Primary Structure of a Human Small Nuclear Ribonucleoprotein Polypeptide as Deduced by cDNA Analysis

Anti-Sm is an antibody specificity often associated with the autoimmune disease systemic lupus erythematosus. The polypeptides Sm-B′/B (estimated molecular mass 27 and 26 kDa, respectively) are primary targets of Sm antibodies. Sm-B′/B are part of the core polypeptides of small ribonucleoprotein particles (snRNP) involved in pre-mRNA splicing. Sm-B′/B share the same amino-terminal sequence as we determined by microsequence analyses of the purified polypeptides. Oligonucleotide probes based on that sequence were used to isolate seven clones from a human lymphoblastoid cDNA library in λgt10. The clones contained a single coding region for a protein of approximately 25 kDa. The predicted amino-terminal sequence was identical to that of the isolated Sm-B′/B polypeptides. In vitro translation experiments produced a protein immunoreactive with human polyclonal anti-Sm antibodies. The isolation of only one unique cDNA sequence suggests that Sm-B′/B may be post-translational variants encoded by a single message. The specific structural features which distinguish Sm-B′ from Sm-B have yet to be determined. Northern blot analysis confirmed the diverse tissue and species distribution expected for these immunologically conserved polypeptides. The Sm-B′/B primary sequence is rich in proline (20%) and glycine (15%) residues. The prolines are concentrated in the carboxyl-terminal half of the protein and display a repetitive unit that is shared with other snRNP and nucleic acid binding proteins. Analysis of these arrays suggests an eight residue proline-rich consensus sequence with potential as either an RNA binding domain, or as a site of protein/protein interaction.