Labeling of Mycoplasma pulmonis cells by iodination with the Bolton-Hunter reagent was shown to efficiently label membrane-associated proteins without significant loss of viability. Labeled proteins proven to be surface exposed by differential proteolytic digestion were analyzed by autoradiography of two-dimensional polyacrylamide gel electrophoresis (PAGE), and seven labeled major polypeptides were identified. To identify all the membrane-associated antigens, pure membranes were isolated by using the surface-labeled proteins as markers. Analysis of the isolated membranes by autoradiography of both sodium dodecyl sulfate-PAGE and the two-dimensional PAGE indicated that the labeled surface proteins were retained in the pure membranes; by immunoblotting with sera from naturally infected animals, these surface proteins were shown to be the predominant antigens recognized by the host during natural infection.

The heat-stable enterotoxins are a family of low molecular weight, cysteine rich peptide toxins which are one of the major causes of watery diarrhea in children and adults. These toxins bind to a cell surface receptor in intestinal cells and mediate their action through elevation of intracellular cyclic GMP. We have generated a monoclonal antibody to these peptide toxins which is able to neutralise the activity of the peptides in a human colonic cell line, the T84 cell line. The monoclonal antibody, ST:G8, appears to be directed to an epitope distinct from antibodies previously generated, and prior incubation of this antibody, or Fab generated from this antibody, with full length STh and STp peptides prevents cGMP accumulation in T84 cells. This inhibition is a direct result of the antibody preventing binding of the peptides to the receptor. ST:G8 Mab does not recognize a 13-mer biologically active analog of STp, comprising the core sequence of STp peptide, suggesting that it is directed to a region in the N-terminus of the peptides, which may modulate receptor interaction/activation. The antibody recognizes a conformational epitope in the ST peptides, since reduction and carboxyamidation of ST abolishes antibody cross-reactivity. Differential cross-reactivity of the Mab with STh and STp peptides which differ markedly only in their N-termini, suggests that this antibody recognizes a distinct conformation in the two peptides, which is essential for receptor interaction.