealed a minor role of this region in the arrest. Considering these results, we conclude that the arrest motif in VemP is mainly composed of the highly conserved multiple residues in the C-terminal region.

The TPR domain of BepA is required for productive interaction with substrate proteins and the β-barrel assembly machinery complex

BepA (formerly YfgC) is an Escherichia coli periplasmic protein consisting of an N-terminal protease domain and a C-terminal tetratricopeptide repeat (TPR) domain. We have previously shown that BepA is a dual functional protein with chaperone-like and proteolytic activities involved in membrane assembly and proteolytic quality control of LptD, a major component of the outer membrane lipopolysaccharide translocon. Intriguingly, BepA can associate with the BAM complex: the β-barrel assembly machinery (BAM) driving integration of β-barrel proteins into the outer membrane. However, the molecular mechanism of BepA function and its association with the BAM complex remains unclear. Here, we determined the crystal structure of the BepA TPR domain, which revealed the presence of two subdomains formed by four TPR motifs. Systematic site-directed in vivo photo-cross-linking was used to map the protein-protein interactions mediated by the BepA TPR domain, showing that this domain interacts both with a substrate and with the BAM complex. Mutational analysis indicated that these interactions are important for the BepA functions. These results suggest that the TPR domain plays critical roles in BepA functions through interactions both with substrates and with the BAM complex. Our findings provide insights into the mechanism of biogenesis and quality control of the outer membrane.

Nascent chain-monitored remodeling of the Sec machinery for salinity adaptation of marine bacteria

SecDF interacts with the SecYEG translocon in bacteria and enhances protein export in a proton-motive-force-dependent manner. Vibrio alginolyticus, a marine-estuarine bacterium, contains two SecDF paralogs, V.SecDF1 and V.SecDF2. Here, we show that the export-enhancing function of V.SecDF1 requires Na+ instead of H+, whereas V.SecDF2 is Na+-independent, presumably requiring H+. In accord with the cation-preference difference, V.SecDF2 was only expressed under limited Na+ concentrations whereas V.SecDF1 was constitutive. However, it is not the decreased concentration of Na+ per se that the bacterium senses to up-regulate the V.SecDF2 expression, because marked up-regulation of the V.SecDF2 synthesis was observed irrespective of Na+ concentrations under certain genetic/physiological conditions: (i) when the secDF1VA gene was deleted and (ii) whenever the Sec export machinery was inhibited. VemP (Vibrio export monitoring polypeptide), a secretory polypeptide encoded by the upstream ORF of secDF2VA, plays the primary role in this regulation by undergoing regulated translational elongation arrest, which leads to unfolding of the Shine-Dalgarno sequence for translation of secDF2VA. Genetic analysis of V. alginolyticus established that the VemP-mediated regulation of SecDF2 is essential for the survival of this marine bacterium in low-salinity environments. These results reveal that a class of marine bacteria exploits nascent-chain ribosome interactions to optimize their protein export pathways to propagate efficiently under different ionic environments that they face in their life cycles.

Activation of Toxin-Antitoxin System Toxins Suppresses Lethality Caused by the Loss of σE in Escherichia coli

σ(E), an alternative σ factor that governs a major signaling pathway in envelope stress responses in Gram-negative bacteria, is essential for growth of Escherichia coli not only under stressful conditions, such as elevated temperature, but also under normal laboratory conditions. A mutational inactivation of the hicB gene has been reported to suppress the lethality caused by the loss of σ(E). hicB encodes the antitoxin of the HicA-HicB toxin-antitoxin (TA) system; overexpression of the HicA toxin, which exhibits mRNA interferase activity, causes cleavage of mRNAs and an arrest of cell growth, while simultaneous expression of HicB neutralizes the toxic effects of overproduced HicA. To date, however, how the loss of HicB rescues the cell lethality in the absence of σ(E) and, more specifically, whether HicA is involved in this process remain unknown. Here we showed that simultaneous disruption of hicA abolished suppression of the σ(E) essentiality in the absence of hicB, while ectopic expression of wild-type HicA, but not that of its mutant forms without mRNA interferase activity, restored the suppression. Furthermore, HicA and two other mRNA interferase toxins, HigB and YafQ, suppressed the σ(E) essentiality even in the presence of chromosomally encoded cognate antitoxins when these toxins were overexpressed individually. Interestingly, when the growth media were supplemented with low levels of antibiotics that are known to activate toxins, E. coli cells with no suppressor mutations grew independently of σ(E). Taken together, our results indicate that the activation of TA system toxins can suppress the σ(E) essentiality and affect the extracytoplasmic stress responses.

IMPORTANCE

σ(E) is an alternative σ factor involved in extracytoplasmic stress responses. Unlike other alternative σ factors, σ(E) is indispensable for the survival of E. coli even under unstressed conditions, although the exact reason for its essentiality remains unknown. Toxin-antitoxin (TA) systems are widely distributed in prokaryotes and are composed of two adjacent genes, encoding a toxin that exerts harmful effects on the toxin-producing bacterium itself and an antitoxin that neutralizes the cognate toxin. Curiously, it is known that inactivation of an antitoxin rescues the σ(E) essentiality, suggesting a connection between TA systems and σ(E) function. We demonstrate here that toxin activation is necessary for this rescue and suggest the possible involvement of TA systems in extracytoplasmic stress responses.

Two copies of the SecY channel and acidic lipids are necessary to activate the SecA translocation ATPase

The SecA ATPase associates with the SecY complex to push preproteins across the bacterial membrane. Because a single SecY is sufficient to create the conducting channel, the function of SecY oligomerization remains unclear. Here, we have analyzed the translocation reaction using nanodiscs. We show that one SecY copy is sufficient to bind SecA and the preprotein, but only the SecY dimer together with acidic lipids supports the activation of the SecA translocation ATPase. In discs, the dimer is predominantly arranged in a back-to-back manner and remains active even if a constituent SecY copy is defective for SecA binding. In membrane vesicles and in intact cells, the coproduction of two inactive SecYs, one for channel gating and the other for SecA binding, recreates a functional translocation unit. These results indisputably argue that the SecY dimer is crucial for the activation of SecA, which is necessary for preprotein transport.

