Cloning and expression of antigenic epitopes of the human 68-kDa (U1) ribonucleoprotein antigen in Escherichia coli.

Autoantibodies directed against the 68-kDa (U1) ribonucleoprotein antigen are mainly found in sera of patients with mixed connective tissue disease. The corresponding cDNA was fragmented into four regions coding for the major antigenic epitopes A', B', C' and D'. All the epitopes were subcloned and expressed as fusion proteins with the glutathione S-transferase in Escherichia coli using the novel expression system pGEX that allows very high yields of recombinant proteins after a single-step purification. The sera of patients with the autoimmune disease were analyzed for the expressed recombinant proteins by an immunoblotting technique. All positive sera showed a patient-specific behavior and could be divided into four groups regarding recognition of the four antigenic epitopes of the 68-kDa (U1) ribonucleoprotein antigen. The epitope B' was reactive to all patient sera positively tested and classified as the marker antigenic epitope for the mixed connective tissue disease.