Immunological crossreactivity of Paramecium surface antigens: conformational differences account for cell surface specificity.

Surface antigens (i-Ags) G and D of Paramecium primaurelia, reported earlier to be completely unrelated on the basis of peptide mapping and immunological tests, were compared and found to be similar in molecular weight (Mr 235,000), isoelectric point (ca. 4.5), and amino acid composition. They are tightly folded proteins with more than 100 intramolecular disulfide bonds. Native i-Ags and their CNBr fragments were blotted onto nitrocellulose and labeled with a battery of homologous and heterologous antibodies fractionated on native and reduced i-Ag immunoadsorbents. It was found the i-Ags retained part of their antigenic structure upon cleavage and reduction. Analysis of labeling revealed that the two proteins share many of their antigenic sites, though they are completely distinct in their native conformation. Immunofluorescence and immunoadsorption on fixed cells confirmed these results. Antigenic crossreactivity also was found with surface antigens of a ciliate outside the Paramecium species complex. It is concluded that surface specificity of paramecia is due, in this case, to the expression of related proteins in different conformations.