The Molecular Architecture of the TRAMP Complex Reveals the Organization and Interplay of Its Two Catalytic Activities

The TRAMP complex is involved in the nuclear sur-veillance and turnover of noncoding RNAs and in-tergenic transcripts. TRAMP is associated with thenuclear exosome and consists of a poly(A)polymer-ase subcomplex (Trf4-Air2) and a helicase (Mtr4).We found that N-terminal low-complexity regions ofTrf4 and Air2 bind Mtr4 in a cooperative manner.The 2.4 A˚ resolution crystal structure of the corre-sponding ternary complex reveals how Trf4 andAir2 wrap around the DExH core of the helicase.Structure-based mutations on the DExH core impairbinding to Trf4 and Air2, and also to Trf5 and Air1.The combination of structural, biochemical, and bio-physical data suggests that the poly(A)polymerasecore of Trf4-Air2 is positioned below the base ofthe helicase, where the unwound 3