Since their first report in 2000, tubulysins have sparked great interest for development as anti-cancer agents due to their exceptionally potent antiproliferative activity. Progress in the discovery and development of tubulysins, especially tubulysin conjugates, has quickly advanced despite limitations in their availability from Nature. In this Highlight, the key research on the isolation and structure determination, biosynthesis, bioactivity, structure-activity relationships (SAR), synthesis, and conjugates of tubulysins is presented.

The Ras proteins are essential GTPases involved in the regulation of cell proliferation and survival. Mutated oncogenic forms of Ras alter effector binding and innate GTPase activity, leading to deregulation of downstream signal transduction. Mutated forms of Ras are involved in approximately 30% of human cancers. Despite decades of effort to develop direct Ras inhibitors, Ras has long been considered ‘undruggable’ due to its high affinity for GTP and its lack of hydrophobic binding pockets. Herein, we report a total chemical synthesis of all L- and all D-amino acid biotinylated variants of oncogenic mutant KRas(G12V). The protein is synthesized using Fmoc-based solid-phase peptide synthesis and assembled using combined native chemical ligation and isonitrile-mediated activation strategies. We demonstrate that both KRas(G12V) enantiomers can successfully fold and bind nucleotide substrates and binding partners with observable enantiodiscrimination. By demonstrating the functional competency of a mirror-image form of KRas bound to its corresponding enantiomeric nucleotide triphosphate, this study sets the stage for further biochemical studies with this material. In particular, this protein will enable mirror-image yeast surface display experiments to identify all-D peptide ligands for oncogenic KRas, providing a useful tool in the search for new therapeutics against this challenging disease target.