The mRNA export machinery requires the novel Sac3p–Thp1p complex to dock at the nucleoplasmic entrance of the nuclear pores

Yra1p and Sub2p are components of the TREX complex, which couples transcription elongation with nuclear export of mRNAs. Here, we report a genetic interaction between Yra1p and a conserved protein Sac3p, which previously was found to interact with Sub2p. In vivo, Sac3p forms a stable complex with Thp1p, which was reported to function in transcription elongation. In addition, Sac3p binds to the mRNA exporter Mex67p–Mtr2p and requires the nucleoporin Nup1p to dock at the nuclear side of the nuclear pore complex (NPC). Significantly, mutations in Sac3p or Thp1p lead to strong mRNA export defects. Taken together, our data suggest that the novel Sac3p–Thp1p complex functions by docking the mRNP to specific nucleoporins at the nuclear entrance of the NPC.