Immunological evidence of a common structure between Paramecium surface antigens and Trypanosoma variant surface glycoproteins.

The surface antigens (SAgs) of Paramecium and the variant surface antigens (VSGs) of Trypanosoma can be purified in two distinct molecular forms: a soluble form (solubilized in dilute ethanolic solution in the case of Paramecium, or in water for Trypanosoma) and a membranal form, amphiphile (solubilized in SDS). In trypanosomes, the enzymatic conversion of the membrane form into the soluble form is accompanied by the unmasking of a particular immunological determinant, called cross-reacting determinant (CRD), which is located in the COOH-terminal phospho-ethanolamine glycopeptide. We demonstrate immunological homologies between Paramecium SAgs and Trypanosoma VSGs. A determinant corresponding to the CRD of VSGs is borne by the ethanol-soluble form of the SAgs and by two cross-reacting light chains also present in ethanolic cellular extracts (together with the soluble form), and not by the membranal form of SAgs. Furthermore, we show that the membranal form of Paramecium SAgs can be converted into soluble form and that this enzymatic conversion also yields cross-reacting light chains. We also demonstrate that the membranal form is the physiological form in paramecia stably expressing a given SAg.