The Ro 60 KDa Autoantigen Comes into Focus: Interpreting Epitope Mapping Experiments on the Basis of Structure

A conserved RNA-binding protein, the Ro 60 kDa (Ro60) autoantigen, is a major target of autoantibodies in patients suffering from the rheumatic diseases Sjogren's syndrome, systemic lupus erythematosus, subacute cutaneous lupus erythematosus and neonatal lupus erythematosus. In both mice and certain bacteria, Ro60 is important for cell survival following ultraviolet irradiation. Although the function of Ro60 was mysterious for many years, recent experiments have demonstrated that this protein binds misfolded noncoding RNAs in vertebrate cells and likely functions in a pathway by which defective RNAs are recognized and targeted for degradation. Recent structural studies have revealed that Ro60 is shaped like a doughnut with an inner hole. Noncoding RNAs called Y RNAs bind on the outer surface of the ring, while the single-stranded ends of misfolded RNAs likely bind within the hole. Comparison of the Ro60 structure with the results of epitope-mapping studies reveals that many of the currently identified epitopes recognized by patient sera overlap regions of Ro60 that function in RNA binding. Moreover, in some patients with anti-Ro60 antibodies, the initial antigenic epitope corresponds to a loop involved in binding single-stranded RNA in the central cavity.