The effectiveness of national influenza vaccination policies for at-risk populations over 5 seasons in a Scottish general practice

AIM

To determine the effectiveness of serial influenza vaccination.

SCOPE

Studied in a Scottish GP population, the overall seroresponse rate increased with annual influenza vaccinations and after 5 years it increased from 45.1% to 93.3% for influenza virus A (H1) and from 48.4% to 98.3% for influenza virus A (H3). However, there was little boosting effect with further doses after becoming a seroresponder. The pre-vaccination titres were significantly higher in previous year's seroresponders compared to non-responders.

CONCLUSIONS

The policy of annual vaccination is supported by our data in order to increase the disappointing response rate after one dose. However, the lack of a boosting response with subsequent doses and the significant residual immunity after becoming a seroresponder suggests a prior serological immunity check in order to better direct the vaccine supply (in the years of no antigenic drift), to those who need it most.

Physical structure of lakes constrains epidemics in Daphnia populations

Parasites are integral parts of most ecosystems, yet attention has only recently focused on how community structure and abiotic factors impact host-parasite interactions. In lakes, both factors are influenced by habitat morphology. To investigate the role of habitat structure in mediating parasitism in the plankton, we quantified timing and prevalence of a common microparasite (Metschnikowia bicuspidata) in its host, Daphnia dentifera, in 18 lakes that vary in basin size and shape. Over three years, we found substantial spatial and temporal variation in the severity of epidemics. Although infection rates reached as high as 50% in some lakes, they did not occur in most lakes in most years. Host density, often considered to be a key determinant of disease spread, did not explain a significant amount of variation in the occurrence of epidemics. Furthermore, host resistance does not fully explain this parasite's distribution, since we easily infected hosts in the laboratory. Rather, basin shape predicted epidemics well; epidemics occurred only in lakes with steep-sided basins. In these lakes, the magnitude of epidemics varied with year. We suggest that biological (predation) and physical (turbulence) effects of basin shape interact with annual weather patterns to determine the regional distribution of this parasite.

Donor strand complementation mechanism in the biogenesis of non-pilus systems

The F1 antigen of Yersinia pestis belongs to a class of non-pilus adhesins assembled via a classical chaperone-usher pathway. Such pathways consist of PapD-like chaperones that bind subunits and pilot them to the outer membrane usher, where they are assembled into surface structures. In a recombinant Escherichia coli model system, chaperone-subunit (Caf1M:Caf1n) complexes accumulate in the periplasm. Three independent methods showed that these complexes are rod- or coil-shaped linear arrays of Caf1 subunits capped at one end by a single copy of Caf1M chaperone. Deletion and point mutagenesis identified an N-terminal donor strand region of Caf1 that was essential for polymerization in vitro, in the periplasm and at the cell surface, but not for chaperone-subunit interaction. Partial protease digestion of periplasmic complexes revealed that this region becomes buried upon formation of Caf1:Caf1 contacts. These results show that, despite the capsule-like appearance of F1 antigen, the basic structure is assembled as a linear array of subunits held together by intersubunit donor strand complementation. This example shows that strikingly different architectures can be achieved by the same general principle of donor strand complementation and suggests that a similar basic polymer organization will be shared by all surface structures assembled by classical chaperone-usher pathways.

Structural and functional similarity between Yersinia pestis capsular protein Caf1 and human interleukin-1 beta

A comparative study of the structural and functional properties of recombinant Yersinia pestis Caf1 and human IL-1beta was performed. According to Fourier transform infrared spectroscopy (FTIR) and circular dichroism (CD) data, IL-1beta and Caf1 are typical beta-structural proteins. Neither protein interacts with the hydrophobic probe ANS (8-anilino-1-naphthalenesulfonate) under physiological conditions. Specific binding of Caf1 [K(d) = (5.4 +/- 0.1) x 10(-10) M] to interleukin-1 receptors (IL-1Rs) on the surface of finite mouse fibroblasts (line NIH 3T3) was observed. Caf1 is able to inhibit high-affinity binding of (125)I-labeled IL-1beta to NIH 3T3 cells, and in the presence of Caf1, the binding of [(125)I]IL-1beta is characterized by a K(d) of (2.0 +/- 0.3) x 10(-9) M. Caf1 binding to IL-1R could reflect adhesive properties of the capsular subunits responsible for the contact of bacteria with the host immunocompetent cells. In its turn, this may represent a signal for the initiation of the expression and secretion of the proteins of Y. pestis Yop virulon. Thus, these results help to explain the importance of Caf1 in the interaction of Y. pestis with the host immune system.

Secretion of recombinant proteins via the chaperone/usher pathway in Escherichia coli

F1 antigen (Caf1) of Yersinia pestis is assembled via the Caf1M chaperone/Caf1A usher pathway. We investigated the ability of this assembly system to facilitate secretion of full-length heterologous proteins fused to the Caf1 subunit in Escherichia coli. Despite correct processing of a chimeric protein composed of a modified Caf1 signal peptide, mature human interleukin-1beta (hIL-1beta), and mature Caf1, the processed product (hIL-1beta:Caf1) remained insoluble. Coexpression of this chimera with a functional Caf1M chaperone led to the accumulation of soluble hIL-1beta:Caf1 in the periplasm. Soluble hIL-1beta:Caf1 reacted with monoclonal antibodies directed against structural epitopes of hIL-1beta. The results indicate that Caf1M-induced release of hIL-1beta:Caf1 from the inner membrane promotes folding of the hIL-1beta domain. Similar results were obtained with the fusion of Caf1 to hIL-1beta receptor antagonist or to human granulocyte-macrophage colony-stimulating factor. Following coexpression of the hIL-1beta:Caf1 precursor with both the Caf1M chaperone and Caf1A outer membrane protein, hIL-1beta:Caf1 could be detected on the cell surface of E. coli. These results demonstrate for the first time the potential application of the chaperone/usher secretion pathway in the transport of subunits with large heterogeneous N-terminal fusions. This represents a novel means for the delivery of correctly folded heterologous proteins to the periplasm and cell surface as either polymers or cleavable monomeric domains.

