Reading the ubiquitin postal code

Polyubiquitin chains are assembled through the formation of an isopeptide bond between a lysine side-chain or terminal amino group of a proximal ubiquitin moiety and the carboxy-terminal of a distal ubiquitin moiety. Protein substrates tagged by polyubiquitin chains of different linkages undergo different fates. Many polyubiquitin chain types have been characterized so far, notably Lys11, Lys48, Lys63 and linear chains. These different types of chains are synthesized, disassembled and recognized by selective enzymes and receptors. Here I survey the structural basis for the selective binding of polyubiquitin chains of specific linkages, with an emphasis on recent advances in our understanding of polyubiquitin chain structure and functions. Recent work suggests linkage-type discrimination by members of the NE-kappa b signalling and DNA repair pathways and a specific role for Lys48-linked polyubiquitin chain recognition by proteasome-associated proteins.