Molecular basis for the interaction between Saccharomyces cerevisiae Rtt103 and the Rat1-Rai1 complex

The Rat1 5'-3' exoribonuclease together with its partner Rai1 have important roles in Saccharomyces cerevisiae RNA polymerase II transcription termination. Rtt103 copurifies with Rat1-Rai1 in S. cerevisiae, but its mechanism of interaction with them is not known. We report here the cryo-EM structure of the S. cerevisiae Rat1-Rai1-Rtt103 ternary complex at 2.9 Å resolution. We found that a short segment of Rtt103 is in close contact with Rai1, while the rest of Rtt103, including its RNA polymerase II C-terminal domain interaction domain, shows no interactions with Rai1 or Rat1. This is in contrast to the observations on the Komagataella phaffii Rat1-Rai1-Rtt103 complex, where only the RNA polymerase II C-terminal domain interaction domain of Rtt103 has contacts with Rai1. Our structure reveals that S. cerevisiae Rtt103 Pro261 and Tyr263 have important contacts with Rai1, and we show that the P261G/Y263A mutation of Rtt103 blocks the interaction with Rat1-Rai1. Our structure suggests that, in yeast, this segment of Rtt103, which we have named the Rai1 interaction segment, likely helps the recruitment of Rat1-Rai1 to RNA polymerase II for termination.