A novel 43-kDa glycoprotein is detected in the nucleus of mammalian cells by autoantibodies from dogs with autoimmune disorders

We have characterized a new antibody specificity in a panel of sera from dogs developing systemic lupus erythematosus (SLE) or clinically related autoimmune disorders. This antibody stains in a speckled fashion the nucleus of cells of different mammalian origins. The target antigen is a basic (pI 9.2) nuclear polypeptide with an apparent molecular weight of 43 kDa (p43) which is detected in various mammalian cell nuclei. p43, as studied in HeLa cells, appears to be cell cycle-independent. It is released from nuclei by salts (0.5 M NaCl or 0.25 M ammonium sulfate). Upon subfractionation of nuclear components, p43 is found in the fraction containing HnRNPs and is recovered in immunoprecipitates obtained with 4F4 monoclonal antibody to HnRNP C proteins. Immunoelectron microscopy revealed that p43 is concentrated over the dense chromatin periphery and interchromatin granule clusters. Another important feature of p43 is its ability to specifically bind wheat germ agglutinin lectin but not concanavalin A nor Ulex europaeus I, supporting the notion that p43 is a glycoprotein bearing an N-acetyl-glucosamine moiety. Consistent with this result, a radio-active p43 band is specifically immunoprecipitated by canine anti-p43 autoantibodies from HeLa cells metabolically labeled with 14C]glucosamine. Finally, anti-p43 antibodies do not immunoprecipitate SnRNA, indicating that p43 has no apparent association with SnRNPs.