Discovery of DTX3L inhibitors through a homogeneous FRET-based assay that monitors formation and removal of poly-ubiquitin chains

Abstract

Ubiquitination is a complex and reversible protein post-translational modification in which the subsequent action of enzymes belonging to three different families, broadly referred to as E1, E2 and E3, results in the covalent linking of ubiquitin to a target protein. While this linkage is canonically an isopeptide bond between the C-terminus of ubiquitin and the lysine residue of the target protein, Ser, Thr, and Tyr can also be susceptible to ubiquitination through an oxyester bond. Once ubiquitinated, multiple units of ubiquitin can be attached to the initial ubiquitin thus extending it to a chain of ubiquitins. Ubiquitination regulates multiple cellular processes, but it is best known as a modification that targets proteins for proteasomal degradation following the formation poly-ubiquitin chains linked through lysine 48 or 63 of ubiquitin. Dysregulation of ubiquitination has been associated with multiple types of cancer and efforts have been carried out to develop technologies that lead to the identification of inhibitors of the enzymes involved in the ubiquitination cascade. Herein, we present the development of a FRET-based assay that allows us to monitor auto-ubiquitination of DTX3L, a RING-type E3 ubiquitin ligase. Our method shows a robust signal window with a robust average Z factor of 0.76. From a validatory screening experiment we have identified the first molecules that inhibit DTX3L with potencies in the low micromolar range. Additionally, we have expanded the system to study deubiquitinases such as USP28 that lead to reduction of FRET due to hydrolysis of fluorescent poly-Ub chains.