Purification and partial sequencing of the nuclear autoantigen RA33 shows that it is indistinguishable from the A2 protein of the heterogeneous nuclear ribonucleoprotein complex

RA33 is a nuclear autoantigen with an apparent molecular mass of33 kD. Autoantibodies against RA33 are found in about 30 % of sera from RA patients, but only occasionally in sera from patients with other connective tissue diseases. To charac-terize RA33, the antigen was purified from HeLa cell nuclear extracts to more than 90 % homogeneity by affinity chromatog-raphy on heparin-Sepharose and by chromatofocusing. Se-quence analysis of five tryptic peptides revealed that their se-quences matched corresponding sequences of the A2 protein of the heterogeneous nuclear ribonucleoprotein (hnRNP) com-plex. Furthermore, RA33 was shown to be present in the 40S hnRNP complex and to behave indistinguishably from A2 in binding to single stranded DNA. In summary, these data strongly indicate that RA33 and A2 are the same protein, and thus identify on a molecular level a new autoantigen. (J. Clin. Invest. 1992. 90:1061-1066.) Key words: rheumatoid arthritis X autoimmune diagnostics * antinuclear antibodies * RNA binding protein * mRNA processin