Antibody recognition and RNA binding of a neuronal nuclear autoantigen associated with paraneoplastic neurological syndromes and small cell lung carcinoma.

The PLE21/HuC neural protein is an autoantigen for anti-neuronal nuclear autoantibodies (ANNA-1/anti-Hu/Type IIa antibodies) from a patient with paraneoplastic limbic encephalomyelitis and small cell lung carcinoma. This antigen belongs to the Hu/ELAV-like protein family, contains three RNA recognition motifs (RRMs) and has RNA binding capacity. In many autoimmune diseases mediated by autoantibodies, antibodies often interfere with the biological functions of their target antigens. To investigate the influences of the autoantibodies on the biological function of the antigen, we mapped the regions which were required for the antibody recognition and for the RNA binding. Deletion analysis of the antigen revealed that the epitopes for the antibodies were localized in the regions of 12 residues, amino acids 161-172, and eight residues, amino acids 29-38, of the first and second RRMs. It was also shown that the eight residues, amino acids 29-38, and the 10 residues, amino acids 187-194, were required for the RNA binding. Although amino acids 29-38 were necessary for both the antibody recognition and the RNA bindings, pre-incubation of the PLE21 antigen with the antibodies did not inhibit the formation of the complex of PLE21, the antibodies and RNA. Thus, the regions required for the antibody recognition are not identical with those for the RNA binding, and it seems unlikely that the autoantibodies interfere with RNA binding of the antigen.