An extended hydrophobic interactive surface of Yersinia pestis Caf1M chaperone is essential for subunit binding and F1 capsule assembly

A single polypeptide subunit, Caf1, polymerizes to form a dense, poorly defined structure (F1 capsule) on the surface of Yersinia pestis. The caf-encoded assembly components belong to the chaperone-usher protein family involved in the assembly of composite adhesive pili, but the Caf1M chaperone itself belongs to a distinct subfamily. One unique feature of this subfamily is the possession of a long, variable sequence between the F1 beta-strand and the G1 subunit binding beta-strand (FGL; F1 beta-strand to G1 beta-strand long). Deletion and insertion mutations confirmed that the FGL sequence was not essential for folding of the protein but was absolutely essential for function. Site-specific mutagenesis of individual residues identified Val-126, in particular, together with Val-128 as critical residues for the formation of a stable subunit-chaperone complex and the promotion of surface assembly. Differential effects on periplasmic polymerization of the subunit were also observed with different mutants. Together with the G1 strand, the FGL sequence has the potential to form an interactive surface of five alternating hydrophobic residues on Caf1M chaperone as well as in seven of the 10 other members of the FGL subfamily. Mutation of the absolutely conserved Arg-20 to Ser led to drastic reduction in Caf1 binding and surface assembled polymer. Thus, although Caf1M-Caf1 subunit binding almost certainly involves the basic principle of donor strand complementation elucidated for the PapD-PapK complex, a key feature unique to the chaperones of this subfamily would appear to be capping via high-affinity binding of an extended hydrophobic surface on the respective single subunits.

Structural and functional significance of the FGL sequence of the periplasmic chaperone Caf1M of Yersinia pestis

The periplasmic molecular chaperone Caf1M of Yersinia pestis is a typical representative of a subfamily of specific chaperones involved in assembly of surface adhesins with a very simple structure. One characteristic feature of this Caf1M-like subfamily is possession of an extended, variable sequence (termed FGL) between the F1 and subunit binding G1 beta-strands. In contrast, FGS subfamily members, characterized by PapD, have a short F1-G1 loop and are involved in assembly of complex pili. To elucidate the structural and functional significance of the FGL sequence, a mutant Caf1M molecule (dCaf1M), in which the 27 amino acid residues between the F1 and G1 beta-strands had been deleted, was constructed. Expression of the mutated caf1M in Escherichia coli resulted in accumulation of high levels of dCaf1M. The far-UV circular dichroism spectra of the mutant and wild-type proteins were indistinguishable and exhibited practically the same temperature and pH dependencies. Thus, the FGL sequence of Caf1M clearly does not contribute significantly to the stability of the protein conformation. Preferential cleavage of Caf1M by trypsin at Lys-119 confirmed surface exposure of this part of the FGL sequence in the isolated chaperone and periplasmic chaperone-subunit complex. There was no evidence of surface-localized Caf1 subunit in the presence of the Caf1A outer membrane protein and dCaf1M. In contrast to Caf1M, dCaf1M was not able to form a stable complex with Caf1 nor could it protect the subunit from proteolytic degradation in vivo. This demonstration that the FGL sequence is required for stable chaperone-subunit interaction, but not for folding of a stable chaperone, provides a sound basis for future detailed molecular analyses of the FGL subfamily of chaperones.

Defined deletion mutants demonstrate that the major secreted toxins are not essential for the virulence of Aeromonas salmonicida

The importance of the two major extracellular enzymes of Aeromonas salmonicida, glycerophospholipid: cholesterol acyltransferase (GCAT) and a serine protease (AspA), to the pathology and mortality of salmonid fish with furunculosis had been indicated in toxicity studies. In this study, the genes encoding GCAT (satA) and AspA (aspA) have been cloned and mutagenized by marker replacement of internal deletions, and the constructs have been used for the creation of isogenic satA and aspA mutants of A. salmonicida. A pSUP202 derivative (pSUP202sac) carrying the sacRB genes was constructed to facilitate the selection of mutants. The requirement of serine protease for processing of pro-GCAT was demonstrated. Processing involved the removal of a short internal fragment. Surprisingly, pathogenicity trials revealed no major decrease in virulence of the A. salmonicida delta satA::kan or A. salmonicida delta aspA::kan mutants compared to the wild-type parent strains when Atlantic salmon (Salmo salar L.) were challenged by intraperitoneal injection. Moreover, using a cohabitation model, which more closely mimics the natural disease, there was also no significant decrease in the relative cumulative mortality following infection with either of the deletion mutants compared to the parent strain. Thus, although these two toxins may confer some competitive advantage to A. salmonicida, neither toxin is essential for the very high virulence of A. salmonicida in Atlantic salmon. This first report of defined deletion mutations within any proposed extracellular virulence factor of A. salmonicida raises crucial questions about the pathogenesis of this important fish pathogen.

