Anti-Sm autoantibodies cross-react with ribosomal protein S10: a common structural unit shared by the small nuclear RNP proteins and the ribosomal protein.

OBJECTIVE

Cross-reactivity of anti-Sm autoantibodies with a certain ribosomal protein has been reported previously. The present study was undertaken to identify the anti-Sm-reactive ribosomal protein, and to characterize the cross-reactive epitope.

METHODS

Two-dimensional gel electrophoresis followed by immunoblotting was used to identify the ribosomal protein (S10) which was reactive with the Y12 anti-Sm monoclonal antibody (MAb). Human anti-Sm antibodies were also tested for cross-reactivity with the Sm-B/B', Sm-D, and isolated S10 proteins by immunoblotting. Epitope analysis was performed by immunoprecipitation of in vitro-translated products of the recombinant S10 and its various mutants.

RESULTS

The Y12 MAb and the affinity-purified human anti-Sm autoantibodies cross-reacted with ribosomal S10 protein. Reactivity of the Y12 MAb with S10 protein was abolished by deletion of 19 amino acids at the carboxyl-terminus of S10, containing the Gly-Arg-Gly sequence motif shared by Sm-B/B' and Sm-D (D1 and D3). Replacements of Arg-158 with Gly and of Arg-158/Arg-160 with Gly/Gly at the carboxyl-terminal 157-Gly-Arg-Gly-Arg-Gly region disrupted the Y12 MAb recognition.

CONCLUSION

At least a part of human anti-Sm antibodies and Y12 MAb show cross-reactivity among Sm-B/B', Sm-D, and ribosomal protein S10. The carboxyl-terminal Gly-Arg-Gly region of S10 protein is involved in constructing the cross-reactive epitope. This demonstrates that a common structural feature is shared by the ribosomal protein and the small nuclear RNP proteins.