Orthogonal ubiquitin transfer identifies ubiquitination substrates under differential control by the two ubiquitin enzymes

Protein ubiquitination is mediated sequentially by ubiquitin activating enzyme E1, ubiquitin conjugating enzyme E2, and ubiquitin ligase E3. Uba1 was thought to be the only E1 until the recent identification of Uba6 as an alternative. To differentiate the biological functions of Uba1 and Uba6, we applied a novel orthogonal ubiquitin transfer (OUT) technology to profile their ubiquitination targets in mammalian cells. By expressing pairs of an engineered ubiquitin and engineered Uba1 or Uba6 that were generated for exclusive interactions, we identified 697 Uba6 targets and 529 Uba1 targets with 258 overlaps. Bioinformatics revealed substantial differences in pathways involving Uba1- and Uba6-specific targets. We demonstrated that polyubiquitination and proteasomal degradation of ezrin and CUGBP1 require Uba6, but not Uba1, and Uba6 is involved in the control of ezrin localization and epithelial morphogenesis. These data reveal the distinctive substrate pools for Uba1 and Uba6 and manifest non-redundant biological roles for Uba6