Placental Sa antigen is citrullinated vimentin. Three identical immunoblots containing semipurified placental Sa antigen (100 μg; lanes 1, 4, 7), human recombinant vimentin (Vim) (50 ng; lanes 2, 5, 8), and human recombinant vimentin that had been citrullinated in vitro (cit-Vim) (50 ng; lanes 3, 6, 9) were stained with either anti-Sa reference serum (left panel), anti-modified citrulline (anti-MC) antibodies (middle panel), or antivimentin (anti-Vim) antibodies (right panel). Sa antigen is recognized both by anti-MC antibodies and by anti-Vim antibodies (lanes 4 and 7, respectively), indicating that the antigen is indeed citrullinated vimentin. Citrullinated vimentin was recognized by the anti-Sa serum, whereas unmodified vimentin was not (lanes 3 and 2, respectively), indicating that the presence of citrulline residues is essential for the autoantigenicity of Sa. Molecular weight markers are indicated on the left.