Influence of the conserved disulphide bond, exposed to the putative binding pocket, on the structure and function of the immunoglobulin-like molecular chaperone Caf1M of Yersinia pestis

The Yersinia pestis protein Caf1M is a typical representative of a subfamily of periplasmic molecular chaperones with characteristic structural and functional features, one of which is the location of two conserved cysteine residues close to the putative binding pocket. We show that these residues form a disulphide bond, the reduction and alkylation of which significantly increases the dissociation constant of the Caf1M-Caf1 (where Caf 1 is a polypeptide subunit of the capsule) complex [from a Kd of (4.77+/-0.50)x10(-9) M for the intact protein to one of (3.68+/-0.68)x10(-8) M for the modified protein]. The importance of the disulphide bond for the formation of functional Caf1M in vivo was demonstrated using an Escherichia coli dsbA mutant carrying the Y. pestis f1 operon. In accordance with the CD and fluorescence measurements, the disulphide bond is not important for maintenance of the overall structure of the Caf1M molecule, but would appear to affect the fine structural properties of the subunit binding site. A three-dimensional model of the Caf1M-Caf1 complex was designed based on the published crystal structure of PapD (a chaperone required for Pap pili assembly) complexed with a peptide corresponding to the C-terminus of the papG subunit. In the model the disulphide bond is in close proximity to the invariant Caf1M Arg-23 and Lys-142 residues that are assumed to anchor the C-terminal group of the subunit. The importance of this characteristic disulphide bond for the orchestration of the binding site and subunit binding, as well as for the folding of the protein in vivo, is likely to be a common feature of this subfamily of Caf1M-like chaperones. A possible model for the role of the disulphide bond in Caf1 assembly is discussed.

The sexual behaviour of international travellers at two Glasgow GUM clinics. Glasgow genitourinary medicine

A survey of patients attending 2 Glasgow genitourinary medicine (GUM) clinics was conducted in 2 3-month periods in 1993 and 1994. Three hundred and twenty-five attendees who had travelled abroad in the preceding 3 months completed anonymous self-administered questionnaires about their sexual behaviour during these recent journeys abroad. There were 112 women and 213 men (185 heterosexuals and 28 homosexuals). Twenty-two (19.6%) women, 56 (31%) heterosexual men and 13 (42%) homosexual men had a sexual contact with a new partner while abroad. Of those who had had a new sexual contact abroad, 11 women (50% of those who had sex with a new partner) and 33 heterosexual men (59% of those who had sex with a new partner) were inconsistent users of condoms. Analysis of data found that homosexual and heterosexual men, and business travellers, are at increased risk of exposure to sexually transmitted diseases, including HIV infection, and should be targeted with safer sex health promotion prior to travel.

The Salmonella virulence plasmid enhances Salmonella-induced lysis of macrophages and influences inflammatory responses

The Salmonella dublin virulence plasmid mediates systemic infection in mice and cattle. Here, we analyze the interaction between wild-type and plasmid-cured Salmonella strains with phagocytes in vitro and in vivo. The intracellular recovery of S. dublin from murine peritoneal and bovine alveolar macrophages cultured in the presence of gentamicin in vitro was not related to virulence plasmid carriage. However, the virulence plasmid increased the lytic activity of S. dublin, Salmonella typhimurium, and Salmonella choleraesuis for resident or activated mouse peritoneal macrophages. Lysis was not mediated by spv genes and was abolished by cytochalasin D treatment. Peritoneal and splenic macrophages were isolated from mice 4 days after intraperitoneal infection with wild-type or plasmid-cured S. dublin strains. The wild-type strain was recovered in significantly higher numbers than the plasmid-cured strain. However, the intracellular killing rates of such cells cultured in vitro for both S. dublin strains were not significantly different. Four days after infection, there was a lower increase of phagocyte numbers in the peritoneal cavities and spleens of mice infected with the wild-type strain compared with the plasmid-cured strain. The virulence plasmid influenced the survival of macrophages in vitro following infection in vivo as assessed by microscopy. Cells from mice infected with the plasmid-cured strain survived better than those from mice infected with the wild-type strain. This is the first report demonstrating an effect of the virulence plasmid on the interaction of Salmonella strains with macrophages. Plasmid-mediated macrophage dysfunction could influence the recruitment and/or the activation of phagocytic cells and consequently the net growth of Salmonella strains during infection.

Study of the intergenic exeF-exeG region and its application as a simple preliminary test for Aeromonas spp

The exeF-exeG intergenic regions from different hybridization groups (HG) of Aeromonas were studied by PCR amplification using a single pair of primers. Six main classes of PCR products were identified according to size: 360 bp, 320 bp, 280 bp, 230-240 bp, 220 bp and 160 bp. Direct sequencing of the PCR products indicated that the shorter intergenic regions had probably originated from deletion of DNA segments between direct repeats. Correlation of certain PCR products with Aeromonas caviae (HG4), A. caviae (HG5), A. veronii (HG8) and A. salmonicida (HG3) was revealed. The PCR reaction was also shown to be generally specific for Aeromonas spp. Thus, the usefulness of this rapid, single colony-based PCR test for both identification and preliminary differentiation of Aeromonas spp. is demonstrated.

Aeromonas salmonicida possesses two genes encoding homologs of the major outer membrane protein, OmpA

Two homologs of the outer membrane protein OmpA were identified in Aeromonas salmonicida by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and amino-terminal sequence analyses. An A. salmonicida genomic DNA library was constructed by using lambda GEM-11 and recombinant phage carrying both genes ompAI and ompAII) selected by immunoscreening. A 5.0-kb BamHI fragment containing the two genes in tandem was subcloned in pBluescript and used for further subcloning and sequencing of the genes. The encoded proteins (Mr = 33,564 and 32,536 for mature OmpAI and OmpAII, respectively) had only 64% identity with each other and otherwise had the highest level of homology to OmpA proteins from the members of the family Enterobacteriaceae. Based on the Escherichia coli OmpA model, an eight-stranded amphipathic beta-barrel model for the membrane assembly of the N-terminal half of OmpAI and OmpAII was predicted. Most variation between the two proteins was localized to the predicted surface loops and periplasmic turns, while the transmembrane strands and C-terminals domains were highly conserved. Expression of ompAI and ompAII separately in E. coli indicated that both genes could be independently transcribed from their own promoters and that both gene products were assembled into the E. coli outer membrane. A survey of different Aeromonas spp. by PCR revealed that possession of two tandem ompA genes was widespread among this genus. This is the first report of any bacterial species possessing two genes for homologs of this major outer membrane protein.

