Interaction of Shigella flexneri IpaC with model membranes correlates with effects on cultured cells

Invasion of enterocytes by Shigella flexneri requires the properly timed release of IpaB and IpaC at the host-pathogen interface; however, only IpaC has been found to possess quantifiable activities in vitro. We demonstrate here that when added to cultured cells, purified IpaC elicits cytoskeletal changes similar to those that occur during Shigella invasion. This IpaC effect may correlate with its ability to interact with model membranes at physiological pH and to promote entry by an ipaC mutant of S. flexneri.

Control of Drosophila retinoid and fatty acid binding glycoprotein expression by retinoids and retinoic acid: northern, western and immunocytochemical analyses

In Drosophila, thorough retinoid deprivation is possible, optimizing investigation of the effects of vitamin A metabolites and retinoic acid on the visual system. Retinoids had been found to control transcription and translation of Drosophila's opsin gene. To follow this line of inquiry, we examined the effect of retinoids on the translation and transcription of a Drosophila Retinoid and Fatty Acid Binding Glycoprotein. Western blots showed that this protein is high in retinoid replete flies and low in deprived flies. Flies grown on media capable of activating the opsin gene's transcription and which contain alternate transcription activators including retinoic acid yielded extracts containing significant amounts of Retinoid and Fatty Acid Binding Glycoprotein. Immunocytochemistry confirmed its absence in deprived flies and its presence in flies reared or replaced on these diverse media containing retinoids or general nutrients. Immunocytochemistry localized Retinoid and Fatty Acid Binding Glycoprotein to the Semper (cone) cells and the intraommatidial matrix (the interphotoreceptor matrix of the ommatidium). Positive staining of Semper cells in mutants of the opsin gene and a mutant lacking receptors suggests that Retinoid and Fatty Acid Binding Glycoprotein does not depend on presence of opsin and that it is not synthesized in receptor cells respectively. Northern blots demonstrated greatly diminished mRNA for Retinoid and Fatty Acid Binding Glycoprotein in flies grown on deprivation food relative to flies grown on normal food. Although the synthesis of Retinoid and Fatty Acid Binding Glycoprotein does not require chromophore precursors as does that of opsin, the control of Retinoid and Fatty Acid Binding Glycoprotein and opsin transcription by retinoids including retinoic acid might very well be the same. Our results suggest that Retinoid and Fatty Acid Binding Glycoprotein may be involved in retinoid transport. Also, Semper cells may be analogous to vertebrate retinal pigment epithelium in retinoid metabolism and/or delivery.

Cloning, expression, and affinity purification of recombinant Shigella flexneri invasion plasmid antigens IpaB and IpaC

Shigella flexneri and related enteropathogenic bacteria are important agents of bacillary dysentery, a potentially life-threatening illness for children in underdeveloped regions of the world. Onset of shigellosis stems from S. flexneri invasion of colonic epithelial cells, leading to localized cell death and inflammation. Invasion plasmid antigens (Ipa) B, C, and D are three secreted proteins encoded by the large virulence plasmid of S. flexneri that have been implicated as essential effectors of this cell invasion process. These proteins are expressed as part of the ipa operon and are among the major targets of the host immune response to shigellosis. Biochemical characterization of the Ipa invasins has been complicated by the fact they have not been purified in the quantities needed for detailed in vitro analysis. Here we describe the first cloning, expression, and extensive purification of IpaB and IpaC fusion proteins from Escherichia coli for use in dissecting of the protein biochemistry of S. flexneri pathogenesis. A variety of approaches were used to prepare significant quantities of these proteins in their soluble forms, including the use of different host cell lines, modification of bacterial growth conditions, and the use of alternative plasmid expression vectors. Now that these Ipa proteins are available in a highly pure form, it will be possible to initiate studies on their important biological and immunological properties as well as their recruitment into high-molecular-weight protein complexes. Together with IpaD (purified as part of a previous study), these purified proteins will be useful for: (a) exploring properties of the host immune response to S. flexneri invasion, (b) elucidating the specific biochemical properties that lead to pathogen internalization, (c) analyzing the importance of specific Ipa protein complexes in host cell invasions, and (d) monitoring, or perhaps even augmenting, the efficacy of live oral vaccines in human trials.

