The yeast Sac3:Cdc31:Sus1:Thp1 (TREX-2) complex facilitates the repositioning and association of actively transcribing genes with nuclear pores (NPCs)-"gene gating"-that is central to integrating transcription, processing, and mRNA nuclear export. We present here the crystal structure of Sus1 and Cdc31 bound to a central region of Sac3 (the CID domain) that is crucial for its function. Sac3CID forms a long, gently undulating α helix around which one Cdc31 and two Sus1 chains are wrapped. Sus1 has an articulated helical hairpin fold that facilitates its wrapping around Sac3. In vivo studies using engineered mutations that selectively disrupted binding of individual chains to Sac3 indicated that Sus1 and Cdc31 function synergistically to promote NPC association of TREX-2 and mRNA nuclear export. These data indicate Sac3CID provides a scaffold within TREX-2 to integrate interactions between protein complexes to facilitate the coupling of transcription and mRNA export during gene expression.
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We are most grateful to our colleagues in Cambridge and Heidelberg, especially Susanna Rodriguez-Navarro, Roger Williams, Olga Perisic, and Ilka Müller for their guidance, assistance, and criticisms, and Maren Schneider for technical assistance. We also thank D. Nurizzo and M. Bowler for assistance at the European Synchrotron Radiation Facility, the group of J. Lechner for mass spectrometry, and E. Schiebel for providing Cdc31 antibody. D.J. holds a Medical Research Council postgraduate studentship, and A.M.E. holds a Marie Curie Fellowship. This work was supported in part by a programme grant to M.S. from the Wellcome Trust and grants from the Deutsche Forschungsgemeinschaft (DFG; SFB638, B3) to E.H.