The Salmonella dublin virulence plasmid does not modulate early T-cell responses in mice

The virulence plasmid in Salmonella dublin mediates systemic infection in mice and cattle. The role of gamma delta T cells or hepatic extrathymic T cells has recently been reported to be important in the control of the early stage of Salmonella choleraesuis infections of mice. Here, we report on T-cell responses in conventional mice after challenge with a virulent strain of S. dublin carrying a virulence plasmid or with a strain cured of the plasmid. Over a period of 4 days postinfection, when both strains could be compared, similar changes in alpha beta and gamma delta T-cell subsets in peritoneal cavities, livers, and spleens were recorded, demonstrating no clear role of the virulence plasmid in modulation of early T-cell responses. To investigate further the role of the virulence plasmid in pathogenesis, the growth of the plasmid-cured strain was assessed in SCID, SCID bg, and irradiated mice. During the first 6 days after infection, there was no statistically difference in the net growth of Salmonella cells in the livers and spleens of SCID and SCID bg mice compared with conventional BALB/mice. This observation excludes a key role for a T- or B-cell-mediated immune response in controlling the initial growth of the plasmid-cured S. dublin strain. Thereafter, the immunocompromised mice were no longer able to control infection, although SCID mice were more efficient at controlling net bacterial multiplication than SCID bg mice, potentially implicating NK cells in the control of infection in SCID mice. The early control of net bacterial multiplication in the spleens and livers of BALB/c mice was ablated by whole-body X-irradiation. Both wild-type and plasmid-cured strains multiplied significantly more rapidly in irradiated than in conventional BALB/c mice. However, the numbers of wild-type bacterial still increased more rapidly than in the numbers of the cured strains. These results are consistent with a role of the S. dublin virulence plasmid in promoting in vivo growth of Salmonella cells.

Specific high affinity binding of human interleukin 1 beta by Caf1A usher protein of Yersinia pestis

Understanding the interaction of Yersinia pestis with the key components of the immune system is important for elucidation of the pathogenesis of bubonic plague, one of the most severe and acute bacterial diseases. Here we report the specific, high affinity binding (Kd = 1.40 x 10(-10) M +/- 0.14 x 10(-10)) of radiolabelled human interleukin 1 beta (hIL-1 beta) to E. coli cells carrying the capsular f1 operon of Y. pestis. Caf1A outer membrane usher protein was isolated to greater than 98% purity. Competition studies with purified Caf1A, together with immunoblotting studies, identified Caf1A as the hIL-1 beta receptor. Competition between Caf1 subunit and hIL-1 beta for the same or an overlapping binding site on Caf1A was demonstrated. Relevance of these results to the pathogenesis of Y. pestis and other Gram negative bacterial pathogens with homologous outer membrane usher proteins is discussed.

Cloning and study of the genetic organization of the exe gene cluster of Aeromonas salmonicida

The Aeromonas salmonicida (As) exe gene cluster, an additional member of the pul-related operon family required for general signal-sequence-dependent secretion of proteins from Gram- bacteria, was cloned in the broad-host-range cosmid pLAFR3. Twelve genes, exeC-N, were identified by partial nucleotide (nt) sequence analyses (exeE-N) or determination of the complete sequence (exeC and exeD). The organisation of the exeC-N genes is similar to that of several other operons of this family. These genes are arranged contiguously and are apparently transcribed in the same direction. On alignment of As and A. hydrophila exe sequences a 73-bp 'silent' deletion was identified close to the end of the As exeF gene. No gene encoding prepilin peptidase (the PulO homolog) was detected in this region. The exeN gene is evidently the last gene of this operon; it is followed by an ORF encoding a putative transcription regulator.

Ozone depletion: ultraviolet radiation and phytoplankton biology in antarctic waters

The springtime stratospheric ozone (O3) layer over the Antarctic is thinning by as much as 50 percent, resulting in increased midultraviolet (UVB) radiation reaching the surface of the Southern Ocean. There is concern that phytoplankton communities confined to near-surface waters of the marginal ice zone will be harmed by increased UVB irradiance penetrating the ocean surface, thereby altering the dynamics of Antarctic marine ecosystems. Results from a 6-week cruise (Icecolors) in the marginal ice zone of the Bellingshausen Sea in austral spring of 1990 indicated that as the O3 layer thinned: (i) sea surface- and depth-dependent ratios of UVB irradiance (280 to 320 nanometers) to total irradiance (280 to 700 nanometers) increased and (ii) UVB inhibition of photosynthesis increased. These and other Icecolors findings suggest that O3-dependent shifts of in-water spectral irradiances alter the balance of spectrally dependent phytoplankton processes, including photoinhibition, photoreactivation, photoprotection, and photosynthesis. A minimum 6 to 12 percent reduction in primary production associated with O3 depletion was estimated for the duration of the cruise.

Requirement of the SecB chaperone for export of a non-secretory polypeptide in Escherichia coli

The SecB protein of Escherichia coli is a cytosolic component of the export machinery which can prevent some precursors from prematurely folding into export-incompatible conformations by binding to the newly synthesised polypeptide. The feature(s) of target proteins recognised by SecB, however, are unclear and have been a matter of controversy. Also, it has not been asked if binding of SecB is specific for secretory proteins. We demonstrate here that a non-secretory polypeptide, a fragment of a tail fiber protein of phage T4, fused to the signal peptide of the outer membrane protein OmpA has a very strong SecB requirement for export and that the signal peptide itself cannot, at least not alone, be responsible for this action of SecB. The data reported, together with those of the literature, suggest that SecB recognizes the polypeptide backbone of the target protein.