Post-liberation cleavage of signal peptides is catalyzed by the site-2 protease (S2P) in bacteria

A signal peptide (SP) is cleaved off from presecretory proteins by signal peptidase during or immediately after insertion into the membrane. In metazoan cells, the cleaved SP then receives proteolysis by signal peptide peptidase, an intramembrane-cleaving protease (I-CLiP). However, bacteria lack any signal peptide peptidase member I-CLiP, and little is known about the metabolic fate of bacterial SPs. Here we show that Escherichia coli RseP, an site-2 protease (S2P) family I-CLiP, introduces a cleavage into SPs after their signal peptidase-mediated liberation from preproteins. A Bacillus subtilis S2P protease, RasP, is also shown to be involved in SP cleavage. These results uncover a physiological role of bacterial S2P proteases and update the basic knowledge about the fate of signal peptides in bacterial cells.

A novel immunity system for bacterial nucleic acid degrading toxins and its recruitment in various eukaryotic and DNA viral systems

The use of nucleases as toxins for defense, offense or addiction of selfish elements is widely encountered across all life forms. Using sensitive sequence profile analysis methods, we characterize a novel superfamily (the SUKH superfamily) that unites a diverse group of proteins including Smi1/Knr4, PGs2, FBXO3, SKIP16, Syd, herpesviral US22, IRS1 and TRS1, and their bacterial homologs. Using contextual analysis we present evidence that the bacterial members of this superfamily are potential immunity proteins for a variety of toxin systems that also include the recently characterized contact-dependent inhibition (CDI) systems of proteobacteria. By analyzing the toxin proteins encoded in the neighborhood of the SUKH superfamily we predict that they possess domains belonging to diverse nuclease and nucleic acid deaminase families. These include at least eight distinct types of DNases belonging to HNH/EndoVII- and restriction endonuclease-fold, and RNases of the EndoU-like and colicin E3-like cytotoxic RNases-folds. The N-terminal domains of these toxins indicate that they are extruded by several distinct secretory mechanisms such as the two-partner system (shared with the CDI systems) in proteobacteria, ESAT-6/WXG-like ATP-dependent secretory systems in Gram-positive bacteria and the conventional Sec-dependent system in several bacterial lineages. The hedgehog-intein domain might also release a subset of toxic nuclease domains through auto-proteolytic action. Unlike classical colicin-like nuclease toxins, the overwhelming majority of toxin systems with the SUKH superfamily is chromosomally encoded and appears to have diversified through a recombination process combining different C-terminal nuclease domains to N-terminal secretion-related domains. Across the bacterial superkingdom these systems might participate in discriminating `self’ or kin from `non-self’ or non-kin strains. Using structural analysis we demonstrate that the SUKH domain possesses a versatile scaffold that can be used to bind a wide range of protein partners. In eukaryotes it appears to have been recruited as an adaptor to regulate modification of proteins by ubiquitination or polyglutamylation. Similarly, another widespread immunity protein from these toxin systems, namely the suppressor of fused (SuFu) superfamily has been recruited for comparable roles in eukaryotes. In animal DNA viruses, such as herpesviruses, poxviruses, iridoviruses and adenoviruses, the ability of the SUKH domain to bind diverse targets has been deployed to counter diverse anti-viral responses by interacting with specific host proteins.

Reconstitution of the SecY translocon in nanodiscs

Secretory proteins are transported across the bacterial envelope using a membrane protein complex called the SecY channel or translocon. Major advances in understanding this transporter have been accomplished with methods including purification, crystallization, and reconstitution of the translocation reaction in vitro. We here describe the incorporation of the SecY complex into supported nanometer scale lipid bilayers called Nanodiscs. These nanoparticles mimic a membrane environment and circumvent many of the technical problems typically observed with liposomes and detergent micelles. The technology is simple, yet should lead to additional new progresses in the field of membrane protein transport.

Structure, binding, and activity of Syd, a SecY-interacting protein

The Syd protein has been implicated in the Sec-dependent transport of polypeptides across the bacterial inner membrane. Using Nanodiscs, we here provide direct evidence that Syd binds the SecY complex, and we demonstrate that interaction involves the two electropositive and cytosolic loops of the SecY subunit. We solve the crystal structure of Syd and together with cysteine cross-link analysis, we show that a conserved concave and electronegative groove constitutes the SecY-binding site. At the membrane, Syd decreases the activity of the translocon containing loosely associated SecY-SecE subunits, whereas in detergent solution Syd disrupts the SecYEG heterotrimeric associations. These results support the role of Syd in proofreading the SecY complex biogenesis and point to the electrostatic nature of the Sec channel interaction with its cytosolic partners.

SecY alterations that impair membrane protein folding and generate a membrane stress

We report on a class of Escherichia coli SecY mutants that impair membrane protein folding. The mutants also up-regulate the Cpx/σ^E stress response pathways. Similar stress induction was also observed in response to a YidC defect in membrane protein biogenesis but not in response to the signal recognition particle–targeting defect or in response to a simple reduction in the abundance of the translocon. Together with the previous contention that the Cpx system senses a protein abnormality not only at periplasmic and outer membrane locations but also at the plasma membrane, abnormal states of membrane proteins are postulated to be generated in these secY mutants. In support of this notion, in vitro translation, membrane integration, and folding of LacY reveal that mutant membrane vesicles allow the insertion of LacY but not subsequent folding into a normal conformation recognizable by conformation-specific antibodies. The results demonstrate that normal SecY function is required for the folding of membrane proteins after their insertion into the translocon.

Proteolytic activity of HtpX, a membrane-bound and stress-controlled protease from Escherichia coli

Escherichia coli HtpX is a putative membrane-bound zinc metalloprotease that has been suggested to participate in the proteolytic quality control of membrane proteins in conjunction with FtsH, a membrane-bound and ATP-dependent protease. Here, we biochemically characterized HtpX and confirmed its proteolytic activities against membrane and soluble proteins. HtpX underwent self-degradation upon cell disruption or membrane solubilization. Consequently, we purified HtpX under denaturing conditions and then refolded it in the presence of a zinc chelator. When supplemented with Zn2+, the purified enzyme exhibited self-cleavage activity. In the presence of zinc, it also degraded casein and cleaved a solubilized membrane protein, SecY. We verified its ability to cleave SecY in vivo by overproducing both HtpX and SecY. These results showed that HtpX is a zinc-dependent endoprotease member of the membrane-localized proteolytic system in E. coli.