Control of Drosophila opsin gene expression by carotenoids and retinoic acid: northern and western analyses

In the fly, thorough retinoid deprivation is possible, to optimize investigation of the effects of vitamin A metabolites and retinoic acid (RA) on visual development. Retinoids had been found to control fly opsin gene transcription, though this finding was contested. Northern blots on Drosophila heads showed that mRNA of Rh1 (the predominant rhodopsin) was high in vitamin A replete controls, very low in deprived flies, and increased upon feeding carrot juice to deprived flies as early as 1 hr. Expression of the ribosomal protein 49 [rp49] gene (the control) was equal both in deprivation and in replacement. Recovery of Rh1 protein upon such carotenoid replacement followed, barely detectable on Western blots at 4 hr but conspicuous by 8 hr. Alternative chromophore deprivation with yeast-glucose food yielded flies with opsin mRNA on Northerns but not rhodopsin, as demonstrated by Western blots, spectrophotometry and the electroretinogram (ERG). Rh1's mRNA but not Rh1 protein resulted from rearing flies from egg to adult on the otherwise deprivational medium supplemented with RA or beef brain-heart infusion. By comparing results from these different media it was concluded that: [1] deprivation and replacement affect opsin gene transcription; and [2] contradictory conclusions were from chromophore deprivation which does not eliminate all retinoid dependent factors which could affect the opsin promoter. Preliminary evidence shows that carotenoid deprivation decreases two proteins relevant to visual function: [1] phospholipase C (PLC); and [2] Drosophila retinoid binding protein (DRBP).

Soluble invasion plasmid antigen C (IpaC) from Shigella flexneri elicits epithelial cell responses related to pathogen invasion

Shigella flexneri invades colonic epithelial cells by pathogen-induced phagocytosis. The three proposed effectors of S. flexneri internalization are invasion plasmid antigens B (IpaB), IpaC, and IpaD, which are encoded on the pathogen's 230-kb virulence plasmid and translocated to the extracellular milieu via the Mxi-Spa translocon. To date, there are no definitive functional data for any purified Ipa protein. Here, we describe the first characterization of highly purified recombinant IpaC, which elicits numerous epithelial cell responses related to events that take place during pathogen invasion. 125I-labeled IpaC binds cultured Henle 407 intestinal cells with an apparent dissociation constant in the low micromolar range. Moreover, incubation of epithelial cells with IpaC results in general changes in cellular phosphoprotein content, demonstrating this protein's ability to influence cellular protein kinase activities. These results contrast dramatically with those seen for recombinant IpaD, which does not bind to or induce detectable changes in the normal activities of cultured epithelial cells. In addition to influencing host cell activities, preincubation of epithelial cells with purified IpaC enhances uptake of S. flexneri by host cells. A similar result is seen when the cells are preincubated with a highly concentrated water extract of virulent S. flexneri 2a (strain 2457T). Interestingly, soluble IpaC also appears to promote uptake of the noninvasive S. flexneri 2a strain BS103. Purified IpaD failed to enhance the uptake of virulent S. flexneri and did not facilitate uptake of BS103. Taken together, the data suggest that IpaC is a potential effector of the host cell biological activities and may be responsible for entry of S. flexneri into target cells.

Antibody response of monkeys to invasion plasmid antigen D after infection with Shigella spp

The antigen preparation most often used for determining the levels of antibodies to virulence-associated proteins of Shigella spp. consists of a mixture of proteins (including IpaB, IpaC, IpaD, and VirG*) extracted from virulent shigellae with water (water extract). To overcome the lack of specificity for individual antigens in the water-extract enzyme-linked immunosorbent assay (ELISA), the ipaD gene from S. flexneri has been cloned, expressed to a high level, and purified for use in a new ELISA for the determination of the levels of antibody against IpaD in monkeys and humans challenged with shigellae. The IpaD ELISA for serum immunoglobulins G and A correlated well with the water-extract ELISA in that monkeys infected with S. flexneri or S. sonnei responded with high serum antibody titers in both assays. The IpaD assay required less antigen per well, had much lower background levels, and did not require correction with antigens from an avirulent organism. In conjunction with the water-extract ELISA, it was possible to identify infected animals that did not respond to IpaD but did produce antibodies that reacted in the water-extract ELISA. This indicates that even though IpaB, IpaC, and IpaD are essential for the invasiveness phenotype, the infected host does not always produce antibodies against all components of the invasiveness apparatus.

Structural analysis of invasion plasmid antigen D (IpaD) from Shigella flexneri

Invaison plasmid antigen D (IpaD) and water-extracted outer-membrane proteins (OMPs) from Shigella flexneri were used to investigate some of the structural relationships of this pathogen's invasions. Extracellular presentation of the three invasion plasmid antigens (Ipa) B, C, and D is required for the S. flexneri invasive phenotype; however, little is known of the structural properties of these essential virulence components. Biochemical data suggest IpaB, C, and D present in S. flexneri OMPs from soluble protein complexes and IpaD may be able to form large homopolymeric complexes.

