Immobilization Antigens from Tetrahymena thermophila Are Glycosyl‐Phosphatidylinositol‐Linked Proteins

We have studied four strains of Tetrahymena thermophila, each of which expresses a different allele of the SerH gene and produces a distinctive surface protein of the immobilization antigen (i‐antigen) class. Following exposure of the strains to [³H]ethanolamine or [³H]myristic acid, a protein corresponding in molecular mass to the characteristic i‐antigen for that strain became highly labeled, as determined by mobility in sodium dodecylsulfate‐polyacrylamide eiectrophoresis gels. Furthermore, antibodies raised to the i‐antigens of the T. thermophila strains selectively immunoprecipitated radioactive proteins having molecular mass identical to that of the i‐antigen characteristic for that particular strain. The lipid moieties labeled by [³H]myristate were not susceptible to hydrolysis by exogenous phosphatidylinositol‐specific phospholipase C from bacteria. However, when protein extraction was carried out in the absence of phospholipase C inhibitors, radioactive fatty acids derived from [³H]myristate were rapidly cleaved from the putative i‐antigens. On the basis of available data, it was concluded that T. thermophila i‐antigens contain covalently‐Iinked glycosyl‐phospha‐tidylinositol anchors.