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

Abstract

In Saccharomyces 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 DEADbox 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.

Glucose stress causes mRNA retention in nuclear Nab2 condensates

“…However, the local ATPase activation through its interaction partner Gle1, which exclusively resides at the NPC, could target Dbp5’s activity largely towards specific RNPs such as Nab2-bound mRNAs that pass the NPC during export. In line with this, Dbp5- ΔN Nup159 localization has been shown to be sufficient for viability (Adams and Wente, 2020), suggesting that the essential role of Dbp5 is restricted to the NPC.…”

“…We next wanted to examine whether Nab2 focus formation is triggered by the loss of Dbp5 from the NPC during long-term stress. Intriguingly, when Dbp5 is prevented to leave the NPC, by using a strain in which Dbp5 is permanently fused to its interaction partner Nup159 (‘Dbp5 tether’, Dbp5- ΔN Nup159, (Adams and Wente, 2020)), Nab2 focus formation is strongly reduced and the mobility of Nab2 is only mildly affected in long-term glucose withdrawal (Fig. 3I-K and S3I).…”

The Great Escape: mRNA Export through the Nuclear Pore Complex

“…The role of the basket in quality control is proven by experiments where deletion of the yeast basket protein Mlp1 causes leakage of immature mRNAs into the cytoplasm [ 100 ]. This interplay between the mRNP and the basket is strengthened by the observation that, while on the nucleoplasmic side, Nab2 can interact directly with Mex67 [ 101 ]. Moreover, Yra1 enhances the interaction between Nab2 and Mex67, and is dispensable if Nab2 or Mex67 are overexpressed [ 86 ].…”

“…Moreover, Yra1 enhances the interaction between Nab2 and Mex67, and is dispensable if Nab2 or Mex67 are overexpressed [ 86 ]. Recent evidences suggest that in yeast, Nab2 is removed from the exported transcript, together with Mex67/MTR2 [ 101 ]. Nab2 is therefore a likely adaptor for Mex67/MTR2, and strengthens the body of data that shapes Yra1 as a crucial cofactor stabilizing the export-competent mRNP in multiple ways.…”

“…Removal of Mex67/MTR2 prevents re-entry of the mRNP in the nucleus, avoiding futile transport cycles, and increases Mex67/MTR2 availability to transport of a new mRNP. At the same time, in yeast, Nab2 is released as well [ 101 ] ( Figure 4 C). Importantly, Dbp5/DDX19 localizes stably to the cytoplasmic side of the NPC, and is part of a wider mRNA export platform that also requires scGle1/hGLE1, scNup42/hNUP42, scNup159/hNUP214, scNup116/hNUP98, and scGle2/hRAE182.…”