Heterologous expression and epitope mapping of a human small nuclear ribonucleoprotein-associated Sm-B'/B autoantigen

The U snRNP associated B'/B polypeptides are primary targets of Sm autoantibodies in patients with systemic lupus erythematosus. We have bacterially expressed a Sm-B'/B autoantigen from Raji cells as a fusion with the anthranilate synthase protein from Escherichia coli. The recombinant Sm-B'/B fusion displays comparable immunologic reactivity to the native protein when tested with both monoclonal and polyclonal antibodies. To map Sm-B'/B epitopes, we constructed a series of 12 anthranilate synthase fusions spanning different regions of Sm-B'/B and tested such fusions on immunoblots against a panel of characterized sera. In this manner, we have identified six epitopes, five of which overlap the proline-rich carboxyl-terminus of the protein. Some of these epitopes appear to be conformational. The human sera tested can be divided, according to the epitopes they recognize, into six groups. Finally, we have shown that anti-Sm recognition of the (U1)RNP-specific A protein is attributable to cross-reactivity between the Sm-B'/B and A autoantigens.