Architectures and biochemical activities of Mtl1-Red1 MTREC helicase complexes

RNA surveillance and decay is carried out in part by helicase containing complexes that identify, capture and sometimes modify RNA before delivering it to the RNA exosome complex for processing or degradation. The MTREC core complex includes a Mtr4-like protein (Mtl1) helicase and Red1 that works with other cofactors and the RNA exosome in Schizosaccharomyces pombe to degrade nuclear transcripts in processes that can result in formation of facultative heterochromatin. The activities of Mtl1 remain uncharacterized as do contributions of Red1 to Mtl1 within MTREC. Here, we reconstitute the MTREC core complex, resolve structures by cryo-electron microscopy, and compare MTREC activities to S. pombe Mtr4 and Mtl1. We show that Mtl1 is more active relative to MTREC and Mtr4, that MTREC binds RNA better than Mtl1, and that Red1 includes an autoinhibitory coiled-coil domain that dimerizes MTREC and contacts the Mtl1 RecA domains to disrupt its ATPase active site. Together, these data suggest that Red1 may endow MTREC to bind RNA while slowing translocation so that it remains associated with RNA long enough to chaperone it to the RNA exosome for processing or degradation.