Receptor Elimination by E3 Ubiquitin Ligase Recruitment (REULR): A Targeted Protein Degradation Toolbox

In recent years, targeted protein degradation (TPD) of plasma membrane proteins by hijacking the ubiquitin proteasome system (UPS) or the lysosomal pathway have emerged as novel therapeutic avenues in drug development to address and inhibit canonically difficult targets. While TPD strategies have been successful to target cell surface receptors, these approaches are limited by the availability of suitable binders to generate heterobifunctional molecules. Here, we present the development of a nanobody (VHH) based degradation toolbox termed REULR (Receptor Elimination by E3 Ubiquitin Ligase Recruitment). We generated human and mouse cross-reactive nanobodies against 5 transmembrane PA-TM-RING type E3 Ubiquitin Ligases (RNF218, RNF130, RNF167, RNF43, ZNRF3) covering a broad range and selectivity of tissue expression with which we characterized expression in human and mouse cell lines and immune cells (PBMCs). We demonstrate that heterobifunctional REULR molecules can enforce transmembrane E3 ligase interaction with a variety of disease relevant target receptors (EGFR, EPOR, PD-1) by induced proximity resulting in effective membrane clearance of the target receptor at varying levels. In addition, we designed E3 Ligase self-degrading molecules, fratricide REULRs (RNF128, RNF130, RENF167, RNF43, ZNRF3), that allow downregulation of one or several E3 Ligases from the cell surface and consequently modulate receptor signaling strength. REULR molecules represent a VHH-based, modular and versatile "mix and match" targeting strategy for facile modulation of cell surface proteins by induced proximity to transmembrane PA-TM-RING E3 ligases.