The Yeast Exosome
The eukaryotic core exosome (CE) is a conserved nine-subunit protein complex important for 3' end trimming and degradation of RNA. In yeast, the Rrp44 protein constitutively associates with the CE and provides the sole source of processive 3'-to-5' exoribonuclease activity. We have obtained EM reconstructions of the core and Rrp44-bound exosome complexes and used bioinformatics tools and docking to generate a pseudo-atomic model of most of the complex (Wang et al. 2007, PNAS 104, 16844-9). The two-lobed Rrp44 protein binds to the RNase PH domain side of the exosome and buttresses the bottom of the exosome-processing chamber. The Rpc44 C-terminal body part containing an RNase II-type active site is anchored to the exosome through a conserved set of interactions mainly to the Rrp45 and Rrp43 subunit, whereas the the Rrp44 N-terminal head part is anchored to the Rrp41 subunit and may function as a roadblock to restrict access of RNA to the active site in the body region. The Rrp44-exosome (RE architecture suggests an active site sequestration mechanism for strict control of 3' exoribonuclease activity in the RE complex.