A pinboard by
Mohit Misra

PhD student, University of Wuerzburg/ Rudolf Virchow Centre for Experimental Biomedicine


Structural and biochemical characterisation of the ubiquitin activating enzyme.

Ubiquitin is one of the most versatile post-translational modifications present throughout eukaryotes which is known for its well-established role for proteasomal degradation but plays an essential role in the myriad of cellular processes including DNA repair, immune response and cell cycle progression to name a few. Three classes of enzymes namely E1, E2 and E3 are required for the transfer of the free ubiquitin onto the target protein via an isopeptide bond. In humans, there are two E1 enzymes, around fifty E2 enzymes and more than six hundred E3 enzymes that facilitate this process. Ubiquitin tagging of the protein leads them to the proteasome, a 2.5 MDa complex for degradation. Since the approval of Bortezomib, a proteasome inhibitor for the treatment of multiple myeloma and mantle cell lymphoma, rigorous research has been carried out to inhibit novel therapeutic target upstream of the proteasome. UBA1 and UBA6 are the two E1 enzymes that activate ubiquitin in humans and thus are the master regulators of ubiquitylation. UBA1 is the only dedicated E1 for ubiquitin as UBA6 can activate both ubiquitin and FAT10, another ubiquitin-like modifier. UBA1 plays a major role in ubiquitylation and research in the last decade has established its inhibition as a promising target for cancer therapy. Two adenosyl sulfamate inhibitors which target UBA1 are currently in phase I clinical trials for the treatment of solid tumours. We have crystallised three inhibitors of this class including the ones that are in clinical trials in the complex with ubiquitin and UBA1 and have solved high-resolution crystal structures of these complexes. These structures serve as a platform for developing highly specific and potent inhibitors that can target individual E1 enzymes responsible for the activation of ubiquitin as well as sixteen other ubiquitin-like modifiers. With the help of in vitro inhibition assays as well as molecular dynamics simulation, we confirmed the structural interpretation of specificity of the inhibitors bound in our complex structures. In addition to targeting the ubiquitin activating enzyme (UBA1) for cancer treatment, we are interested in understanding the molecular mechanism of E1 enzyme catalysis. For this purpose, we have utilized biochemical tools in addition to X-ray crystallography to learn the structural and mechanistic insights of UBA1 enzymatic activity. We recently published the findings of the adenosyl sulfamate inhibitors of UBA1 in Structure.


An inhibitor of ubiquitin conjugation and aggresome formation.

Abstract: Proteasome inhibitors have revolutionized the treatment of multiple myeloma, and validated the therapeutic potential of the ubiquitin proteasome system (UPS). It is believed that in part, proteasome inhibitors elicit their therapeutic effect by inhibiting the degradation of misfolded proteins, which is proteotoxic and causes cell death. In spite of these successes, proteasome inhibitors are not effective against solid tumors, thus necessitating the need to explore alternative approaches. Furthermore, proteasome inhibitors lead to the formation of aggresomes that clear misfolded proteins via the autophagy-lysosome degradation pathway. Importantly, aggresome formation depends on the presence of polyubiquitin tags on misfolded proteins. We therefore hypothesized that inhibitors of ubiquitin conjugation should inhibit both degradation of misfolded proteins, and ubiquitin dependent aggresome formation, thus outlining the path forward toward more effective anticancer therapeutics. To explore the therapeutic potential of targeting the UPS to treat solid cancers, we have developed an inhibitor of ubiquitin conjugation (ABP A3) that targets ubiquitin and Nedd8 E1 enzymes, enzymes that are required to maintain the activity of the entire ubiquitin system. We have shown that ABP A3 inhibits conjugation of ubiquitin to intracellular proteins and prevents the formation of cytoprotective aggresomes in A549 lung cancer cells. Furthermore, ABP A3 induces activation of the unfolded protein response and apoptosis. Thus, similar to proteasome inhibitors MG132, bortezomib, and carfilzomib, ABP A3 can serve as a novel probe to explore the therapeutic potential of the UPS in solid and hematological malignancies.

Pub.: 01 Sep '15, Pinned: 27 Jul '17