Practical Chemical Synthesis of Atypical Ubiquitin Chains by Using an Isopeptide‐Linked Ub Isomer

Chemical ubiquitination is an effective approach to synthesize structurally defined, atypical ubiquitin (Ub) chains that are difficult to prepare by other technologies. Herein, we describe an isoUb-based strategy that uses a readily accessible, premade, isopeptide-linked 76-mer isoUb, with an N-terminal Cys and a C-terminal hydrazide as the key building blocks, to assemble atypical Ub chains in a modular fashion. This method avoids the use of auxiliary-modified Lys and instead employs the canonical and therefore more robust, Cys-based native chemical ligation technique. The efficiency and capacity of this isoUb-based strategy is exemplified by the cost-effective synthesis of several linkage- and length-defined atypical Ub chains, including K27-linked tetra-Ub, K11/K48-branched tri-, tetra-, penta-, and hexa-Ubs.