A wide range of cellular processes are modulated through the generation and attachment of polyubiquitin (polyUb) chains to target proteins. Increasing evidence suggests that polyUb chains joined through linear peptide bonds between the C-terminus of a ubiquitin and the N-terminus of another play important functional roles. The enzyme machinery responsible for the generation of linear polyUb chains has been termed LUBAC, consisting of HOIL-1L and HOIP. Chains of these type have been determined to have an open conformation, similar to polyUb K63, but with very distinct functional properties. Linear polyUb chains are cleaved by the deubiquitylases CYLD, USP5 (IsoT), USP2 and have been shown to bind to many UBDs including NEMO and Trabin-n (3xnzf). Recombinant linear chains of defined length are expressed in E. coli and purified to homogeneity. Amide linkages join the N- and C-terminus of each ubiquitin molecule to each other. This molecule is HIS-tagged at the N-terminus of the most distal ubiquitin.
