Convergent Assembly of Homo- and Heterotypic Ubiquitin Chains from Functionalized, Expressed Monomers via Thiol-Ene Chemistry.

Nature constructs ubiquitin tags with high spatiotemporal precision to execute defined functions that critically rely on the exact molecular composition of the ubiquitin chain. Deciphering the complex ubiquitin code is of paramount interest in biology and requires flexible access to homogeneous ubiquitin tags. As enzymatic approaches suffer from inherent drawbacks such as hardly controllable chain length or connectivity and substrate-specificity, we apply a combination of expression and chemical tools to assemble ubiquitin chains. Our strategy includes expression of ubiquitin-intein fusion constructs to obtain large quantities of defined ubiquitin monomers with C-terminal modifications such as hydrazides and propargylamides. Linkages between ubiquitins are generated via photoinitiated thiol-ene click (TEC) chemistry, resulting in a nearly native isopeptide bond. We demonstrate the generation of homo- and heterotypic ubiquitin oligomers with K27, 29, 48, and 63 linkages up to a K48-linked tetramer. The presented toolbox allows selective installation of ubiquitin on target peptides and proteins with reactive cysteine residues as demonstrated for segments of the microtubule-associated protein tau. Such segments can be implemented into protein semisyntheses as shown here for ubiquitylated full-length Tau4. The presented work combines minimal synthetic effort with high fidelity linkage chemistry, paving the way toward homogeneously ubiquitylated proteins.