Molecular cloning of cDNA encoding Sm autoantigen: derivation of a cDNA for a B polypeptide of the U series of small nuclear ribonucleoprotein particles.

Abstract

The Sm snRNPs play a central role in the processing of pre-mRNA. Anti-Sm antibodies, the diagnostic hallmark of systemic lupus erythematosus, target the B/B and D polypeptides of these snRNPs. We have used patient autoantibodies to clone a cDNA from a human fibroblast cDNA library that encodes the full length of a polypeptide identical with, or closely related to, polypeptide B. This cDNA is comprised of 1139 bases and contains an open reading frame of 855 nucleotides that is capable of encoding 285 amino acids. The first 223 amino acids at the NH2 terminus exhibit nearly complete homology with polypeptide N, a newly recognized brain-and heart-specific component of Sm snRNPs. The derived amino acid sequence for B differs from that of the N polypeptide primarily by a 50-amino acid insert 12 residues upstream from the homologous COOH termini of these polypeptides. The structural differences in these cDNAs for B and N may regulate tissue-specific alternative splicing mechanisms for mRNA. In addition, these clones make it possible to map in fine detail the most characteristic autoimmune responses of systemic lupus erythematosus.The Sm group of small nuclear ribonucleoproteins (the U1, U2, U5, and U4/U6 snRNPs) have a central role in the splicing of pre-mRNA (1-3). These particles are also dominant targets for autoimmune responses in systemic lupus erythematosus (SLE) (4,5). For example, previous studies have demonstrated that anti-Sm autoantibodies, which are diagnostic markers for SLE (6), are directed against the B/B and D polypeptides of these particles (7). Similarly, anti-Ul RNP autoantibodies, which occur in SLE and other connective tissue diseases, are directed against the 70-kDa, A, B/B, and C polypeptides of the U1 snRNP (7-10). The other U1 snRNP polypeptides known as E, F, and G are rarely targets for autoimmune responses. These studies have made it clear that the B/B polypeptides are pivotal in the autoimmune responses of SLE.Recent studies have provided some insights into the structure of the B polypeptides and their relationship to the Sm snRNPs. Only a single B polypeptide is apparent in rodent tumor cell lines, whereas human cells contain the B/B doublet described above (11-13). These latter polypeptides are closely related and possibly represent variants of a single protein (14). In studies based on immunoblots, anti-Sm antibodies were noted to recognize a polypeptide in rat brain tissue referred to as N, which appeared slightly larger than polypeptide B' in NaDodSO4 polyacrylamide gels. Subsequent studies showed that N is an Sm snRNP component specific to brain and heart tissue of both humans and rodents (15). It is currently thought that such unique snRNP polypeptides might provide a basis for tissue-specific mRNA splicing mechanisms (15). Other studies have shown that the B7/B polypeptides likely associate with the U-series small RNAs indirectly through binding to the D, E, F, and G polypeptides (16).In the present study we have cloned and sequenced the entire length of the cDNA that enco...

A conserved idiotype and antibodies to retroviral proteins in systemic lupus erythematosus.

“…In support of this hypothesis is the deduced amino acid sequence of the B/B' subunit of Sm and HIV-1 gag polypeptide. Both molecules contain several regions rich in proline residues (11,12,13), a potential site for cross-reactivity.…”

Electron microscopy of assembly intermediates of the snRNP core: morphological similarities between the RNA-free (E.F.G) protein heteromer and the intact snRNP core

“…Proteins were extracted by incubation of the snRNPs with DE53 ion-exchange resin in the presence of EDTA. The molecular masses of the proteins derived from their cDNA sequences are 31.3 (A), 28.5 (A'), 25.5 (B"), 24.6 (B'), 23.7 (B), 13.4 (D1), 13.5 (D2), 13.9 (D3), 10.8 (E), 9.7 (F) and 8.5 kDa (G) Hermann et al, 1995;Lehmeier et al, 1994;Ohosone et al, 1989;Rokeach et al, 1988;Sillekens et al, 1987Sillekens et al, , 1989Sharpe et al, 1989;Stanford et al, 1988;van Dam et al, 1989).…”

Determination of Protein-Protein Interactions of ICln by the Yeast Two-Hybrid System

“…Human SnRNP core protein SmB/B Another clone obtained by the two-hybrid system is identical to the human SnRNP core protein SmB/B [31]. As for the SmD1 protein, Pu et al also demonstrated an interaction of in vitro translated SmB/B with ICln Xenopus [14].…”