Characterization of the mouse autoantigen La (SS-B). Identification of conserved RNA-binding motifs, a putative ATP binding site and reactivity of recombinant protein with poly(U) and human autoantibodies.

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Abstract
To facilitate the study of autoimmunity to the nuclear Ag La (SS-B) we have isolated and characterized cDNA encoding the mouse La (SS-B) protein. Mouse La (SS-B) protein has 76.7% identity to both human and bovine La (SS-B) proteins and the previously recognized RNP-binding motifs were noted to be highly conserved across the three species. Examination of the primary sequence of human and bovine La (SS-B) newly identifies the site of a potential ATP-binding motif G/AXXXXGK which is preserved in mouse La (SS-B) despite a unique 16 amino acid insertion present in this region of the mouse protein. Analysis of mouse genomic DNA suggests a single gene encodes mouse La (SS-B) and no restriction fragment polymorphisms were identified in the three mouse strains investigated. Two alternate 5' untranslated regions were identified in mouse La (SS-B) cDNA and La (SS-B) mRNA was identified in a wide range of murine tissues consistent with its ubiquitous expression. Recombinant mouse La (SS-B) protein purified from Escherichia coli was shown to bind to poly(U) agarose and to react with sera containing anti-La (SS-B) antibodies obtained from patients with Sjögren's syndrome. The ability to produce recombinant mouse La (SS-B) should provide a valuable reagent for the study of the cellular autoimmune response as well as immunologic tolerance to La (SS-B) Ag in mice.

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