Structural analysis of the yeast Dhh1–Pat1 complex reveals how Dhh1 engages Pat1, Edc3 and RNA in mutually exclusive interactions

Translational repression and deadenylation of eukaryotic mRNAs result either in the sequestration of the transcripts in a nontranslatable pool or in their degradation. Removal of the 5' cap structure is a crucial step that commits deadenylated mRNAs to 5'-to-3' degradation. Pat1, Edc3 and the DEAD-box protein Dhh1 are evolutionary conserved factors known to participate in both translational repression and decapping, but their interplay is currently unclear. We report the 2.8 Å resolution structure of yeast Dhh1 bound to the N-terminal domain of Pat1. The structure shows how Pat1 wraps around the C-terminal RecA domain of Dhh1, docking onto the Phe-Asp-Phe (FDF) binding site. The FDF-binding site of Dhh1 also recognizes Edc3, revealing why the binding of Pat1 and Edc3 on Dhh1 are mutually exclusive events. Using co-immunoprecipitation assays and structure-based mutants, we demonstrate that the mode of Dhh1-Pat1 recognition is conserved in humans. Pat1 and Edc3 also interfere and compete with the RNA-binding properties of Dhh1. Mapping the RNA-binding sites on Dhh1 with a crosslinking-mass spectrometry approach shows a large RNA-binding surface around the C-terminal RecA domain, including the FDF-binding pocket. The results suggest a model for how Dhh1-containing messenger ribonucleoprotein particles might be remodeled upon Pat1 and Edc3 binding.