Molecular relationships between U snRNP proteins as investigated by rabbit antisera and peptide mapping

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Abstract
Each of the major U snRNP polypeptides from human cells was purified by electroelution from SDS-polyacrylamide gels. Rabbit antisera could be obtained against the individual proteins 70K, A, B', B and D, although rabbits failed to elicit antibodies against E, F and G. A strong structural horaology was found between proteins B' and B, against which patients with connective tissue diseases produce predominantly anti-Sm autoantibodies. Thus, rabbit antisera against B' strongly crossreact with B and vice versa. Peptide patterns of the proteins B' and B obtained with chymotrypsin are identical with the exception of one fragment in each case. Polypeptide D, the third major Sm-antigenic protein, is structurally distinct from B' and B, as evidenced by the failure of anti-D antisera to crossreact with B' or B and vice versa, as well as by the different peptide patterns observed for proteins D and B' or B. The Ul specific polypeptide A and the U2 specific polypeptide B' share homologous regions, as indicated by the crossreactivity of anti-A antisera with protein B', and the occurrence of common fragments in the peptide patterns of the two proteins. Further homologies between other snRNP protein pairs were not detected.