Twins take the job.

The exosome, one of the main cellular ribonucleases in eukaryotes, is a multi-subunit complex of deep evolutionary origin. This complex has been extensively characterized in Saccharomyces cerevisae. Given the good conservation of exosome subunit sequences, it was widely accepted that the yeast exosome provided a good model for other eukaryotic species. Recent analysis of higher eukaryotic exosomes reveals some differences challenging this idea. In this issue of The EMBO Journal, Tomecki et al (2010) and Staals et al (2010) provide new insights into the composition and function of the human exosome, with implications for our understanding on the organization, function and localization of this complex in these cells. The exosome is responsible for many RNA processing and degradation reactions in the nucleus and cytoplasm (Lykke-Andersen et al, 2009). Yeast exosome is composed of a nine subunit ring structure to which Dis3 binds to form the exosome core. Additional co-factors associate to the exosome core in the nucleus and the cytoplasm. Of the 10 core subunits, only Dis3 possesses enzymatic activity with an RNB domain acting as exoribonuclease and a PIN domain