Smith (Sm) Antigen

Sm antigen is a non-histone nuclear protein composed of several polypeptides of differing molecular weights. They include B (26 kD), B'(27 k D), and D (13 kD). The principle reactivity has been shown to reside in the B, B’, and D polypeptides (3). Anti-Sm autoantibodies were described originally as precipitating autoantibodies in sera of patients with Systemic Lupus Erythematosus (1). Anti-Sm antibodies are also usually accompanied by antinuclear ribonucleoprotein (nRNP) antibodies (2). The U1 RNP particle has both Sm and RNP binding specificities. The difference is that the RNP particles bound by U2, U4/6 and U5 are bound by anti-Sm autoantibodies, but not by anti-nRNP autoantibodies. Autoantibodies against the Sm antigen precipitate the U1, U2, U4/6, and U5, small nuclear RNAs (4). The Sm antigen is involved in normal post-transcriptional, premessenger RNA processing to excise introns (5). It has been demonstrated that the Sm antigenicity is both RNase and DNase resistant and partia