On the Biochemical Nature of the ‘Sm’ Nucleoplasmic Antigen

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Abstract
The [human] nucleoplasmic autoantigens RNP [ribonucleoprotein] and Sm are of particular interest because of their associations with certain symptoms of mixed connective tissue disease and systemic lupus erythematosus. The RNP is generally thought to be a ribonucleoprotein and there is evidence that its RNA may be single-stranded. Experiments support the concept that the Sm-antigen may also be an RNA protein. Purified Sm.sbd.anti-Sm precipitates have high RNA contents and treatment of Sm-antigen with RNAse in a hypotonic medium strongly reduced its antigenicity. Apparently the Sm-antigen may, in contrast to the RNP, contain double-stranded RNA, also suggested by the finding that the Sm-antigen was soluble in 2 M LiCl. The Sm-antigen differed from RNP in being selectively absorbed on BD-Sephadex, while RNP remained active in the supernatant. Cytochemical studies involving stimulation and inhibition experiments with lectins [phytohemagglutinin] and RNA polymerase inhibitors showed that the Sm-antigen was, in distinction to RNP, sensitive to rifampicin but not to .alpha.-amanitine. Apparently the RNA of the nucleoplasmic antigens may be synthesized by different RNA polymerases.