Peculiar properties of DsbA in its export across the Escherichia coli cytoplasmic membrane

Export of DsbA, a protein disulfide bond-introducing enzyme, across the Escherichia coli cytoplasmic membrane was studied with special reference to the effects of various mutations affecting translocation factors. It was noted that both the internalized precursor retaining the signal peptide and the periplasmic mature product fold rapidly into a protease-resistant structure and they exhibited anomalies in sodium dodecyl sulfate-polyacrylamide gel electrophoresis in that the former migrated faster than the latter. The precursor, once accumulated, was not exported posttranslationally. DsbA export depended on the SecY translocon, the SecA ATPase, and Ffh (signal recognition particle), but not on SecB. SecY mutations, such as secY39 and secY205, that severely impair translocation of a number of secretory substrates by interfering with SecA actions only insignificantly impaired the DsbA export. In contrast, secY125, affecting a periplasmic domain and impairing a late step of translocation, exerted strong export inhibition of both classes of proteins. These results suggest that DsbA uses not only the signal recognition particle targeting pathway but also a special route of translocation through the translocon, which is hence suggested to actively discriminate pre-proteins.

SecM facilitates translocase function of SecA by localizing its biosynthesis

"Arrest sequence" of Escherichia coli SecM interacts with the ribosomal exit tunnel and arrests its own translation elongation, which is released by cotranslational export of the nascent SecM chain. This property of SecM is essential for the basal and regulated expression of SecA. Here we report that SecM has an additional role of facilitating SecA activities. Systematic determinations of the SecA-abundance-protein export relationships of cells with different SecA contents revealed that SecA was less functional when SecM was absent from the upstream region of the secM-secA message, when SecM had the arrest-defective mutation, and also when SecM lacked the signal sequence. These results suggest that cotranslational targeting of nascent SecM to the translocon plays previously unrecognized roles of facilitating the formation of functional SecA molecules. Biosynthesis in the vicinity of the membrane and the Sec translocon will be beneficial for this multiconformation ATPase to adopt ready-to-function conformations.

Translation arrest of SecM is essential for the basal and regulated expression of SecA

The SecM protein of Escherichia coli contains an arrest sequence (F(150)XXXXWIXXXXGIRAGP(166)), which interacts with the ribosomal exit tunnel to halt translation elongation beyond Pro-166. This inhibition is reversed by active export of the nascent SecM chain. Here, we studied the physiological roles of SecM. Arrest-alleviating mutations in the arrest sequence reduced the expression of secA, a downstream gene on the same mRNA. Among such mutations, the arrest-abolishing P166A substitution mutation on the chromosomal secM gene proved lethal unless the mutant cells are complemented with excess SecA. Whereas secretion defect due either to azide addition, a secY mutation, or low temperature leads to up-regulated SecA biosynthesis, this regulation was lost by a secM mutation, which synergistically retarded growth of cells with lowered secretion activity. Finally, an arrest-alleviating rRNA mutation affecting the constricted part of the exit tunnel lowered the basal level of SecA as well as its secretion defect-induced up-regulation. Thus, the arrest sequence of SecM has at least two roles in SecA translation. First, the transient elongation arrest in normal cells is required for the synthesis of SecA at levels sufficient to support cell growth. Second, the prolonged SecM elongation arrest under conditions of unfavorable protein secretion is required for the enhanced expression of SecA to cope with such conditions.

Role of YidC in folding of polytopic membrane proteins

YidC of Echerichia coli, a member of the conserved Alb3/Oxa1/YidC family, is postulated to be important for biogenesis of membrane proteins. Here, we use as a model the lactose permease (LacY), a membrane transport protein with a known three-dimensional structure, to determine whether YidC plays a role in polytopic membrane protein insertion and/or folding. Experiments in vivo and with an in vitro transcription/translation/insertion system demonstrate that YidC is not necessary for insertion per se, but plays an important role in folding of LacY. By using the in vitro system and two monoclonal antibodies directed against conformational epitopes, LacY is shown to bind the antibodies poorly in YidC-depleted membranes. Moreover, LacY also folds improperly in proteoliposomes prepared without YidC. However, when the proteoliposomes are supplemented with purified YidC, LacY folds correctly. The results indicate that YidC plays a primary role in folding of LacY into its final tertiary conformation via an interaction that likely occurs transiently during insertion into the lipid phase of the membrane.

YaeL proteolysis of RseA is controlled by the PDZ domain of YaeL and a Gln-rich region of RseA

sigmaE is an alternative sigma factor involved in a pathway of extracytoplasmic stress responses in Escherichia coli. Under normal growth conditions, sigmaE activity is down-regulated by the membrane-bound anti-sigmaE protein, RseA. Extracytoplasmic stress signals induce degradation of RseA by two successive proteolytic events: DegS-catalyzed first cleavage at a periplasmic site followed by YaeL-mediated second proteolysis at an intramembrane region. Normally, the second reaction (site-2 proteolysis) only occurs after the first cleavage (site-1 cleavage). Here, we show that YaeL variants with the periplasmic PDZ domain deleted or mutated allows unregulated cleavage of RseA and consequent sigmaE activation. It was also found that a glutamine-rich region in the periplasmic domain of RseA was required for the avoidance of the YaeL-mediated proteolysis in the absence of site-1 cleavage. These results indicate that multiple negative elements both in the enzyme (PDZ domain) and in the substrate (glutamine-rich region) determine the strict dependence of the site-2 proteolysis on the site-1 cleavage.

Importance of transmembrane segments in Escherichia coli SecY

To assess the functional importance of the transmembrane regions of SecY, we constructed a series of SecY variants, in which the six central residues of each transmembrane segment were replaced by amino acid residues from either transmembrane segment 3 or 4 of LacY. The SecY function, as assessed by the ability to complement cold-sensitive secYmutants with respect to their growth and translocase defects, was eliminated by the alterations in transmembrane segments 2, 3, 4, 7, 9 and 10. Among them, those in segments 3 and 4 had especially severe effects. In contrast, transmembrane segments 1, 5, 6, and 8 were more tolerant to the sequence alterations. The purified protein with an altered transmembrane segment 6 retained, in large measure, the ability to support SecA-dependent preprotein translocation in vitro. These results will help us to further understand how the SecYEG protein translocation channel functions.

