Ubiquitin-binding domains

Nine ubiquitin-binding domains (UBDs) have been identified and characterized so far by various approaches. There are probably more UBDs yet to be discovered.Interactions between UBDs and ubiquitin are typically weak (Kd = 2–500 μM), and are likely to be dynamic and tightly regulated in cells.Although all UBDs bind to a similar surface of ubiquitin — a hydrophobic patch that includes Ile44 of ubiquitin — the structures of different UBDs are surprisingly diverse.Many UBDs are required for the ubiquitylation of the protein within which they are carried. The mechanism and functions of UBD-dependent ubiquitylation have not yet been defined.There are only a few defined functions for UBDs in cellular proteins, which include a role in polyubiquitin chain formation, roles as receptors for polyubiquitin chains that target proteins to the proteasome for degradation, and proposed roles as receptors for ubiquitin sorting signals in the late endosomal pathway.