Expression and DNA binding of the human 52 kDa Ro/SSA autoantigen

The 52 kDa Ro/SSA protein is an intracellular autoantigen that is frequently recognized by antibodies in sera of patients with systemic lupus erythematosus or Sjogren's syndrome. While the function of this molecule is not known, zinc finger and leucine zipper motifs have been identified in its predicted amino acid sequence which suggest that it may interact with nucleic acids. To test this hypothesis, the human gene which encodes this protein was cloned in a baculovirus and expressed in Spodoptera frugipoda cells. Extracts from these infected insect cells were used as a source of protein for this study. The protein is similar in size and antigenicity to that expressed in human cells. This protein binds to DNA at physiological temperature and is eluted with high concentrations of sodium chloride. Striking similarities were found between the sequence in, and adjacent to, the nucleic acid-binding motifs of 52 kDa Ro/SSA and a growing family of zinc finger proteins which have been shown to bind to DNA or regulate gene expression. The findings presented here place this protein structurally and functionally in this family and demonstrate a biochemical assay which can be used to study its function.