Interfering mutations provide in vivo evidence that Escherichia coli SecE functions in multimeric states

SecY, SecE and SecG form a heterotrimer, which functions as a protein translocation channel in Escherichia coli. The cytosolic loop of SecE contains a segment that is conserved among different organisms. Here we show that mutational alterations in this segment not only inactivate the SecE function but confer dominant interfering properties on the altered SecE molecule. Such effects were especially evident in mutant cells in which the requirement for SecE function was increased. Overproduction of SecE, but not of SecY, alleviated the dominant negative effects. These results suggest that the inactive SecE molecule sequesters wild-type SecE. It was also found that an amino acid substitution, D112P, in the C-terminal periplasmic region intragenically suppressed the dominant interference. These results are consistent with a notion that there is significant SecE-SecE interaction in vivo, in which the C-terminal region has an important role. The data hence suggest that dimeric SecE participates in the formation of the functional translocation channel.

A SecE mutation that modulates SecY-SecE translocase assembly, identified as a specific suppressor of SecY defects

The SecY39(Cs) (cold-sensitive) alteration of Arg357 results in a defect of translocation initiation. As a means to dissect the Sec translocation machinery, we isolated mutations that act as suppressors of the secY39 defect. A specific secE mutation, designated secE105, was thus isolated. This mutation proved to be identical with the prlG2 mutation and to suppress a number of cold-sensitive secY mutations. However, other prlG mutations did not effectively suppress the secY defects. Evidence indicates that the Ser105-to-Pro alteration in the C-terminal transmembrane segment of SecE weakens SecY-SecE association. In vitro analyses showed that the SecE(S105P) alteration preferentially stimulates the initial phase of translocation. It is suggested that the S105P alteration affects the SecYEG channel such that it is more prone to open and to accept the translocation initiation domain of a preprotein molecule.

Superactive SecY variants that fulfill the essential translocation function with a reduced cellular quantity

The fifth and the sixth cytoplasmic regions (C5 and C6) of SecY are important for the SecA-driven preprotein translocation reaction. A cold-sensitive mutation, secY205 (Tyr-429 --> Asp), in C6 impairs the ATP- and precursor-dependent SecA insertion into the membrane. We now identified second site mutations that suppressed the defect. Cis-placement of these mutations proved to suppress mutations at another essential residue (Arg-357) of SecY as well. Thus, they tolerate the otherwise defective SecY alterations in the same molecule. Two alterations (Ile-195 to Ser in TM5 region and Ile-408 to Leu in TM10 region) were found to make the translocation channel more active, because it enabled cells to survive with reduced content of the SecYE complex. These mutations only very weakly suppressed a signal sequence defect of the lambda receptor protein. The mutant SecYEG translocase exhibited higher than normal activity in vitro, being accompanied by striking independence of the proton motive force as well as by stabilization of a bound and active SecA species against urea treatment. These results have been interpreted in terms of balance shifts between channel closing and channel opening alterations in the SecYEG translocase.

YaeL (EcfE) activates the sigma(E) pathway of stress response through a site-2 cleavage of anti-sigma(E), RseA

Escherichia coli YaeL (EcfE) is a homolog of human site-2 protease (S2P), a membrane-bound zinc metalloprotease involved in regulated intramembrane proteolysis. We have shown previously that YaeL, having essential metalloprotease active site motifs in the cytoplasmic domain, is indispensable for viability. Here, we obtained rpoE, encoding an extracytoplasmic stress response sigma factor (sigma(E)), as a multicopy suppressor against the yaeL disruption. Whereas sigma(E) is thought to be activated by regulated cleavage of RseA on the periplasmic side by the DegS protease, we found that a degradation intermediate of RseA consisting of the transmembrane and the cytoplasmic domains accumulated in the YaeL-depleted cells. This intermediate was degraded on expression of YaeL but not of its metalloprotease motif mutants. Cells depleted of YaeL were incapable of activating a sigma(E)-dependent promoter in response to an envelope stress. It is suggested that sigma(E) activation involves two successive proteolytic cleavages: first, at a periplasmic site by DegS; second, at a cytoplasmic or intramembrane site by YaeL. Thus, YaeL is positively required for the sigma(E) extracytoplasmic stress response.

Roles of the C-terminal end of SecY in protein translocation and viability of Escherichia coli

SecY, a central component of the membrane-embedded sector of protein translocase, contains six cytosolic domains. Here, we examined the importance of the C-terminal cytosolic region of SecY by systematically shortening the C-terminal end and examining the functional consequences of these mutations in vivo and in vitro. It was indicated that the C-terminal five residues are dispensable without any appreciable functional defects in SecY. Mutants missing the C-terminal six to seven residues were partially compromised, especially at low temperature or in the absence of SecG. In vitro analyses indicated that the initial phase of the translocation reaction, in which the signal sequence region of the preprotein is inserted into the membrane, was affected by the lack of the C-terminal residues. SecA binding was normal, but SecA insertion in response to ATP and a preprotein was impaired. It is suggested that the C-terminal SecY residues are required for SecA-dependent translocation initiation.

The ribosomal exit tunnel functions as a discriminating gate

Translation of SecM stalls unless its N-terminal part is "pulled" by the protein export machinery. Here we show that the sequence motif FXXXXWIXXXXGIRAGP that includes a specific arrest point (Pro) causes elongation arrest within the ribosome. Mutations that bypass the elongation arrest were isolated in 23S rRNA and L22 r protein. Such suppressor mutations occurred at a few specific residues of these components, which all face the narrowest constriction of the ribosomal exit tunnel. Thus, we suggest that this region of the exit tunnel interacts with nascent translation products and functions as a discriminating gate.

