RNA decay machines: The exosome

► The eukaryotic exosome is a major ribonuclease for RNA decay and processing. ► The repertoire of catalytic subunits varies among Eukaryota. ► Some catalytic subunits are compartment-specific and give rise to exosome isoforms. ► Substrate threading modulates ribonuclease activities in the holocomplex. ► The many cellular functions of the exosome are mediated by auxiliary factors.