Linear ubiquitin chain-binding domains.

Ubiquitin modification (ubiquitination) of target proteins can vary with respect to chain lengths, linkage type, and chain forms, such as homologous, mixed, and branched ubiquitin chains. Thus, ubiquitination can generate multiple unique surfaces on a target protein substrate. Ubiquitin-binding domains (UBDs) recognize ubiquitinated substrates, by specifically binding to these unique surfaces, modulate the formation of cellular signaling complexes and regulate downstream signaling cascades. Among the eight different homotypic chain types, Met1-linked (also termed linear) chains are the only chains in which linkage occurs on a non-Lys residue of ubiquitin. Linear ubiquitin chains have been implicated in immune responses, cell death and autophagy, and several UBDs - specific for linear ubiquitin chains - have been identified. In this review, we describe the main principles of ubiquitin recognition by UBDs, focusing on linear ubiquitin chains and their roles in biology.