Identification of glutamic acid 479 as the gluzincin coordinator of zinc in FtsH (HflB)

Escherichia coli FtsH (HflB) is a membrane-bound and ATP-dependent metalloprotease. Its cytoplasmic domain contains a zinc-binding motif, H(417)EXXH, whose histidine residues have been shown to be functionally important. Although they are believed to be involved directly in zinc coordination, nothing is known about the third zinc ligand of this protease. Sequence alignment indicates that glutamic acid residues are conserved among the FtsH homologues at positions corresponding to Glu(479) and Glu(585) of E. coli FtsH. We replaced each of them by Gln, Asp, Lys, or Val. Mutations at position 479 compromised the proteolytic functions of FtsH in vivo. In vitro proteolytic activities of the E479Q, E479V, and E479D mutant enzymes were much lower than that of the wild-type protein and were significantly stimulated by a high concentration of zinc ion. These mutant proteins retained the wild-type levels of ATPase activities, and their trypsin susceptibilities as well as CD spectra were essentially indistinguishable from those of the wild-type protein, indicating that the mutations did not cause gross conformational changes in FtsH. They exhibited reduced zinc contents upon purification. From these results, we conclude that Glu(479) is a zinc-coordinating residue.

Characterization of the yaeL gene product and its S2P-protease motifs in Escherichia coli

An Escherichia coli open reading frame, yaeL, encodes a predicted homolog of human site-2 protease (S2P), a putative membrane-bound zinc metalloproteinase involved in the proteolytic activation of regulatory factors for sterol biosynthesis and for stress responses. The potential importance of YaeL in processes analogous to the regulated intramembrane proteolysis in E. coli prompted us to characterize it. Cell fractionation and alkaline phosphatase fusion experiments established that YaeL has four transmembrane segments with both termini orienting toward the periplasm. A strain in which a chromosomal disruption of yaeL was combined with arabinose promoter-controlled yaeL on a plasmid enabled us to deplete this protein from the cell. The depletion was found to cause rapid loss of viability, cell elongation and growth cessation. Mutations affecting the HEXXH metalloproteinase motif and those affecting the LDG motif, conserved among S2Ps, abolished the ability of YaeL to support cell growth. These results indicate that YaeL is indispensable in E. coli, and probably functions as a metalloproteinase at the membrane.

Roles of homooligomerization and membrane association in ATPase and proteolytic activities of FtsH in vitro

Escherichia coli FtsH is a membrane-bound and ATP-dependent protease which degrades some soluble and integral membrane proteins. The N-terminal region of FtsH mediates membrane association as well as homooligomeric interaction of this enzyme. Previously, we studied in vivo functionality of FtsH derivatives, in which the N-terminal membrane region was either deleted (FtsH(DeltaTM)), replaced by a leucine zipper (Zip-FtsH(DeltaTM)), or replaced by a lactose permease transmembrane segment (LacY-FtsH). It was indicated that homooligomerization is required for the minimum proteolytic activity, whereas a transmembrane sequence is required for membrane protein degradation. Here we characterized these proteins in vitro. Although these mutant enzymes were very low in their activities, they were significantly stimulated by dimethyl sulfoxide, which enabled us to characterize their activities. LacY-FtsH degraded both soluble and membrane proteins, but Zip-FtsH(DeltaTM) only degraded soluble proteins. These proteins also exhibited significant ATPase activities. However, FtsH(DeltaTM) remained inactive both in ATPase and in protease activities even in the presence of dimethyl sulfoxide. The monomeric FtsH(DeltaTM) was able to bind ATP and a denatured protein. These results indicate that subunit association is important for the enzymatic catalysis by FtsH and that the additional presence of the transmembrane sequence is required for this enzyme to degrade a membrane protein even under detergent-solubilized conditions.

Secretion monitor, SecM, undergoes self-translation arrest in the cytosol

The product of the Escherichia coli secM gene (secretion monitor, formerly gene X), upstream of secA, is involved in secretion-responsive control of SecA translation. In wild-type cells, SecM is rapidly degraded by the periplasmic tail-specific protease. It is also subject to a transient translation pause at a position close to the C terminus. The elongation arrest was strikingly prolonged when translocation of SecM was impaired. SRP was not required for this arrest. Instead, the nascent SecM product itself may participate, as the arrest was diminished when it incorporated a proline analog, azetidine. We propose that cytosolically localized nascent SecM undergoes self-translation arrest, thereby enhancing translation of secA through an altered secondary structure of the secM-secA messenger RNA.

Roles of multimerization and membrane association in the proteolytic functions of FtsH (HflB)

FtsH (HflB) is an Escherichia coli ATP-dependent protease that degrades some integral membrane and cytoplasmic proteins. While anchored to the cytoplasmic membrane by the two transmembrane (TM) segments near the N-terminus, it has a large cytoplasmic domain. The N-terminal region also has a role in homo-oligomerization of this protein. To study the significance of the membrane integration and oligomer formation, we constructed FtsH derivatives in which the N-terminal region had been deleted or replaced with either the leucine zipper sequence from Saccharomyces cerevisiae GCN4 protein or TM regions from other membrane proteins. The cytoplasmic domain, which was monomeric and virtually inactive, was converted, by the attachment of the leucine zipper, to an oligomer with proteolytic function against a soluble, but not a membrane-bound substrate. In contrast, chimeric TM-FtsH proteins were active against both substrate classes. We suggest that the cytoplasmic domain has intrinsic but weak self-interaction ability, which becomes effective with the aid of the leucine zipper or membrane tethering, and that membrane association is essential for FtsH to degrade integral membrane proteins.

Length recognition at the N-terminal tail for the initiation of FtsH-mediated proteolysis

FtsH-mediated proteolysis against membrane proteins is processive, and presumably involves dislocation of the substrate into the cytosol where the enzymatic domains of FtsH reside. To study how such a mode of proteolysis is initiated, we manipulated N-terminal cytosolic tails of three membrane proteins. YccA, a natural substrate of FtsH was found to require the N-terminal tail of 20 amino acid residues or longer to be degraded by FtsH in vivo. Three unrelated sequences of this segment conferred the FtsH sensitivity to YccA. An artificially constructed TM9-PhoA protein, derived from SecY, as well as the SecE protein, were sensitized to FtsH by addition of extra amino acid sequences to their N-terminal cytosolic tails. Thus, FtsH recognizes a cytosolic region of sufficient length (approximately 20 amino acids) to initiate the processive proteolysis against membrane proteins. Such a region is typically at the N-terminus and can be diverse in amino acid sequences.

