Antibodies directed against the 68-kDa subunit of signal recognition particle (SRP) precipitate an Alu RNA X protein complex formed during in vitro transcription of a plasmid containing an Alu family sequence. The same Alu RNA X protein complex is precipitated by anti-Alu sera from certain patients with systemic lupus erythematosus and related autoimmune diseases (Kole, R., Fresco, L. D., Keene, J. D., Cohen, P. L., Eisenberg, R. A., and Andrews, P. G. (1985) J. Biol. Chem. 260, 11781-11786). Similarly to anti-SRP antibodies human, anti-Alu sera precipitate SRP from HeLa cell extract and detect a 68-kDa SRP subunit on immunoblots. These results indicate that the Alu antigen and the 68-kDa SRP subunit are the same polypeptide
