Conserved mRNP remodeling mechanism of the TREX-2.1 complex.

Processing, packaging, and nuclear export of messenger ribonucleoprotein particles (mRNPs) are critical for eukaryotic gene expression, with the DEAD-box ATPase DDX39B (yeast Sub2) playing a central role in mRNP processing and remodeling. Our recent studies identified human TREX-2 (GANP•PCID2•DSS1), yeast TREX-2 (Sac3•Thp1•Sem1), and a related human TREX-2.1 complex (LENG8•PCID2•DSS1) as key regulators of DDX39B/Sub2. Here, we characterize the yeast TREX-2.1 (scTREX-2.1) complex, composed of Thp3, Csn12, and Sem1. We show that the scTREX-2.1 complex directly interacts with Sub2 and co-occupies a fraction of CBC-containing mRNPs with Sub2. Using cryo-electron microscopy , we determined the structure of scTREX-2.1 bound to Sub2, revealing a conserved "trigger loop" mechanism by which scTREX-2.1 regulates Sub2 activity. Functional assays show that disruption of scTREX-2.1 leads to the accumulation of intron-containing pre-mRNAs. These findings uncover a conserved mechanism from yeast to humans by which TREX-2 and TREX-2.1 complexes regulate Sub2/DDX39B during nuclear mRNP maturation, providing insights into the coordination of mRNP remodeling and processing prior to nuclear export.