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V Koronakis

Publications and source records attributed to V Koronakis.

At least 37 records · Page 2Linked to original sources
PubMed1998

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function.

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

Acyl Carrier Protein↗
PubMed1997

RfaH and the ops element, components of a novel system controlling bacterial transcription elongation.

The RfaH protein controls the transcription of a specialized group of Escherichia coli and Salmonella operons that direct the synthesis, assembly and export of the lipopolysaccharide core, exopolysaccharide, F conjugation pilus and haemolysin toxin. RfaH is a specific regulator of transcript elongation; its loss increases transcription polarity in these operons without affecting initiation from the operon promoters. The operons of the RfaH-dependent regulon contain a short conserved 5' sequence, the ops element, deletion of which increases operon polarity to an extent similar to that caused by loss of RfaH. The ops element is also present upstream of polysaccharide gene clusters of Shigella flexneri, Yersinia enterocolitica, Vibrio cholerae and Klebsiella pneumoniae and the RP4 fertility operon of Pseudomonas aeruginosa, suggesting that this is a widely spread control system. The mechanistic coupling of RfaH and the ops element has been demonstrated in vitro and in vivo, and we suggest that the ops element recruits RfaH and potentially other factors to the RNA polymerase complex, modifying the complex to increase its processivity and allowing transcription to proceed over long distances.

Bacterial Proteins↗
PubMed1997

Structure of TolC, the outer membrane component of the bacterial type I efflux system, derived from two-dimensional crystals.

TolC is an outer membrane protein required for the export of virulence proteins and toxic compounds without a periplasmic intermediate. We show that TolC is an integral part of the translocator, interacting with inner membrane components, by demonstrating a need for TolC in protein export not only from intact cells but also from sphaeroplasts. To establish the structure of TolC, and thus gain information on how this might be achieved, the protein was purified from the Escherichia coli outer membrane, as a trimer, and crystallized in two-dimensional lattices by reconstitution in phospholipid bilayers. The projection structure at 12A resolution showed a threefold symmetric molecule of 58A outer diameter, and a single pool of stain filling its centre. Side views parallel to the membrane plane revealed an additional domain outside the membrane. Eighteen membrane-spanning beta-strands were predicted for the 51.5 kDa monomer, excluding a 7 kDa C-terminal segment, and this segment was shown to contain a proteinase K-sensitive site that was exposed in reconstituted membranes and sphaeroplasts, but which was protected in intact cells. The combined data suggest that TolC is a trimeric outer membrane protein with each monomer comprising a membrane domain, predicted to be beta-barrel, and a C-terminal periplasmic domain. The latter could form part of the bridge to the energized inner membrane component of the translocation complex.

Bacterial Outer Membrane Proteins↗
PubMed1996

Suppression of transcription polarity in the Escherichia coli haemolysin operon by a short upstream element shared by polysaccharide and DNA transfer determinants.

Expression of the Escherichia coli hlyCABD operon encoding synthesis, maturation and export of haemolysin toxin was strongly dependent upon a 35 bp DNA sequence, spanning the element GGCGGTAG, located 2 kbp upstream. When the hly operon was placed under the control of the inducible tac promoter, expression remained dependent upon this element, when transcribed in its native orientation 3' of the promoter. The increase in ptac-directed transcription was strongest for the distal, export genes of the hly operon, and was particularly striking when ptac and the element were placed far upstream. The element did not influence transcript stability, and we suggest that it is a key component of a novel regulatory mechanism may suppresses transcription polarity within operons. The mechanism that be of widespread importance in bacterial gene expression because the 8 bp element is present in many Gram-negative species as an upstream component of operons encoding the production of toxins and the surface assembly of polysaccharides and components required for the conjugal transfer of DNA. We name it the ops element for operon polarity suppressor.

Bacterial Proteins↗
PubMed1996

Increased distal gene transcription by the elongation factor RfaH, a specialized homologue of NusG.

The Escherichia coli RfaH protein is required for the expression of operons directing synthesis and export of the toxin haemolysin, the lipopolysaccharide core, and the F-factor sex pilus. Mutation of rfaH increases transcriptional polarity along all three operons. By demonstrating strong RfaH-dependent suppression of transcription polarity in vitro, we have established RfaH as a novel transcriptional activator, and we reveal that RfaH is a homologue of the essential protein NusG that modulates general transcriptional pausing and termination in prokaryotes. Full transcription of the distal genes from an upstream promoter required RfaH and the 5' cls-acting ops element, both in vivo and in vitro. In vivo the requirement for the ops element was suppressed by overexpressing RfaH, and in vitro the presence of ops lowered the concentration of RfaH required to stimulate transcript elongation. We suggest that RfaH directs transcript elongation in an analogous way to NusG, but does so in a subset of bacterial operons primarily engaged in the production of extracellular components required for virulence and fertility.

