Abstract
Monitoring Editor: Thomas Sommer The ubiquitin-associated (UBA) domain of the mRNA nuclear export receptor Mex67 helps coordinating transcription elongation and nuclear export by interacting both with ubiquitin conjugates and specific targets, such as Hpr1, a component of the THO complex. Here we analyzed substrate specificity and ubiquitin selectivity of the Mex67 UBA domain. UBA-Mex67 is formed by three helices arranged in a classical UBA fold plus a fourth helix, H4. Deletion or mutation of helix H4 strengthens the interaction between UBA-Mex67 and ubiquitin, but decreases its affinity for Hpr1. Interaction with Hpr1 is required for Mex67 UBA domain to bind poly-ubiquitin, possibly by inducing an H4-dependent conformational change. In vivo, deletion of helix H4 reduces cotranscriptional recruitment of Mex67 on activated genes and also shows a mRNA export defect. Based on these results, we propose that H4 functions as a molecular switch that coordinates the interaction of Mex67 with ubiquitin bound to specific substrates, defines the selectivity of the Mex67 UBA domain for poly-ubiquitin and prevents its binding to nonspecific substrates.