Ubiquitin chains bearing genetically encoded photo-crosslinkers enable efficient covalent capture of (poly)ubiquitin-binding domains.

Ubiquitin-mediated signaling pathways regulate essentially every aspect of cell biology in eukaryotes. Ubiquitin receptors typically contain ubiquitin-binding domains (UBDs) that have the ability to recognize monomeric ubiquitin (Ub) and polymeric Ub (polyUb) chains. However, how signaling specificity is achieved remains poorly understood, and many of the UBDs that selectively recognize polyUb chains of particular linkages still need to be identified and characterized. Here we report the incorporation of a genetically encoded photo-cross-linker, p-benzoyl-L-phenylalanine (Bpa), into recombinant Ub and enzymatically synthesized polyUb chains. This allows photo-cross-linking (covalent bond formation) of monoUb and K48- and K63-linked diUb chains to UBDs. This approach provides a framework for understanding Ub cellular signaling through the capture and identification of (poly)Ub-binding proteins.