Characterization of the La (SS-B) antigen from several mammalian sources

The La or SS-B antigen is associated with rheumatic diseases, systemic lupus erythematosus, and Sjogren's syndrome, and is part of a larger ribonucleoprotein complex. Immunoaffinity chromatography allowed for the efficient separation of the La antigen from the bulk of the cellular proteins, with a minimum of protease exposure. Protein blot analysis of the affinity-isolated material indicated a major immunoreactive polypeptide of 50,000 m.w. A comparison of this antigen in a number of mammalian sources (human, rabbit, and rat) suggested strong conservation of the native polypeptide m.w. Likewise, in a direct comparison of this antigen from Epstein-Barr virus-infected cells in which there are distinct differences in the antigen-associated RNA species, the immunoreactive polypeptide species were of similar size. The La protein is readily susceptible to endogenous proteolysis, with the resulting generation of smaller, discrete polypeptides that still retain antigenicity. By using the La protein to monitor potential degradation, we have developed a simple two-step procedure to isolate the La-associated snRNP complex. The complexes thus isolated provide material suitable as a source of both the active antigen and of the functional ribonucleoprotein complex.