To analyze the evolutionary stability of the Sm B polypeptides, the cDNA nucleotide (nt) sequence was derived for the Drosophila melanogaster Sm B polypeptide and compared to the cDNAs encoding human and murine Sm B. The three cDNAs were transcribed and translated in reticulocyte lysates followed by analysis of the synthesized proteins by SDS-PAGE. D. melanogaster B migrated at approximately 25 kDa, in comparison to 28 kDa for the murine and human B proteins, although all three proteins were immunoprecipitated by human anti-Sm autoantibodies and by the Y12 anti-Sm murine monoclonal antibody (Y12 mAb). Immunoblots and immunoprecipitations of [35S]methionine-labeled D. melanogaster S2/M3 cells confirmed the smaller size of the D. melanogaster protein, and revealed that B' was absent in this cell line, as in murine cells. In comparison to the 231 amino acids (aa) of human and murine B, the deduced sequence for the D. melanogaster clone was 199 aa (predicted M(r) of 24,598) with two 5-aa deletions and a 19-aa truncation at the 3' end, compared to the other two clones. D. melanogaster protein B shared 65% aa sequence identity with the human and mouse clones, and 80% similarity when conservative aa substitutions were noted. The C-terminal portion of the D. melanogaster protein was the most evolutionarily variable in comparison to the deduced aa sequences for the other two proteins; however, autoantigenic epitopes bound by human anti-Sm antibodies and the Y12 mAb in this region of the protein were conserved across species lines.(ABSTRACT TRUNCATED AT 250 WORDS)