The Zn2+Binding Domain of the Human Ro 52 kDa Protein is a Target for Conformation-dependent Autoantibodies

Abstract

The Ro 52kDa protein was originally identified using autoimmune sera directed against the Ro/SS-A antigen, also known as Ro ribonucleoprotein particles (Ro RNPs). In most human cells these consist of one of four small human cytoplasmic (hY) RNA molecules complexed with the Ro 60kDa protein. This protein interacts directly with the hY RNAs, whereas the Ro 52kDa protein is thought to associate via protein–protein interactions. The Ro RNPs are present in all mammalian cells, but their intracellular location and function remain unclear.The primary structure shows that the Ro 52kDa protein is a member of a small family of proteins sharing two features, a leucine zipper region and an aminoterminal cysteine–histidine rich region with two different putative zinc finger motifs.To study if the Ro 52kDa protein actually binds Zn2+, both the full-length Ro 52kDa clone and various subclones were expressed as recombinant proteins and assayed for Zn2+bindingin vitro. A fragment containing the first cysteine–histidine cluster at residues 14–54 and other larger overlapping fragments were found to bind Zn2+.In this report we also demonstrate that the Zn2+binding domain is a target for conformation-dependent anti-Ro 52kDa autoantibodies. The antigenicity is dramatically increased by the same reducing conditions that promote Zn2+binding. This is in contrast to the previously described immunodominant Ro 52kDa domain.