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 generat e the tum- antigens P91A and P198, respectively, rabbit antisera were raised with ovalbumin-coupled synthetic peptides that correspond to th eir respective C terminus. In immunoadsorption tests using immobilized protein A the antisera recognized the translation products synthesize d by rabbit reticulocyte lysates programmed with the SP6 polymerase tr anscripts 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 sep arates from murine catechol oxidase activity in rate zonal sedimentati on analysis. P198p is a ribosomal constituent, or a factor firmly link ed to both the free and membrane-bound ribosomes. These subcellular lo calizations strengthen other evidence that the antigens presented to T lymphocytes by class I products of the major histocompatibility compl ex derive from proteins of the cytosol, or in direct contact with it.