Antibodies to small nuclear RNAs complexed with proteins are produced by patients with systemic lupus erythematosus

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Abstract
Patients with systemic lupus erythematosus often possessed antibodies against 2 nuclear antigens called Sm and RNP (ribonucleoprotein). The molecular identity of these antigens was established by analyzing immune precipitates of nuclear extracts from mouse Ehrlich ascites cells labeled with 32P and 35S. Anti-Sm serum selectively precipitated 6 small nuclear RNA molecules (snRNA); anti-RNP serum reacted with only 2 of these; and a 3rd serum, characterized as mostly anti-RNP, precipitated a subset of 3 snRNA bands. Three of the 6 RNA were identified by fingerprint analysis as the previously characterized and highly abundant nucleoplasmic snRNA species U1a (171 nucleotides), U1b and U2 (196 nucleotides). The other 3 RNA (U4, U5 and U6) likewise were uridine rich and contained modified nucleotides, but they were smaller, with lengths of about 145, 120 and 95 residues, respectively. Each of the 6 snRNA was complexed with and apparently antigenic by virtue of association with specific proteins. All 3 sera precipitated an identical complement of 7 different polypeptides ranging in MW from 12,000-35,000; these proteins were abundant in nuclear extracts, but were neither histones nor the major polypeptides comprising the 30S heterogenous nuclear RNP particles of mammalian nuclei. Each of the 6 snRNA apparently exists in a separate small nuclear ribonucleoprotein (snRNP) complex with a total MW of about 175,000. Human antisera also precipitated snRNA from a wide range of vertebrate species and from arthropods. The antigenic snRNP was discussed in relation to the published literature on snRNA and nuclear RNP and possible functions of snRNP in nuclear processes were considered.