CYTOPLASMIC DETECTION OF A NOVEL PROTEIN CONTAINING A NUCLEAR-LOCALIZATION SEQUENCE BY HUMAN AUTOANTIBODIES

A great diversity of antibodies directed to cell proteins has been des cribed in sera of patients with autoimmune diseases. Most of these ser a recognize nuclear components, but some others are directed against c ytoplasmic autoantigens. Some of the antibodies directed to cytoplasmi c autoantigens are well characterized, such as anti-mitochrondial, ant i-ribosomal, anti-microsomal and anti-Golgi complex autoantibodies, bu t the target of many others remains unknown. In the last 5 years we ha ve selected 32 sera with a characteristic speckled cytoplasmic pattern in indirect immunofluorescence (IIF) assay among a total of more than 31 000 sera from patients with any kind of autoimmune manifestation w ho attend our Connective Tissue Disease Clinic. Using a human cDNA exp ression library, we have identified a new autoantibody specificity nam ed RCD-8 in five of these sera, directed to one cytoplasmic autoantige n. Affinity-purified antibodies eluted from a positive clone reproduce d the same IIF cytoplasmic staining pattern as native serum and reacte d with one single band of 160 kD on an immunoblot of HeLa cell extract . The sequence was found homologous to an autoantigen recently reporte d named Ge-1, and contains a nuclear localization sequence (NLS), an a ctive protein domain made by a contiguous stretch of amino acids which allows the selective entry of the protein into the nucleus. The five patients whose sera exhibited this new autoantibody specificity displa yed different autoimmune pathological profiles.