Export of altered forms of an Escherichia coli K-12 outer membrane protein (OmpA) can inhibit synthesis of unrelated outer membrane proteins

Expression of mutant ompA genes, encoding the 325 residue Escherichia coli outer membrane protein OmpA, caused an inhibition of synthesis of the structurally unrelated outer membrane porins OmpC and OmpF and of wild-type OmpA, but not of the periplasmic beta-lactamase. There was no accumulation of precursors of the target proteins and the inhibitory mechanism operated at the level of translation. So far only alterations around residue 45 of OmpA have been found to affect this phenomenon. Linkers were inserted between the codons for residues 45 and 46. A correlation between size and sequence of the resulting proteins and presence or absence of the inhibitory effect was not found, indicating that the added residues acted indirectly by altering the conformation of other parts of the mutant OmpA. To be effective, the altered polypeptides had to be channelled into the export pathway. Internal deletions in effector proteins, preventing incorporation into the membrane, abolished effector activity. The results suggest the existence of a periplasmic component that binds to OmpA prior to membrane assembly; impaired release of this factor from mutant OmpA proteins may trigger inhibition of translation. The factor could be a See B-type protein, keeping outer membrane proteins in a form compatible with membrane assembly.

Export incompatibility of N-terminal basic residues in a mature polypeptide of Escherichia coli can be alleviated by optimising the signal peptide

Export of the outer membrane protein, OmpA, across the cytoplasmic membrane of Escherichia coli was severely inhibited by the presence of two, three, four or six additional basic residues at the N-terminus of the mature polypeptide, but not by three similarly positioned acidic residues. Because a few bacterial proteins do possess basic residues close to the leader peptidase cleavage site and because the type of inhibition described here could pose problems in the construction of hybrid secretory proteins, we also studied means of alleviating this form of export incompatibility. Inhibition was abolished when basic residues were preceded by acidic ones. Also, the processing rates of the mutants with two or six basic residues could be partially restored by increasing the length of the hydrophobic core of the signal peptide. Taking this as a precedent, it is suggested that the structure of the signal peptide is an important feature for maintenance of a reasonable rate of translocation of those exported proteins which possess basic residue(s) at the N-terminus of the mature polypeptide.

The role of the mature part of secretory proteins in translocation across the plasma membrane and in regulation of their synthesis in Escherichia coli

Presently available data are reviewed which concern the role of the mature parts of secretory precursor proteins in translocation across the plasma membrane of Escherichia coli. The following conclusions can be drawn; i) signals, acting in a positive fashion and required for translocation do not appear to exist in the mature polypeptides; ii) a number of features have been identified which either affect the efficiency of translocation or cause export incompatibility. These are: alpha) protein folding prior to translocation; beta) restrictions regarding the structure of N-terminus; gamma) presence of lipophilic anchors; delta) too low a size of the precursor. Efficiency of translocation is also enhanced by binding of chaperonins (SecB, trigger factor, GroEL) to precursors. Binding sites for chaperonins appear to exist within the mature parts of the precursors but the nature of these sites has remained rather mysterious. Mutant periplasmic proteins with a block in release from the plasma membrane have been described, the mechanism of this block is not known. The mature parts of secretory proteins can also be involved in the regulation of their synthesis. It appears that exported proteins are already recognized as such before they are channelled into the export pathway and that their synthesis can be feed-back inhibited at the translational level.

Topological analysis of the amino-terminal region of lactose permease using the Escherichia coli outer membrane protein, OmpA, as a marker

LacY-ompA fusions, encoding the N-terminal 50, 71 or 143 residues of lactose permease, were constructed. The observed orientation of the OmpA part of each hybrid protein with respect to the plasma membrane supports current models of the N-terminus of Lac permease. Hybrids possessing the entire mature OmpA were very stable; those with only a part thereof were much less stable. Due to their in vivo stability and accessibility to antibody it is proposed that such hybrids may represent potential models to investigate the assembly pathway of lactose permease.

An artificial hydrophobic sequence functions as either an anchor or a signal sequence at only one of two positions within the Escherichia coli outer membrane protein OmpA

The 325-residue outer membrane protein, OmpA, of Escherichia coli, like most other outer membrane proteins with known sequence, contains no long stretch of hydrophobic amino acids. A synthetic oligonucleotide, encoding the sequence Leu-Ala-Leu-Val, was inserted four times between the codons for amino acid residues 153 and 154 and two, three, or four times between the codons for residues 228 and 229, resulting in the OmpA153-4, OmpA-228-2, -3, and -4 proteins, respectively. In the first case, the lipophilic sequence anchored the protein in the plasma membrane. In the OmpA228 proteins, 16 but not 12 or 8 lipophilic residues most likely also acted as an anchor. By removal of the NH2-terminal signal peptide, the function of the insert in OmpA153-4 was converted to that of a signal-anchor sequence. Possibly due to differences in amino acid sequences surrounding the insert, no signal function was observed with the insert in OmpA228-4. Production of the OmpA153-4 protein, with or without the NH2-terminal signal sequence, resulted in a block of export of chromosomally encoded OmpA. Clearly, long hydrophobic regions are not permitted within proteins destined for the bacterial outer membrane, and these proteins, therefore, have had to evolve another mechanism of membrane assembly.