Fluorescence analysis of the interaction between ganglioside GM1-containing phospholipid vesicles and the B subunit of cholera toxin

Binding of cholera toxin B protomer (CT-B) to a pyrene-labeled analogue of its ganglioside GM1 receptor (pyrene-GM1) in the absence and presence of phosphatidylcholine vesicles was monitored using steady-state fluorescence spectroscopy. CT-B association with pyrene-GM1 micelles induces changes in the fluorescence properties of this ganglioside analogue that are consistent with its conversion from an excimer to a monomer form. Incubation of pyrene-GM1 with preformed vesicles of phosphatidylcholine (PC) results in complete conversion of pyrene-GM1 to its monomer form, however, unlike with CT-B binding, incorporation of pyrene-GM1 into PC vesicles occurs with a concomitant loss of fluorescence quenching by the small polar quenching agent acrylamide. Subsequent binding of CT-B to the PC-GM1 composite vesicles causes no further change in the pyrene fluorescence emission spectrum but does appear to increase acrylamide accessibility. These data lead to the conclusion that cholera toxin binding to a cell membrane alters membrane packing at the site of attachment. Furthermore, this phenomenon appears to be influenced by environmental conditions such as pH. A pH of about 4.0 or less causes acrylamide quenching to decrease to approximately the levels observed in the absence of CT-B. These results may be useful in describing the dynamics of the interaction between cholera toxin and target cell membranes. Moreover, these data could provide clues to the mechanism by which the toxic portion of CT is able to enter the cytoplasm of target cells.

Fluorescence characterization of the environment encountered by nascent polyalanine and polyserine as they exit Escherichia coli ribosomes during translation

The fate of the amino termini of nascent polyalanine, polyserine, and polylysine was monitored by fluorescence techniques as each was translated on Escherichia coli ribosomes. A coumarin probe was placed at the alpha-amino group of a synthetic elongator alanyl-tRNA or a synthetic initiator alanyl-tRNA or at the epsilon-amino group of natural lysyl-tRNA, and each was used to nonenzymatically initiate peptide synthesis. The fluorescent alanyl-tRNAs containing an AAA anticodon were used to initiate polyserine (with a synthetic tRNA(Ser] or polyalanine synthesis from a poly(uridylic acid) template. The fluorescent lysyl-tRNA was used to initiate polylysine synthesis from poly(adenylic acid). Changes in the fluorescence of the amino-terminal coumarin were examined to characterize the environment of the probe as the nascent peptides were extended. Protection from proteolysis and the binding of anti-coumarin antibodies or Fab fragments suggest that the amino terminus of each polypeptide is protected from interaction with proteins (Mr greater than 28,000) until the peptides are extended to an average length of 40-50 residues; however, the fluorescence from the amino terminus of shorter nascent polyalanine and polyserine peptides was readily quenched by methyl viologen (Mr = 257), indicating ribosomes do not shield the nascent peptide from molecules of this size. The data appear to indicate that polyalanine, polyserine, and polylysine are extended from the peptidyl transferase into a protected region of the ribosome such as a groove or tunnel but that this region is readily accessible to small molecules.

A synthetic alanyl-initiator tRNA with initiator tRNA properties as determined by fluorescence measurements: comparison to a synthetic alanyl-elongator tRNA

Two synthetic tRNAs have been generated that can be enzymatically aminoacylated with alanine and have AAA anticodons to recognize a poly(U) template. One of the tRNAs (tRNA(eAla/AAA)) is nearly identical to Escherichia coli elongator tRNA(Ala). The other has a sequence similar to Escherichia coli initiator tRNA(Met) (tRNA(iAla/AAA)). Although both tRNAs can be used in poly(U)-directed nonenzymatic initiation at 15 mM Mg2+, only the elongator tRNA can serve for peptide elongation and polyalanine synthesis. Only the initiator tRNA can be bound to 30S ribosomal subunits or 70S ribosomes in the presence of initiation factor 2 (IF-2) and low Mg2+ suggesting that it can function in enzymatic peptide initiation. A derivative of coumarin was covalently attached to the alpha amino group of alanine of these two Ala-tRNA species. The fluorescence spectra, quantum yield and anisotropy for the two Ala-tRNA derivatives are different when they are bound to 70S ribosomes (nonenzymatically in the presence of 15 mM Mg2+) indicating that the local environment of the probe is different. Also, the effect of erythromycin on their fluorescence is quite different, suggesting that the probes and presumably the alanine moiety to which they are covalently linked are in different positions on the ribosomes.

Anti-RNA polymerase I antibodies in the urine of patients with systemic lupus erythematosus

Urine samples from patients with systemic lupus erythematosus (SLE) (n = 80), patients with rheumatoid arthritis (RA) (n = 21), and healthy controls (n = 36) were analyzed by radio-immunoassay (RIA) for anti-RNA polymerase I (RPI) antibodies. Significant levels of anti-RPI antibodies were detected in the urine of 46% of the patients with SLE but in only 19% of the patients with RA and in no sample from healthy individuals. The presence of anti-RPI antibodies in the urine was confirmed by demonstrating that IgG purified from the urine of patients with SLE was capable of inhibiting the transcriptional activity of RPI in vitro. If the quantity of anti-RPI antibodies excreted is related to disease activity, analysis of urine for these antibodies may be a useful alternative for the purpose of monitoring the progression of disease in individuals with SLE because of the ease by which the sample can be collected.