ABSTRACT The post-translational modification of proteins by ubiquitination is a highly regulated process that involves a dynamic, three-step enzymatic cascade, where more than 600 E3 ligases play a critical role in recognizing specific substrates for modification. Separating bona fide targets of E3s from E3-interacting proteins remains a major challenge in the field. In this study, we present BioE3, a novel approach for identifying substrates of ubiquitin-like (UbL) E3 ligases of interest. Using BirA-E3 ligase fusion proteins and bioUbLs, the method facilitates site-specific biotinylation of UbL-modified substrates of particular E3s for proteomic identification. We demonstrate that the BioE3 system can identify both known and novel targets of two RING-type ubiquitin E3 ligases: RNF4, known to be involved in DNA damage response and the regulation of PML nuclear bodies, and MIB1, implicated in endocytosis, autophagy, and centrosomal protein homeostasis. We further show the versatility of BioE3 by identifying targets of an organelle-specific E3 (MARCH5) and a relatively uncharacterized E3 (RNF214). Furthermore, we show that BioE3 works with HECT-type E3 ligases and identify novel targets of NEDD4 involved in vesicular trafficking. BioE3 is a powerful tool that enables identification of bona fide substrates of UbL E3 ligases and how they change with chemical perturbations. BioE3 may also be applicable for UbLs beyond Ub and SUMO, as well as other E3 ligase classes. The resulting knowledge can shed light on the regulation of cellular processes by the complex UbL network and provide information useful for strategies such as targeted protein degradation (TPD), advancing our understanding of fundamental biological mechanisms and their applications.

BioE3 enables the identification ofbona fidetargets of E3 ligases