Amino Acid Sequence↗
PubMed1996

Independent interaction of the acyltransferase HlyC with two maturation domains of the Escherichia coli toxin HlyA.

The apparently unique fatty acylation mechanism that underlies activation (maturation) of Escherichia coli haemolysin and related toxins is further clarified by investigation of the interaction of protoxin with the specific acyltransferase HlyC. Using deleted protoxin variants and protoxin peptides as substrates in an in vitro maturation reaction dependent upon HlyC and acyl-acyl carrier protein, two independent HlyC recognition domains were identified on the 1024-residue protoxin, proA, and they were shown to span the two target lysine residues K564 (KI) and K690 (KII) that are fatty acylated. Each domain required 15-30 amino acids for basal recognition and 50-80 amino acids for wild-type acylation. The two domains (FAI and FAII) competed with each other in cis and in trans for HlyC. The affinity of FAI for HlyC is approximately four times greater than that of FAII resulting in an overall 80% acylation at KI and 20% acylation at KII in both whole toxin and peptide derivatives. No other proA sequences were required for toxin maturation, and excess Ca2+ prevented acylation of both lysines. The lack of primary sequence identity between FAI and FAII domains in proA and among corresponding sites on related protoxins currently precludes an explanation of the basis of HlyC recognition by proA.

Acylation↗
PubMed1995

Protein exporter function and in vitro ATPase activity are correlated in ABC-domain mutants of HlyB.

The Escherichia coli toxin exporter HlyB comprises an integral membrane domain fused to a cytoplasmic domain of the ATP-binding cassette (ABC) super-family, and it directs translocation of the 110kDa haemolysin protein out of the bacterial cell without using an N-terminal secretion signal peptide. We have exploited the ability to purify the soluble HlyB ABC domain as a fusion with glutathione S-transferase to obtain a direct correlation of the in vivo export of protein by HlyB with the degree of ATP binding and hydrolysis measured in vitro. Mutations in residues that are invariant or highly conserved in the ATP-binding fold and glycine-rich linker peptide of prokaryotic and eukaryotic ABC transporters caused a complete loss of both HlyB exporter function and ATPase activity in proteins still able to bind ATP effectively and undergo ATP-induced conformational change. Mutation of less-conserved residues caused reduced export and ATP hydrolysis, but not ATP binding, whereas substitutions of poorly conserved residues did not impair activity either in vivo or in vitro. The data show that protein export by HlyB has an absolute requirement for the hydrolysis of ATP bound by its cytoplasmic domain and indicate that comparable mutations that disable other prokaryotic and eukaryotic ABC transporters also cause a specific loss of enzymatic activity.

ATP-Binding Cassette Transporters↗
PubMed1994

Fatty acylation of two internal lysine residues required for the toxic activity of Escherichia coli hemolysin.

Hemolysin of Escherichia coli is activated by fatty acylation of the protoxin, directed by the putative acyl transferase HlyC and by acyl carrier protein (ACP). Mass spectrometry and Edman degradation of proteolytic products from mature toxin activated in vitro with tritium-labeled acylACP revealed two fatty-acylated internal lysine residues, lysine 564 and lysine 690. Resistance of the acylation to chemical treatments suggested that fatty acid was amide linked. Substitution of the two lysines confirmed that they were the only sites of acylation and showed that although each was acylated in the absence of the other, both sites were required for in vivo toxin activity.

Acyl Carrier Protein↗
PubMed1993

Bacterial signal peptide-independent protein export: HlyB-directed secretion of hemolysin.

Bacterial ABC transporter proteins are associated with a great number of cellular processes. While many of the best characterised examples are involved in the import of small molecules into the cell, others mediate export of substrates, especially hydrophilic proteins. This article focuses on these bacterial ABC exporters, in particular the HlyB protein which is central to secretion of the 110 kDa hemolysin across both the cytoplasmic and outer membranes. This is an example of non-conventional protein export which does not require an N-terminal signal sequence or the cellular SecA-dependent machinery.

Adenosine Triphosphatases↗
PubMed1993

ATPase activity and ATP/ADP-induced conformational change in the soluble domain of the bacterial protein translocator HlyB.