The plasmid F OmpP protease, a homologue of OmpT, as a potential obstacle to E. coli-based protein production

OmpT, an outer membrane-localized protease of Escherichia coli, cleaves a number of exogenous and endogenous proteins during their purification. SecY, an endogenous membrane protein, is a target of this artificial proteolysis in vitro. Here we report that SecY cleavage occurs even in cell extracts from ompT-disrupted cells, if they carry an F plasmid derivative. A gene, ompP, on the F plasmid was shown to be responsible for this proteolysis. These results indicate that the absence of an F-like plasmid should be checked when choosing a host strain for E. coli-based protein production.

Self-processing of FtsH and its implication for the cleavage specificity of this protease

FtsH, a membrane-bound and ATP-dependent protease of Escherichia coli, is involved in degradation of some of uncomplexed integral membrane proteins and short-lived cytoplasmic proteins. It is composed of an N-terminal membrane-spanning region and a following large cytoplasmic domain that contains ATPase and protease active sites. In the present study, it was found that FtsH undergoes C-terminal processing in vivo. The processing was blocked by loss of function mutations of FtsH. Purified FtsH-His(6)-Myc, a C-terminally tagged derivative of FtsH, was self-processed in vitro. This in vitro processing was observed only in the presence of ATP and not in the presence of adenosine 5'-(beta,gamma-imino)triphosphate (AMP-PNP). Moreover, such processing did not occur in the case of the ATPase motif mutant protein. These results indicated that this processing is a self-catalyzed reaction that needs ATP hydrolysis. Mutations in the hflKC genes that encode a possible modulator of FtsH, and the growth phase of the cells as well, affected the processing. Complementation experiments with genetically constructed variants suggested that both the processed and the unprocessed forms of FtsH are functional. The cleavage was found to occur between Met-640 and Ser-641, removing a heptapeptide from the C-terminus of FtsH. Systematic mutational analyses of Met-640 and Ser-641 revealed preferences for positively charged and hydrophobic amino acid residues at these positions for processing. This cleavage specificity may be shared by the self-cleavage and the substrate-cleavage reactions of this protease.

Dislocation of membrane proteins in FtsH-mediated proteolysis

Escherichia coli FtsH degrades several integral membrane proteins, including YccA, having seven transmembrane segments, a cytosolic N-terminus and a periplasmic C-terminus. Evidence indicates that FtsH initiates proteolysis at the N-terminal cytosolic domain. SecY, having 10 transmembrane segments, is also a substrate of FtsH. We studied whether and how the FtsH-catalyzed proteolysis on the cytosolic side continues into the transmembrane and periplasmic regions using chimeric proteins, YccA-(P3)-PhoA-His6-Myc and SecY-(P5)-PhoA, with the alkaline phosphatase (PhoA) mature sequence in a periplasmic domain. The PhoA domain that was present within the fusion protein was rapidly degraded by FtsH when it lacked the DsbA-dependent folding. In contrast, both PhoA itself and the TM9-PhoA region of SecY-(P5)-PhoA were stable when expressed as independent polypeptides. In the presence of DsbA, the FtsH-dependent degradation stopped at a site near to the N-terminus of the PhoA moiety, leaving the PhoA domain (and its C-terminal region) undigested. The efficiency of this degradation stop correlated well with the rapidity of the folding of the PhoA domain. Thus, both transmembrane and periplasmic domains are degraded by the processive proteolysis by FtsH, provided they are not tightly folded. We propose that FtsH dislocates the extracytoplasmic domain of a substrate, probably using its ATPase activity.

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.

Global regulation by the small RNA-binding protein CsrA and the non-coding RNA molecule CsrB

Csr (carbon storage regulator) is a recently discovered global regulatory system that controls bacterial gene expression post-transcriptionally. Its effector is a small RNA-binding protein referred to as CsrA or, in phytopathogenic Erwinia species, RsmA (repressor of stationary phase metabolites). Numerous genes whose expression occurs in the stationary phase of growth are repressed by csrA/rsmA, and csrA activates certain exponential-phase metabolic pathways. Glycogen synthesis and catabolism, gluconeogenesis, glycolysis, motility, cell surface properties and adherence are modulated by csrA in Escherichia coli, while the production of several secreted virulence factors, the plant hypersensitive response elicitor HrpN(Ecc) and, potentially, other secondary metabolites are regulated by rsmA in Erwinia carotovora. CsrA represses glycogen synthesis by binding to and destabilizing glgCAP mRNA and is hypothesized to repress other genes by a similar mechanism. The second component of the Csr system is CsrB (AepH in Erwinia species), a non-coding RNA molecule that forms a large globular ribonucleoprotein complex with approximately 18 CsrA subunits and antagonizes the effects of CsrA in vivo. Highly repeated sequence elements found within the loops of predicted stem-loops and other single-stranded segments of CsrB RNA may facilitate CsrA binding. Current information supports a model in which CsrA exists in an equilibrium between CsrB and CsrA-regulated mRNAs, which predicts that CsrB levels may be a key determinant of CsrA activity in the cell. The presence of csrA homologues in phylogenetically diverse species further suggests that this novel kind of regulatory system is likely to play a broad role in modulating eubacterial gene expression.

Roles of the periplasmic domain of Escherichia coli FtsH (HflB) in protein interactions and activity modulation

FtsH is a membrane-bound and ATP-dependent protease of Escherichia coli, known to degrade SecY, a membrane protein for protein translocation, and CII, a soluble transcription factor for lysis/lysogeny decision of phage lambda. FtsH forms a homo-oligomeric complex as well as a hetero-oligomeric complex with HflKC, a putative modulator of FtsH. Although FtsH has a small periplasmic region, HflKC is mostly exposed to the periplasmic space. We studied the roles of the periplasmic region of FtsH by engineering mutations in this protein. FtsHDelta236, lacking most of the periplasmic region, retained the in vivo ability to degrade SecY but not CII, resulting in high frequency lysogenization of lambda. Several insertion mutations in the periplasmic region of FtsH also differentially affected the proteolytic activities of FtsH. Interestingly, purified and detergent-solubilized FtsHDelta236 was as active as the wild-type enzyme in degrading SecY and CII, although its ATPase activity was lowered 5-fold. Affinity chromatography using histidine-tagged derivatives showed that the periplasmic domain-deleted FtsH no longer interacted with FtsH or HflKC. Although FtsHDelta236-His6-Myc lost the static FtsH-FtsH interaction, it retained the ability to change its conformation in an ATP-dependent manner at 37 degreesC, leading to a limited oligomerization. These results suggest that the periplasmic region of FtsH has crucial roles in the protein-protein interactions of this complex and in the modulation of its proteolytic functions against different substrates.

