D58 is a residue that has been identified as being important for binding and recognition by proteins that contain ubiquitin binding domains (UBDs), and represents a new hydrophilic interaction surface on ubiquitin. This residue may be crucial for the interaction and recognition of poly-ubiquitin chains by the 26S proteasome and other enzymes involved in ubiquitination. Ub D58A can form an E1-catalyzed active thioester at the C-terminus allowing the molecule to be transferred to the lysines of substrate proteins.