The human exosome and PNPase. Molecular machines for controlled RNA degradation

Abstract

The human exosome is a multi-protein complex present in all eukaryotic cells, which is involved in the processing and degradation of a large variety of RNA molecules. In patients suffering from an overlap syndrome of the autoimmune diseases polymyositis (PM) and scleroderma (Scl), autoantibodies are found that are reactive with protein subunits of this complex. These antibodies are referred to as anti-PM/Scl antibodies. The goal of this study was to characterize in more detail the composition and structure of the human exosome and to gain more insight in the reason why this complex is targeted by auto-antibodies in patients with the PM/Scl overlap syndrome. In order to analyze the structure of the exosome complex, a systematic analysis of interactions between human exosome subunits was performed. Together with the identification of an interesting homology between the exosome and the bacterial protein polyribonucleotide phosphorylase (PNPase), this allowed the construction of a model for the exosome complex, the main feature of which is its ring-shaped structure. In addition, a human homologue of the bacterial PNPase protein is described, which localizes specifically in the mitochondria. The evolutionary relationships between the exosome complex and both prokaryotic and eukaryotic PNPases are discussed. Finally, one of the exosome subunits, PM/Scl-75, is shown to be a significantly longer protein than was believed until recently. This protein was found to be a substrate for caspase-mediated cleavage in apoptotic cells, suggesting a role for the protein in the generation of patient autoantibodies that recognize the exosome complex. In addition, it is shown that the autoantibodies from patients with the PM/Scl overlap syndrome are mainly targeted to this longer form of the PM/Scl-75 protein, whereas until now another exosome subunit, PM/Scl-100, was believed to be the main autoantigen of the complex.