The influence of amino substitutions within the mature part of an Escherichia coli outer membrane protein (OmpA) on assembly of the polypeptide into its membrane

The membrane part of the 325-residue outer membrane protein OmpA of Escherichia coli encompasses residues 1-177. This part is thought to cross the membrane eight times in antiparallel beta-strands, forming four loops of an amphipathic beta-barrel. With the aim of gaining some insight into the mechanism of sorting, i.e. the way the protein recognizes and assembles into its membrane, a set of point mutants in the ompA gene has been generated. Selection for toxicity of ompA expression following mutagenesis with sodium bisulfite yielded genes with multiple base pair substitutions, the majority of which resulted in amino acid substitutions in the membrane moiety of the protein. None of the altered proteins was blocked in membrane incorporation. A proline residue exists at or near each of the presumed turns at the inner side of the outer membrane. Using oligonucleotide-directed mutagenesis, each of them was replaced by a leucine residue which is thought to be a turn blocking residue. None of these proteins had lost the ability to be incorporated into the membrane. Apparently, leucine residues are tolerated at turns in this protein. To interfere with the formation of antiparallel beta-strands, four double mutants were prepared: ompA-ON3 (Ala11----Pro, Leu13----Pro), -ON4 (Ala11----Asp, Leu13----Pro), -ON5 (Gly160----Val, Leu162----Arg), and -ON6 (Leu164----Pro, Val166----Asp). The former three proteins and even quadruple mutants consisting of a combination of ompA-ON2 or -ON4 with -ON5 were not defective in membrane assembly. In contrast, the OmpA-ON6 protein was translocated across the plasma membrane but could not be incorporated into the outer membrane. It is concluded that at least one rather small area of the polypeptide is of crucial importance for the assembly of OmpA into the outer membrane.

Internal deletions in the gene for an Escherichia coli outer membrane protein define an area possibly important for recognition of the outer membrane by this polypeptide

A series of overlapping deletions has been constructed in the ompA gene which encodes the 325-residue Escherichia coli outer membrane protein OmpA. Immunoelectron microscopy showed that the OmpA fragments were either located in the periplasmic space or were associated with the outer membrane. Apparently an area between residues 154 and 180 is required for this association; all proteins missing this area were found to be periplasmic. The nature of this association remained unknown; no membrane-protected tryptic fragments could be identified for any of these polypeptides. Hybrid genes were constructed encoding parts of the periplasmic maltose binding protein and an area of the ompA gene coding for residues 154-274. The corresponding proteins were not localized to the outer membrane but remained attached to the outer face of the plasma membrane, possibly because the normal mechanism of release from this membrane was impaired. In the OmpA protein the conspicuous sequence Ala180-Pro-Ala-Pro-Ala-Pro-Ala-Pro187 exists. Frameshift mutants were constructed to eliminate this sequence. There was no effect on the incorporation of the mutant proteins into the outer membrane. Thus, this "hinge" region is not involved in sorting. A proposal suggesting the existence of a sorting signal common to several outer membrane proteins (Benson, S. A., Bremer, E., and Silhavy, T. J. (1984) Proc. Natl. Acad. Sci. U. S. A. 81, 3830-3834) was subsequently rejected (Bosch, D., Leunissen, J., Verbakel, J., de Jong, M., van Erp, H., and Tommassen, J. (1986) J. Mol. Biol. 189, 449-455; Freudl, R., Schwarz, H., Klose, M., Movva, N. R., and Henning, U. (1985) EMBO J. 4, 3593-3598). Although it is not known whether or not the outer membrane association observed represents a step in the normal sorting mechanism, it is concluded that it remains an open question whether or not a sorting signal, as proposed originally, exists in outer membrane proteins.

Alterations to the signal peptide of an outer membrane protein (OmpA) of Escherichia coli K-12 can promote either the cotranslational or the posttranslational mode of processing

The signal sequence of the precursor of the Escherichia coli outer membrane protein OmpA was altered by oligonucleotide insertions into the corresponding gene. In one case, OmpA-S1, the hydrophobic core of the signal peptide, is reduced from 12 to 10 residues, and one positive charge is added near the NH2-terminus. In another case, OmpA-P1, the hydrophobic core is extended from 12 to 16 residues. The pro-OmpA protein is normally processed partially co- and partially posttranslationally. Processing of the pro-OmpA-S1 protein was entirely posttranslational and that of the pro-OmpA-P1 protein strictly cotranslational. Evidence is presented which strongly suggests that posttranslational processing reflects posttranslational translocation across the plasma membrane. The generation times of cells expressing pro-OmpA-P1 or pro-OmpA-S1 were identical and the pro-OmpA-S1 polypeptide could be chased into the mature protein in the absence of protein synthesis. Hence, it does not matter which mode of processing, or rather translocation, is used. The same oligonucleotides were inserted into the ompA gene of plasmid pRD87; a plasmid which leads to overproduction of the protein and to massive accumulation of both the mature protein and the precursor. In the OmpA signal sequence encoded by pRD87-P2 the hydrophobic core is extended from 12 to 20 residues. This peptide was also rapidly removed. Therefore, regardless of whether the hydrophobic core contains 12, 16, or 20 lipophilic residues, not only does the signal sequence always function correctly to mediate export, but in each case, the cleavage site is always accessible to the signal peptidase.

The signal sequence of an Escherichia coli outer membrane protein can mediate translocation of a not normally secreted protein across the plasma membrane

The distal part of the long tail fibers of the Escherichia coli phage T4 consists of a dimer of protein 37. A fragment of the corresponding gene, encoding 253 amino acids, was inserted into several different sites within the cloned gene for the 325-residue outer membrane protein OmpA. In plasmid pTU T4-5 the fragment was inserted once and in pTU T4-10 tandemly twice between the codons for residues 153 and 154 of the OmpA protein. In pTU T4-22 two fragments were present, in tandem, between the codons for residues 45 and 46 of this protein. In pIN T4-6 one fragment was inserted into the ompA gene immediately following the part encoding the signal sequence. The corresponding mature proteins consist, in this order, of 605, 860, 835, and 279 amino acid residues. All precursor proteins were processed and translocated across the plasma membrane. Hence, not only can the OmpA protein serve as a vehicle for export of a nonsecretory protein, but the signal sequence alone can also mediate export of such a protein. Export of the pro-OmpA protein depends on the SecA protein. Export of the tail fiber fragment expressed from pIN T4-6 remained SecA dependent. Thus, the secA pathway in this case is chosen by the signal peptide. It is proposed that a signal peptide can mediate translocation of nonsecretory proteins as long as they are export-compatible. The inability of a signal sequence to mediate export of some proteins appears to be due to export incompatibility of the protein rather than to the absence of information, within the mature part of the polypeptide, which would be required for translocation.

