Structural basis for differential receptor binding of cholera and
            Escherichia coli
            heat‐labile toxins: influence of heterologous amino acid substitutions in the cholera B‐subunit

ABSTRACT Heat-labile enterotoxin from enterotoxinogenic Escherichia coli is not only an important cause of diarrhea in humans and domestic animals but also possesses potent immunomodulatory properties. Recently, the nontoxic, receptor-binding B subunit of heat-labile enterotoxin (EtxB) was found to induce the selective death of CD8+ T cells, suggesting that EtxB may trigger activation of proapoptotic signaling pathways. Here we show that EtxB treatment of CD8+ T cells but not of CD4+ T cells triggers the specific up-regulation of the transcription factorc-myc, implicated in the control of cell proliferation, differentiation, and death. A concomitant elevation in Myc protein levels was also evident, with peak expression occurring 4 h posttreatment. Preincubation with c-myc antisense oligodeoxynucleotides demonstrated that Myc expression was necessary for EtxB-mediated apoptosis. Myc activation was also associated with an increase of IκBα turnover, suggesting that elevated Myc expression may be dependent on NF-κB. When CD8+ T cells were pretreated with inhibitors of IκBα turnover and NF-κB translocation, this resulted in a marked reduction in both EtxB-induced apoptosis and Myc expression. Further, a non-receptor-binding mutant of EtxB, EtxB(G33D), was shown to lack the capacity to activate Myc transcription. These findings provide further evidence that EtxB is a signaling molecule that triggers activation of transcription factors involved in cell survival.

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Multiple amino acid substitutions suggest a structural basis for the separation of biological activity and receptor binding in a mutant human interleukin-1.beta. protein

Biochemistry ◽

10.1021/bi00144a002 ◽

1992 ◽

Vol 31 (29) ◽

pp. 6632-6638 ◽

Cited By ~ 16

Author(s):

Philip E. Auron ◽

Gary J. Quigley ◽

Lanny J. Rosenwasser ◽

Lee Gehrke

Keyword(s):

Biological Activity ◽

Amino Acid ◽

Receptor Binding ◽

Interleukin 1 ◽

Structural Basis ◽

Amino Acid Substitutions ◽

Interleukin 1 Beta ◽

Multiple Amino Acid

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Cloning and Characterization of Genes Encoding Homologues of the B Subunit of Cholera Toxin and the Escherichia coli Heat-Labile Enterotoxin from Clinical Isolates of Citrobacter freundii and E. coli

Infection and Immunity ◽

10.1128/iai.70.12.7153-7155.2002 ◽

2002 ◽

Vol 70 (12) ◽

pp. 7153-7155 ◽

Cited By ~ 14

Author(s):

Tadahiro Karasawa ◽

Hideaki Ito ◽

Teizo Tsukamoto ◽

Shinji Yamasaki ◽

Hisao Kurazono ◽

...

Keyword(s):

Escherichia Coli ◽

Amino Acid ◽

Cholera Toxin ◽

Dna Probes ◽

Citrobacter Freundii ◽

Gene Encoding ◽

E Coli ◽

B Subunit ◽

Intergenic Space ◽

Heat Labile

ABSTRACT We identified and characterized a gene encoding a homologue of the B subunits of cholera toxin (CTB) and heat-labile enterotoxin (LTB) of Escherichia coli from a clinical isolate of Citrobacter freundii that was found to produce a factor in the culture supernatant that cross-reacted with antibodies to CTB and LTB when assayed by enzyme-linked immunosorbent assay (ELISA). The gene encoding the ELISA-positive factor, cfxB, consisted of 375 nucleotides and was located downstream of an 852-nucleotide open reading frame, cfxA, with a 56-nucleotide intergenic space. The cfxB gene was predicted to encode a 125-amino-acid polypeptide, which had 73.8 and 72.8% identities with the amino acid sequences of LTB and CTB, respectively. However, the amino acid sequence of the deduced polypeptide CFXA had no homologies to those of the A subunits of CT or LT. DNA probes developed from the sequences of cfxA and cfxB were used to screen 67 C. freundii isolates and 152 E. coli isolates from diarrheal patients by colony blot hybridization. Two strains, C. freundii 48 and E. coli 176, reacted with both DNA probes under conditions of high stringency. We cloned homologues of the cfxA and cfxB genes from E. coli 176 and designated them ecxA and ecxB, respectively. The ecxA gene and the ecxB gene comprise 855 and 375 nucleotides, respectively, with a 50-nucleotide intergenic space, and encode a 285- and a 125-amino-acid residue polypeptides, respectively. The results of the present study may provide important clues to the origin and evolution of immunologically related factors sharing a common enterotoxin-like A and B subunit structures.

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