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Patients affected with systemic lupus erythematosus and mixed connective tissue disease produce antibodies directed against two nuclear antigens, Sm and nuclear ribonucleoprotein (nRNP), respectively. The two antigens exhibit a relationship of partial identity in serologic assays, but the molecular basis of this relationship was not understood. This report describes the isolation of a nRNP particle containing both nRNP and Sm antigens. The particle was isolated by sucrose density gradient centrifugation of a rat liver nuclear extract followed by anti-nRNP affinity chromatography of a 14S gradient fraction. The protein moiety of the isolated particle consisted primarily of two polypeptides, P13 (Mr, 13,000) and P30 (Mr, 30,000). The immunoreactivity of P13 and P30 was demonstrated directly by transfer of these proteins from gels to nitrocellulose paper, followed by immunoautoradiography. Anti-nRNP sera reacted only with P30, whereas anti-Sm sera reacted with P13. Anti-nRNP sera were previously found to react with P13, but only in the presence of RNA [Douvas, A. S., Stumph, W. E., Reyes, P. R. &amp; Tan, E. M. (1979) J. Biol. Chem. 254, 3608--3616]. From these observations it was concluded that P13 is the Sm antigen. The precipitating nRNP antigen is composed of P30--RNA complexes or P13--RNA complexes, with a RNA-independent reaction occurring with P30. The partial identity between Sm and nRNP antigens can be explained on the basis of a common reactivity to the P30--P13--RNA particle, with anti-Sm sera capable of binding additionally to RNA-free P13.

Autoantibodies occurring in two different rheumatic diseases react with the same nuclear ribonucleoprotein particle.