Investigation of the interaction between UPF1 and the THO/TREX complex

Up-frameshift 1 (UPF1) is a central factor in nonsense-mediated mRNA decay (NMD), a well-known quality-control pathway that detects and degrades mRNA molecules containing premature termination codons and in doing so prevents the accumulation of potentially harmful truncated proteins. In addition, UPF1 has been implicated in DNA damage response and telomere maintenance. Seeking to get more insight into the diverse functions of UPF1, we used a combination of stable isotope labeling of amino acids in cell culture (SILAC) experiments to determine by quantitative proteomics novel UPF1 interactors. We used this approach to distinguish between RNA-mediated and protein-mediated UPF1 interactors and to determine proteins that preferentially bind the hypo- or the hyper-phosphorylated form of UPF1. Among other findings, our results indicated that phosphorylated UPF1 interacts with all members of THO complex (THOC), a sub-complex of the transcription export complex (TREX). Validation of this interaction by immunoprecipitations and western blotting is ongoing. Given that current models for NMD postulate UPF1 phosphorylation to be the defining step for NMD activation, a phosphorylation-dependent interaction with the mainly nuclear THOC is unexpected and its investigation may provide novel insights in the phosphorylation-dephosphorylation regulation of UPF1 and its functional consequences.