Structural insights into specificity and diversity in mechanisms of ubiquitin recognition by ubiquitin-binding domains

UBDs [Ub (ubiquitin)-binding domains], which are typically small protein motifs of <50 residues, are used by receptor proteins to transduce post-translational Ub modifications in a wide range of biological processes, including NF-kappa B (nuclear factor kappa B) signalling and proteasomal degradation pathways. More than 20 families of UBDs have now been characterized in structural detail and, although many recognize the canonical Ile(44)/Val(70)-binding patch on Ub, a smaller number have alternative Ub-recognition sites. The A20 Znf (A20-like zinc finger) of the ZNF216 protein is one of the latter and binds with high affinity to a polar site on Ub centred around Asp(58)/Gln(62). ZNF216 shares some biological function with p62, with both linked to NF-kappa B signal activation and as shuttle proteins in proteasomal degradation pathways. The UBA domain (Ub-associated domain) of p62, although binding to Ub through the Ile(44)/Val(70) patch, is unique in forming a stable dimer that negatively regulates Ub recognition. We show that the A20 Znf and UBA domain are able to form a ternary complex through independent interactions with a single Ub molecule, supporting functional models for Ub as a 'hub' for mediating multi-protein complex assembly and for enhancing signalling specificity.