Cancer and other disease states can change the landscape of
proteins post-translationally tagged with
ubiquitin (Ub) chains. Molecules capable of modulating Ub chains are potential therapeutic agents, but their discovery represents a significant challenge. Recently, it was shown that de novo
cyclic peptides, selected from trillion-member random libraries, are capable of binding particular Ub chains. However, these
peptides were overwhelmingly proteinogenic, so the prospect of in vivo activity was uncertain. Here, we report the discovery of small, non-proteinogenic
cyclic peptides, rich in non-canonical features like N-methylation, which can tightly and specifically bind Lys48-linked Ub chains. These
peptides engage three Lys48-linked Ub units simultaneously, block the action of
deubiquitinases and the
proteasome, induce apoptosis in vitro, and attenuate
tumor growth in vivo. This highlights the potential of non-proteinogenic
cyclic peptide screening to rapidly find in vivo-active leads, and the targeting of
ubiquitin chains as a promising anti-
cancer mechanism of action.