Cell surface exposure of the outer membrane protein OmpA of Escherichia coli K-12

The 325-residue OmpA protein is one of the major outer membrane proteins of Escherichia coli K-12. A model, in which this protein crosses the membrane eight times in an antiparallel beta-sheet conformation and in which regions around amino acids 25, 70, 110 and 154 are exposed at the cell surface, had been proposed. Linkers were inserted into the ompA gene with the result that OmpA proteins, carrying non-OmpA sequences between residues 153 and 154 or 160 and 162, were synthesized. Intact cells possessing these proteins were treated with proteases. Insertion of 15 residues between residues 153 and 154 made the protein sensitive to proteinase K and the sizes of the two cleavage products were those expected following proteolysis at the area of the insertion. Addition of at least 17 residues between residues 160 and 162 left the protein completely refractory to protease action. Thus, the former area is cell surface exposed while the latter area appears not to be. The insertions did not cause a decrease in the concentration of the hybrid proteins as compared to that of the OmpA protein, and in neither case was synthesis of the protein deleterious to cell growth. It is suggested that this method may serve to carry peptides of practical interest to the cell surface and that it can be used to probe surface-located regions of other membrane proteins.

Smooth lipopolysaccharide is the major protective antigen for mice in the surface extract from IATS serotype 6 contributing to the polyvalent Pseudomonas aeruginosa vaccine PEV

The nature of the protective antigen in one of the sixteen monovalent extracts (viz., extract-6) contributing to the pseudomonas polyvalent extract vaccine (PEV) was studied in a mouse challenge assay. Selective removal, by filtration through Sep-Pak C18 cartridges, of two major protein antigens with molecular weights of 16,200 and 21,000 had no effect on the protection afforded by extract-6. When analyzed on the basis of 2-keto-3-deoxyoctonate, lipopolysaccharide (LPS) purified by hot phenol extraction (LPS-A) from Pseudomonas aeruginosa (International Antigenic Typing System serotype 6) could account in full for the protective capacity of extract-6. Comparative analysis of LPS heterogeneity by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining indicated that both extract-6 and LPS-A possessed similar spectra of smooth LPS molecules, containing between 10 and approximately equal to 50 O-antigen repeating units. Differences in the profiles of heterogeneity displayed by LPS in LPS-A and extract-6 were restricted to molecular species with short O-antigen chains. Subfractionation of LPS molecules on the basis of number of O-antigen repeating units was achieved by gel filtration in the presence of deoxycholate. Protection experiments performed on the subfractionated species of LPS-A revealed a relationship between O-antigen chain length and protective capacity; molecules with over 18 O-antigen repeating units being 50 to 100 times more protective than those with zero-two repeating units. The results indicate that most of the protection afforded by LPS-A and extract-6 can be accounted for by LPS molecules possessing extended (10 or more) O-antigen repeating units.

Immunochemical and biochemical analysis of the polyvalent Pseudomonas aeruginosa vaccine PEV

The Pseudomonas aeruginosa polyvalent vaccine PEV and its 16 constituent monovalent extracts from International Antigenic Typing System serotypes 1 through 13 and 15 through 17 (J. J. Miler, J. F. Spilsbury, R. J. Jones, E. A. Roe, and E. J. L. Lowbury, J. Med. Microbiol. 10:19-27, 1977) were subjected to biochemical analysis and to detailed immunochemical analysis with rabbit anti-PEV immunoglobulins. The results of chemical analysis, of analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis performed in conjunction with silver staining, and of analysis by crossed immunoelectrophoresis, sodium dodecyl sulfate-polyacrylamide gel-crossed immunoelectrophoresis, and Western blotting showed clearly that lipopolysaccharide was a major constituent of each monovalent extract and that it was probably the dominant antigen present in at least 15 of the 16 monovalent extracts. A 16.2-kilodalton protein, which was pronase resistant and nonsedimentable at 105,000 X g and which appeared to be biochemically and antigenically unrelated to pili, was a common although minor antigen for all extracts. Several other proteins, some of outer membrane origin, were also detected in unformalinized extracts, but these were also minor antigenic constituents of the vaccine. Neither pilin nor flagellin appeared to be major protein constituents of tested monovalent extracts, although anti-flagella antibodies could be demonstrated in rabbit anti-PEV by Western blotting. Preliminary analysis by crossed immunoelectrophoresis of serum raised in volunteers to PEV also indicated the presence therein of antibodies to lipopolysaccharide antigens.

Purification and reconstitution in lipid bilayer membranes of an outer membrane, pore-forming protein of Aeromonas salmonicida

We have purified a major outer membrane protein from Aeromonas salmonicida. This 42-kilodalton protein shared several physical characteristics with enterobacterial porins in that it was noncovalently associated with the peptidoglycan, it was released from the peptidoglycan in the presence of 0.1 M NaCl and sodium dodecyl sulfate, and its mobility on sodium dodecyl sulfate-polyacrylamide gels was dependent on the solubilization temperature before electrophoresis. When added to the aqueous solution bathing a planar bilayer membrane it caused the conductance of the membrane to increase by several orders of magnitude. At lower protein concentrations, single channels with an average conductance of 1.6 nS in 1 M KCl were incorporated into the membrane in a stepwise fashion. Evidence that the protein formed a large, relatively nonselective, water-filled channel was obtained by performing single-channel experiments at different NaCl concentrations and in a variety of different salts. Current through the channel was a linear function of the applied voltage, and no evidence of voltage gating was observed. In addition, we obtained evidence for a 43-kilodalton channel-forming protein in the outer membrane of A. hydrophila with a similar single-channel conductance as the 42-kilodalton protein in 1 M NaCl.

