The tum- antigens P91A and P198 derive from proteins located in the cytosolic compartment of cells.

To characterize the proteins P91Ap and P198p, of which mutants generate the tum- antigens P91A and P198, respectively, rabbit antisera were raised with ovalbumin-coupled synthetic peptides that correspond to their respective C terminus. In immunoadsorption tests using immobilized protein A the antisera recognized the translation products synthesized by rabbit reticulocyte lysates programmed with the SP6 polymerase transcripts of the P91A and P198 cDNA. The presence of the two proteins was demonstrated by SDS-PAGE and immunoblotting in all the mouse cells and organs examined. P91Ap is a constituent of the cytosol; despite a remarkable homology to the Drosophila diphenol oxidase DOX-A2, it separates from murine catechol oxidase activity in rate zonal sedimentation analysis. P198p is a ribosomal constituent, or a factor firmly linked to both the free and membrane-bound ribosomes. These subcellular localizations strengthen other evidence that the antigens presented to T lymphocytes by class I products of the major histocompatibility complex derive from proteins of the cytosol, or in direct contact with it.