The haemolysin exporter HlyB and its homologues are central to the unconventional signal-peptide-independent secretion of toxins, proteases and nodulation proteins by bacteria. HlyB is a member of the ATP-binding cassette (ABC) or traffic ATPase superfamily, and resembles closely in structure and function mammalian exporters such as the multidrug-resistance P-glycoprotein, combining both integral membrane and cytosolic domains. Overproduction of the HlyB cytoplasmic domain as a C-terminal peptide fused to glutathione S-transferase allowed the direct affinity purification and concentration of 30-50 mg ml-1 of soluble protein (GST-Bctp) in an apparently dimeric form possessing both transferase and ATPase activity. GST-Bctp bound to ADP-agarose and was eluted specifically by ATP and ADP, affinity behaviour which was confirmed in both the full-length HlyB and the unfused HlyB cytoplasmic domain synthesized in vitro. The stoichiometry of binding to MgATP and MgADP was close to equimolar and both ligands induced substantial conformational change in the protein. Mg(2+)-dependent ATPase activity of GST-Bctp (Vmax 1 mumol min-1 mg-1, Km 0.2 mM) was comparable with the activity of the bacterial importer MalK and human P-glycoprotein reconstituted into proteoliposomes, and over an order of magnitude higher than in vitro measurements of disaggregated MalK purified from inclusion bodies. Activity was unaffected by inhibitors of F- and V-type ATPases, non-hydrolysable ATP analogues, or translocation substrate, but was severely inhibited by inhibitors of E1E2 (P-type) ATPases, and the acidic phospholipid phosphatidyl glycerol.

Adenosine Diphosphate↗
PubMed1992

E. coli hemolysin interactions with prokaryotic and eukaryotic cell membranes.

The hemolysin toxin (HlyA) is secreted across both the cytoplasmic and outer membranes of pathogenic Escherichia coli and forms membrane pores in cells of the host immune system, causing cell dysfunction and death. The processes underlying the interaction of HlyA with the bacterial and mammalian cell membranes are remarkable. Secretion of HlyA occurs without a periplasmic intermediate and is directed by an uncleaved C-terminal targetting signal and the HlyB and HlyD translocator proteins, the former being a member of a transporter superfamily central to import and export of a wide range of substrates by prokaryotic and eukaryotic cells. The separate process by which HlyA is targetted to mammalian cell membranes is dependent upon fatty acylation of a non-toxic precursor, proHlyA. This is achieved by a novel mechanism directed by the activator protein HlyC, which binds to an internal proHlyA recognition sequence and provides specificity for the transfer of fatty acid from cellular acyl carrier protein.

Acylation↗
PubMed1992

Escherichia coli HlyT protein, a transcriptional activator of haemolysin synthesis and secretion, is encoded by the rfaH (sfrB) locus required for expression of sex factor and lipopolysaccharide genes.

Synthesis and secretion of the 110kDa haemolysin toxin of Escherichia coli and other pathogenic Gram-negative bacteria are governed by the four genes of the hly operon. We have identified, by transposon mutagenesis, an E. coli cellular locus, hlyT, required for the synthesis and secretion of haemolysin encoded in trans by intact hly operons carrying the hly upstream regulatory region. Mutation of the hlyT locus specifically reduced the level of hlyA structural gene transcript 20-100-fold and thus markedly lowered both intracellular and extracellular levels of the HlyA protein. Genetic and structural analysis of the hlyT locus mapped it at co-ordinate 3680 kbp (minute 87) on the chromosome adjacent to the fadBA operon, and identified it specifically as the rfaH (sfrB) locus which is required for transcription of the genes encoding synthesis of the sex pilus and also the lipopolysaccharide core for attachment of the O-antigen of E. coli and Salmonella. Expression of the hly operon in the E. coli hlyT mutant was restored in trans by both the hlyT and rfaH genes, suggesting that the rfaH gene is an important activator of regulon structures that are central to the fertility and virulence of these pathogenic bacteria. DNA sequencing of the hlyT locus identifies the HlyT/RfaH transcriptional activator as a protein of 162 amino acids (Mr 18325) which shows no identity to characterized transcription factors.

Amino Acid Sequence↗
PubMed1992

Activation of Escherichia coli prohemolysin to the membrane-targetted toxin by HlyC-directed ACP-dependent fatty acylation.

