Dbp5 associates with RNA-bound Mex67 and Nab2 and its localization at the nuclear pore complex is sufficient for mRNP export and cell viability

InSaccharomyces cerevisiae, the mRNA export receptor Mex67 is recruited to mature nuclear transcripts to mediate mRNA export through the nuclear pore complex (NPC) to the cytoplasm. Mex67 binds transcripts through adaptor proteins such as the poly(A) binding protein Nab2. When a transcript reaches the cytoplasmic face of the NPC, the DEAD-box protein Dbp5 acts to induce a local structural change to release Nab2 and Mex67 in an essential process termed mRNP remodeling. It is unknown how certain proteins (Nab2, Mex67) are released during Dbp5-mediated mRNP remodeling, whereas others remain associated. Here, we demonstrate that Dbp5 associates in close proximity with Mex67 and Nab2 in a cellular complex. Further, fusion of Dbp5 to Nup159 anchors Dbp5 at the cytoplasmic face of the NPC and is sufficient for cell viability. Thus, we speculate that the essential role of Dbp5 in remodeling exporting mRNPs requires its localization to the NPC and is separable from other subcellular functions of Dbp5. This work supports a model where the diverse nuclear, cytoplasmic and NPC functions of Dbp5 in the mRNA lifecycle are not interdependent and that Dbp5 is locally recruited through complex protein-protein interactions to select regions of transcripts for specific removal of transport proteins at the NPC. Author summary An mRNA's progress through the gene expression pathway is dictated by its subcellular localization and collection of proteins bound to it. At each stage in the mRNA lifecycle, members of the DEAD-box protein (Dbp) family displace specific proteins from the mRNA-protein complex (mRNP) by generating kinks in the local RNA structure, a process termed remodeling. Dbp5 is a uniquely multifunctional regulator of mRNPs, with roles in transcription, export from the nucleus, and translation. The most well-understood of these functions is during mRNA export, where Dbp5 releases the transport proteins that mediate export through the nuclear pore complex (NPC). It was not known whether this function at the NPC is linked to Dbp5 actions in the nucleus, or how Dbp5 targets a specific protein for removal. Based on the results of a combination of genetic and cellular experiments, we speculate that Dbp5 remodels mRNPs at the NPC independently of its nuclear function, indicating separability of these functions. Furthermore, our data support a model wherein Dbp5 is locally recruited at the NPC through protein-protein interactions to select regions of transcripts for specific removal of transport proteins. These studies further elucidate the function of Dbp5, a regulator of mRNA fate during mRNA export and an important transition in the mRNA lifecycle.