Syd, a SecY-interacting protein, excludes SecA from the SecYE complex with an altered SecY24 subunit

Syd is an Escherichia coli cytosolic protein that interacts with SecY. Overproduction of this protein causes a number of protein translocation-related phenotypes, including the strong toxicity against the secY24 mutant cells. Previously, this mutation was shown to impair the interaction between SecY and SecE, the two fundamental subunits of the membrane-embedded part of protein translocase. We have now studied in vitro the mechanisms of the Syd-directed inhibition of protein translocation. Pro-OmpA translocation into inverted membrane vesicles (IMVs) prepared from the secY24 mutant cells as well as the accompanied translocation ATPase activity of SecA were rapidly inhibited by purified Syd protein. In the course of protein translocation, high affinity binding of preprotein-bearing SecA to the translocase on the IMV is followed by ATP-driven insertion of the 30-kDa SecA segment into the membrane. Our experiments using 125I-labeled SecA and the secY24 mutant IMV showed that Syd abolished both the high affinity SecA binding and the SecA insertion. Syd was even able to release the inserted form of SecA that had been stabilized by a nonhydrolyzable ATP analog. Syd affected markedly the proteolytic digestion pattern of the IMV-integrated SecY24 protein, suggesting that Syd exerts its inhibitory effect by interacting directly with the SecY24 protein. In accordance with this notion, a SecY24 variant with a second site mutation (secY249) resisted the Syd action both in vivo and in vitro. Thus, Syd acts against the SecY24 form of translocase, in which SecY-SecE interaction has been compromised, to exclude the SecA motor protein from the SecYE channel complex.

Different pathways for protein degradation by the FtsH/HflKC membrane-embedded protease complex: an implication from the interference by a mutant form of a new substrate protein, YccA

Escherichia coli FtsH (HflB) is a membrane-bound and ATP-dependent zinc-metalloproteinase, which forms a complex with a pair of periplasmically exposed membrane proteins, HflK and HflC. It is the protease that degrades uncomplexed forms of the SecY subunit of protein translocase. Here, we characterized a new class of SecY-stabilizing mutation on the E. coli chromosome. The mutation (yccA11) is an internal deletion within a gene (yccA) known as an open reading frame for a hydrophobic protein with putative seven transmembrane segments. The YccA protein was found to be degraded in an FtsH-dependent manner in vivo and in vitro, whereas the YccA11 mutant protein, lacking eight amino acid residues within the amino-terminal cytoplasmic domain, was refractory to the degradation. The yccA11 mutation exhibited partial dominance when overexpressed. Cross-linking, co-immunoprecipitation, and histidine tagging experiments showed that YccA11 as well as YccA can associate with both the FtsH and the HflKC proteins. Thus, the mutant YccA protein appeared to compete with SecY for recognition by the FtsH proteolytic system and the residues deleted by the yccA mutation are required for the initiation of proteolysis by FtsH. Interestingly, the inhibitory action of YccA11 was mediated by HflKC, since the deletion of hflK-hflC suppressed the yccA11 phenotype. The yccA11 mutation stabilized subunit a of the proton ATPase F0 segment as well, but not the CII protein of bacteriophage lambda or the sigma 32 protein. From these results we suggest that there are at least two pathways for FtsH-dependent protein degradation, only one of which (probably for membrane proteins) is subject to the HflKC-dependent interference by the YccA11 mutant substrate.

Polypeptide binding of Escherichia coli FtsH (HflB)

The Escherichia coli FtsH protein is a membrane-bound and ATP-dependent protease. In this study, we describe ATP-dependent conformational changes in FtsH as well as a polypeptide binding ability of this protein. A 33 kDa segment of FtsH became trypsin resistant in the presence of ATP. ATP and ATPgammaS prevented self-aggregation of detergent-solubilized FtsH-His6-Myc at 37 degrees C, again suggesting that the binding of ATP induces a conformational change in FtsH. Affinity chromatography showed that FtsH-His6-Myc can associate with denatured alkaline phosphatase (PhoA) but not with the native enzyme. Denatured PhoA also prevented the aggregation of FtsH, and these two proteins co-sedimented through a sucrose gradient. Binding between FtsH-His6-Myc and detergent-solubilized SecY was also demonstrated. Although FtsH-bound SecY was processed further for ATP-dependent proteolysis, FtsH-bound PhoA was not. Thus, FtsH association with denatured PhoA is uncoupled from proteolysis. Overproduction of FtsH significantly increased the cytoplasmic localization of the PhoA moiety of a MalF-PhoA hybrid protein, in which a charged residue had been introduced into a transmembrane segment. Thus, denatured PhoA binding of FtsH may also occur in vivo.

In vitro analysis of the stop-transfer process during translocation across the cytoplasmic membrane of Escherichia coli

In this study, using a derivative of proOmpA containing an artificial stop-transfer sequence (proOmpA2xH1), we analyzed the process of stop-transfer during translocation across the cytoplasmic membrane of Escherichia coli. ProOmpA2xH1 did not interfere with the transit of wild-type proOmpA. When proOmpA2xH1 was anchored in the membrane, membrane-inserted SecA was deinserted with the reversion of the inverted topology of SecG. Cross-linking experiments revealed that the anchored proOmpA2xH1 that does not interact with either SecY or SecA. These results, taken together, suggest that proOmpA2xH1 leaves the translocation pathway by means of a specific interaction between the stop-transfer sequence and the translocational channel.

