The N-recognin UBR4 of the N-end rule pathway is targeted to and required for the biogenesis of the early endosome

Abstract

The N-end rule pathway is a proteolytic system in which single N-terminal residues of proteins act as N-degrons. These degrons are recognized by N-recognins, facilitating substrate degradation via the ubiquitin (Ub) proteasome system (UPS) or autophagy. We have previously identified a set of N-recognins (UBR1, UBR2, UBR4/p600, and UBR5/EDD) that bind N-degrons through their UBR boxes to promote proteolysis by the proteasome. Here, we show that the 570 kDa N-recognin UBR4 is associated with maturing endosomes through the interaction with Ca2+-bound calmodulin. The endosomal recruitment of UBR4 is essential for the biogenesis of early endosomes (EEs) and endosome-related processes such as the trafficking of endocytosed protein cargoes and degradation of extracellular cargoes by endosomal hydrolases. In mouse embryos, UBR4 marks and plays a role in the endosome-lysosome pathway that mediates the heterophagic proteolysis of endocytosed maternal proteins into amino acids. By screening 9,591 drugs through DrugBank database, we identify picolinic acid as a putative ligand to UBR4, which inhibits the biogenesis of EEs. Our results suggest that UBR4 is an essential modulator in the endosome-lysosome system.