A CK2-β-TrCP kinase-E3 ubiquitin ligase cascade is a metabolic sensor regulating tryptophan 2,3-dioxygenase stability.

Small molecules toggling the ubiquitin-proteasome system (UPS) are powerful regulators of protein degradation, yet how endogenous ligands gate UPS decisions remains poorly understood. Here, we define control of UPS access to human tryptophan 2,3-dioxygenase (TDO2), which converts the amino acid tryptophan (Trp) to N-formylkynurenine. When Trp concentrations are limiting, TDO2 is degraded to avert hypotryptophanemia. We discovered that a CK2-β-TrCP kinase-E3 ligase cascade generates and recognizes phosphodegrons in TDO2 unless they are shielded by Trp binding at an exosite distal from the catalytic site. Effects of Trp analogs on CK2-β-TrCP-dependent ubiquitylation indicated that the indole, amino, and carboxylate groups collectively protect TDO2. Cryo-EM reveals that these moieties organize a Trp-ordered flap that covers the exosite and thereby restricts generation of adjacent phosphodegrons. In the absence of Trp, the flap is flexible, enabling phosphorylation-coupled ubiquitylation. Overall, our data uncovered an endogenous small molecule conformationally stabilizing its own metabolizing enzyme through protection from a phosphorylation-ubiquitylation cascade.