Structures of S. pombe Lsm2–8 bound to pentauridylate with a 2′,3′ OH or 2′,3′ cyclic phosphate. (A) Overview of Lsm2–8 bound to “unprocessed” U6 snRNA 3′ terminus. (B) Overview of Lsm2–8 bound to “mature” U6 snRNA 3′ terminus. (C) Detail of Lsm2–8 interface with “unprocessed” RNA. The 3′ terminal uridine is disordered and thus not visible in the final electron density maps. (D) In contrast, the mature U6 3′ end, with a 2′,3′ cyclic phosphate, shows electron density for the terminal nucleotide. (E) The Sm-like pocket in Lsm3 binds RNA as observed previously in other Lsm2–8 complexes from S. cerevisiae. (F) In contrast, the 3′ uridine cyclic phosphate has a unique binding mechanism relative to the other four uridines, including a stacking interaction with the carboxy-terminal histidine of Lsm8.