The RNA molecule CsrB binds to the global regulatory protein CsrA and antagonizes its activity in Escherichia coli

The RNA-binding protein CsrA (carbon storage regulator) is a new kind of global regulator, which facilitates specific mRNA decay. A recombinant CsrA protein containing a metal-binding affinity tag (CsrA-H6) was purified to homogeneity and authenticated by N-terminal sequencing, matrix-assisted laser desorption/ionization time of flight mass spectrometry, and other studies. This protein was entirely contained within a globular complex of approximately 18 CsrA-H6 subunits and a single approximately 350-nucleotide RNA, CsrB. cDNA cloning and nucleotide sequencing revealed that the csrB gene is located downstream from syd in the 64-min region of the Escherichia coli K-12 genome and contains no open reading frames. The purified CsrA-CsrB ribonucleoprotein complex was active in regulating glg (glycogen biosynthesis) gene expression in vitro, as was the RNA-free form of the CsrA protein. Overexpression of csrB enhanced glycogen accumulation in E. coli, a stationary phase process that is repressed by CsrA. Thus, CsrB RNA is a second component of the Csr system, which binds to CsrA and antagonizes its effects on gene expression. A model for regulatory interactions in Csr is presented, which also explains previous observations on the homologous system in Erwinia carotovora. A highly repeated nucleotide sequence located within predicted stem-loops and other single-stranded regions of CsrB, CAGGA(U/A/C)G, is a plausible CsrA-binding element.

SecE-depleted membranes of Escherichia coli are active. SecE is not obligatorily required for the in vitro translocation of certain protein precursors

Membrane vesicles were prepared from Escherichia coli cells in which SecE was depleted to 2% of wild-type membranes. SecE depletion had pleiotropic effects; SecD, SecF, SecG, and SecY were decreased 4-6-fold, whereas SecA was increased about 16-fold over that of wild-type membranes. These membranes were substantially active in the in vitro translocation of proOmpA, which was mediated by the SecA pathway since it was inhibited by azide. Similar substantial translocation activities were observed for proLamB and proLpp in the SecE-depleted membranes. However, the translocation of proPhoA was more severely impaired. These data indicate that SecE may enhance but is not obligatorily required for the translocation of at least certain precursors, and suggest that the effects of the SecE depletion on protein translocation may be precursor-dependent.

Subunit interactions in the Escherichia coli protein translocase: SecE and SecG associate independently with SecY

We used hexahistidine-tagged SecE and SecY to study how the core subunits (SecY, SecE and SecG) of Escherichia coli protein translocase interact with each other. Detergent extracts were prepared from the plasma membranes and fractionated by Ni2+-NTA agarose affinity binding. Although His6-SecE, expressed in wild-type cells, brought down both SecY and SecG, neither of them was brought down when the same protein was expressed in the secY24 mutant cells. His6-SecY brought down both SecE and SecG, as expected. Interestingly, His6-SecY24 was able to bring down SecG but not SecE. These results confirm our previous conclusion that the secY24 alteration impairs the SecY-SecE interaction, and demonstrate that SecY and SecG can form a complex that does not contain SecE. Likewise, SecY-SecE complex could be isolated from the secG-deleted strain. The trimeric complex, in detergent extracts, dissociated at a critical temperature between 23 and 26 degrees C, whereas the SecY-SecE complex without SecG dissociated at a slightly lower temperature (20-23 degrees C). We conclude that each of SecE and SecG independently binds to SecY, the central subunit of protein translocase, although the trimeric complex is more stable than the binary complexes.

Differential in vivo roles played by DsbA and DsbC in the formation of protein disulfide bonds

Several Escherichia coli proteins participate in protein disulfide bond formation. Among them, DsbA is the primary factor that oxidizes target cysteines. Biochemical evidence indicates that DsbC has disulfide isomerization activity. To study intracellular functions of DsbA and DsbC, we used an alkaline phosphatase mutant, PhoA[SCCC], with the most amino-terminal cysteine replaced by serine. It was found that the remaining 3 cysteines in PhoA[SCCC] form a disulfide bond of incorrect as well as correct combinations. An aberrant disulfide bond was preferentially formed in wild-type cells, which was converted slowly to the normal disulfide bond. This conversion did not occur in the dsbC-disrupted cells. Overproduction of DsbC stimulated the formation of the correct disulfide bond. In contrast, the inefficiently formed disulfide bonds in the dsbA-disrupted cells, and the more efficiently formed disulfide bonds in the same strain in the presence of oxidized glutathione were mostly in the correct form. These results suggest that the DsbA-catalyzed reaction can be too rapid for some proteins. DsbA may simply oxidize available pairs of cysteines, which happen to be in an incorrect combination in the case of PhoA[SCCC]. In contrast, DsbC stimulates the formation of correct disulfide bonds and corrects previously introduced aberrant ones. Thus, DsbC acts to isomerize disulfide bonds in vivo.

Roles of disulfide bonds in bacterial alkaline phosphatase

Alkaline phosphatase of Escherichia coli (a homodimeric protein found in the periplasmic space) contains two intramolecular disulfide bonds (Cys-168-Cys-178 and Cys-286-Cys-336) that are formed after export to the periplasmic space. The location-specific folding character of this enzyme allowed its wide usage as a reporter of protein localization in prokaryotic cells. To study the roles of disulfide bonds in alkaline phosphatase, we eliminated each of them by Cys to Ser mutations. Intracellular stability of alkaline phosphatase decreased in the absence of either one or both of the disulfide bonds. The mutant proteins were stabilized in a DegP protease-deficient strain, allowing accumulation at significant levels and subsequent characterization. A mutant protein that lacked the N-terminally located disulfide bond (Cys-168-Cys-178) was found to have Cys-286 and Cys-336 residues disulfide-bonded, to have a dimeric structure, and to have almost full enzymatic activity. Nevertheless, the mutant protein lost the trypsin-resistant conformation that is characteristically observed for the wild-type enzyme. In contrast, mutants lacking Cys-286 and Cys-336 were monomeric and inactive. These results indicate that the Cys-286-Cys-336 disulfide bond is required and is sufficient for correctly positioning the active site region of this enzyme, but such an active conformation is still insufficient for the conformational stability of the enzyme. Thus, a fully active state of this enzyme can be formed without full protein stability, and the two disulfide bonds differentially contribute to these properties.