Effect of IgG and C-reactive protein on complement depletion by monosodium urate crystals

Complement activity in normal human serum was rapidly depleted by the addition of monosodium urate crystals (MSUC), while the same MSUC preparation had little effect on complement activity in 4 of 8 sera from patients with common variable immunodeficiency (CVID). The degree of complement depletion in the CVID sera correlated with their C-reactive protein (CRP) concentrations but not with their concentrations of IgG. MSUC-induced complement depletion in the 4 poorly reactive sera was partially restored by addition of CRP or by normalizing the IgG concentration, but not by addition of IgM or IgA. It is proposed that CRP, an acute phase protein, may play a role in the pathogenesis of gouty arthritis attacks occurring in some patients following surgery or acute physical illness.

Purification and partial characterization of a bacterial phospholipid: cholesterol acyltransferase

A glycerophospholipid:cholesterol acyltransferase has been purified to near homogeneity from cell-free culture supernatants of Aeromonas salmonicida. The characteristics of the enzyme distinguish it from bacterial phospholipases; however, it shares several properties with the lecithin:cholesterol acyltransferase of mammalian plasma. Thus, the enzyme inhibits 2-positional specificity as an acyltransferase and it will act as a phospholipase A2 in the absence of cholesterol. Furthermore, it has no divalent cation requirement and it is stimulated both by albumin and by human apolipoprotein A-I. Unlike the mammalian acyltransferase, however, the bacterial enzyme is not specific for phosphatidylcholine and in addition it can use human erythrocyte membranes as substrates. Similar to Naja naja phospholipase A2, it acts asymmetrically on intact erythrocytes.

Modified Mapleson D system for long-term mechanical ventilation of infants and children

The Bain circuit, a modified Mapleson D system, is a lightweight, simple circuit which has been used primarily for anesthesia. This report describes its use for long-term mechanical ventilation for infants and children. The use of this circuit improved warming of inspired gas reducing patient heat loss and, additionally, it was believed to have resulted in increased humidity of the inspired gas. There were no instances of accidental extubation nor were any of the endotracheal tubes blocked by inspissated secretions. Temperatures of the humidifiers had to be lower than conventionally recommended but this did not result in any nosocomial infections. This circuit is an effective and safe circuit to use for long-term mechanical ventilation of children.

Purification and some properties of the hemolytic toxin aerolysin

Aerolysin, the hemolytic toxin produced by Aeromonas hydrophila, has been purified by a combination of salt fractionation, gel filtration, and ion-exchange and hydroxyapatite chromatography. The resulting protein has a molecular weight of 51 500 and appears homogeneous by polyacrylamide gel electrophoresis in sodium dodecyl sulphate. It is free of detectable protease and phospholipase activities. The purified protein can be separated into two active components with pIs of 5.39 and 5.46 by isoelectric focusing. Both components are found in the original culture supernatant indicating that the multiplicity is not due to proteolysis during isolation. Purified aerolysin is unstable even at 25 degrees C and its hemolytic action is inhibited by certain reducing agents including ferrous iron and cysteine. It appears to be the only toxin hemolytic to human cells that is produced by A. hydrophila under the conditions described.

Identification and characterization of outer membrane fragments released by Aeromonas sp

Aeromonas hydrophila and Aeromonas salmonicida were shown to release "blebs" or fragments averaging 20-30 nm in diameter. The protein composition of the fragments was very similar to that of the corresponding outer membrane by sodium dodecyl sulphate--polyacrylamide gel electrophoresis and the fragments were shown to contain phospholipid and lipopolysaccharide. The results therefore indicate that the blebs are derived from the outer membrane of these organisms. Fragments isolated directly by differential ultracentrifugation were indistinguishable from fragments isolated by salt fractionation and gel filtration in chemical composition, protein composition, and density. However fragments isolated directly contained much less glycerophospholipid-cholesterol acyltransferase than those isolated by salt fractionation. The potential usefulness of membrane fragments to the bacteria and as a tool in the study of outer membrane structure and function is discussed.

Distribution of glycerophospholipid-cholesterol acyltransferase in selected bacterial species

The distribution of glycerophospholipid-cholesterol acyltransferase in selected bacterial species was examined. Enzyme activity was demonstrated in cell-free growth media from all members of the family Vibrionaceae which were tested except Plesiomonas shigelloides. In each case, enzyme was produced in exponential to early stationary phase and was excluded from Sepharose 6B, indicating a complex of high molecular weight. In a limited survey of other families, Stahylococcus aureus was the only organism outside the Vibrionaceae which was shown to produce the enzyme. In this case, however, the enzyme exhibited much less activity against erythrocyte membranes and appeared to have a lower molecular weight. The reasons for these differences and the importance of the acyltransferase as a biochemical identification tool are discussed.

Presence of glycerophospholipid: cholesterol acyltransferase and phospholipase in culture supernatant of Aeromonas hydrophila

Human erythrocyte membrane glycerophospholipids are deacylated by Aeromonas hydrophila 13-h culture supernatants, resulting in the production of cholesterol ester, free fatty acid, and water-soluble phosphates. This activity appears to be due to the actions of an acyltransferase (phosphatide:cholesterol acyltransferase, EC 2.3.1 group) and a phospholipase (phosphatide acyl-hydrolase). The enzyme activities are produced simultaneously in late exponential/early stationary phase, are precipitated together from the culture supernatant with 85% ammonium sulfate, and are eluted together near the void volume during gel filtration on Sepharose 6B. These results suggest that A. hydrophila produces a multienzyme complex with an unusual mode of action on membrane lipids. The complex is distinct from the hemolytic factor aerolysin, which is also produced by A. hydrophila.