Hemolysin (HlyA) and related toxins of Escherichia coli and other Gram-negative pathogenic bacteria form membrane pores in cells of the host immune system, causing cell dysfunction and death. An insight into the mechanism by which HlyA is targetted to mammalian cell membranes was achieved by establishing in vitro activation of the non-toxic precursor proHlyA. By this approach we have discovered that conversion of proHlyA to the post-translational active HlyA toxin is determined by fatty acylation of proHlyA in an apparently novel process directed by the HlyC homodimer activator protein, and dependent upon the cellular acyl carrier protein (ACP). By further exploiting the in vitro activation system it is now possible to obtain direct evidence that HlyC binds to an internal recognition sequence in the proHlyA precursor, in this way providing specificity for the transfer to proHlyA of a fatty acid moiety carried by the ACP. It is possible that the fatty acid modification determines directly the binding of HlyA to mammalian membrane lipids, thus initiating the toxin interaction with the target cells.

Acyl Carrier Protein↗
PubMed1992

The HlyB/HlyD-dependent secretion of toxins by gram-negative bacteria.

Hemolysin (HlyA) and related toxins are secreted across both the cytoplasmic and outer membranes of Escherichia coli and other pathogenic Gram-negative bacteria in a remarkable process which proceeds without a periplasmic intermediate. It is directed by an uncleaved C-terminal targetting signal and the HlyD and HlyB translocator proteins, the latter of which are members of a transporter superfamily central to import and export of a wide range of substrates by prokaryotic and eukaryotic cells. Our mutational analyses of the HlyA targetting signal and definition for the first time of stages and intermediates in the HlyB/HlyD-dependent translocation allow a discussion of the hemolysin export process in the wider context of protein translocation.

Adenosine Triphosphate↗
PubMed1991

Activation of Escherichia coli prohaemolysin to the mature toxin by acyl carrier protein-dependent fatty acylation.

Haemolysin secreted by pathogenic Escherichia coli binds to mammalian cell membranes, disrupting cellular activities and lysing cells by pore-formation. It is synthesized as nontoxic prohaemolysin (proHlyA), which is activated intracellularly by a mechanism dependent on the cosynthesized HlyC. Haemolysin is one of a family of membrane-targeted toxins, including the leukotoxins of Pasteurella and Actinobacillus and the bifunctional adenylate cyclase haemolysin of Bordetella pertussis, which require this protoxin activation 1-5. HlyC alone cannot activate proHlyA, but requires a cytosolic activating factor6. Here we report the cytosolic activating factor is identical to the acyl carrier protein and that activation to mature toxin is achieved by the transfer of a fatty acyl group from acyl carrier protein to proHlyA. Only acyl carrier protein, not acyl-CoA, can promote HlyC-directed proHlyA acylation, but a range of acyl groups are effective.

Acyl Carrier Protein↗
PubMed1991

Energetically distinct early and late stages of HlyB/HlyD-dependent secretion across both Escherichia coli membranes.

The alternative secretion pathway which exports hemolysin across both Escherichia coli membranes into the surrounding medium is directed by an uncleaved C-terminal targeting signal and the membrane translocator proteins HlyD and HlyB. In order to identify stages and intermediates in this unconventional secretion process we have examined the effect of inhibition of the total proton motive force (delta P) and its components during the in vivo HlyB/HlyD-dependent export of a 22.4 kDa secretion competent HlyA C-terminal peptide (Actp). Secretion of Actp was severely inhibited by the proton ionophore carbonylcyanide m-chlorophenylhydrazone (CCCP), which collapses simultaneously membrane potential delta psi and the proton gradient delta pH, and also by valinomycin/K+, a potassium ionophore which disrupts delta psi. The inhibition of secretion by valinomycin/K+ was ameliorated by imposition of a pH gradient, the second component of the delta P, and selective depletion of delta pH by nigericin also blocked secretion. This indicates that, as in the secretion of beta-lactamase to the periplasm, HlyB/D-directed secretion requires delta P itself and not specifically one of its components. However, inhibition of HlyB/D-dependent secretion was only marked when CCCP, valinomycin/K+ or nigericin were present during the early stage of Actp secretion; at a later stage the secretion was not significantly inhibited. HlyB/D-dependent secretion appears therefore to share with conventional secretion across the cytoplasmic membrane an early requirement for delta P, but comprises in addition a late stage which does not require delta P, delta psi or delta pH. The translocation intermediate identified in the delta P-independent late stage of secretion was associated with the membrane fraction. Analysis of the protease accessibility of this intermediate in whole cells and spheroplasts showed that it was not in the periplasm, nor was it exposed on the cell surface or on the periplasmic faces of either the inner or outer membranes. This may reflect its close association with the inner membrane or a membrane translocation complex.

Amino Acid Sequence↗