An emerging principle of cellular signaling is the regulation of metabolism by E3 ligases. The ubiquitin-proteasome system regulates protein turnover through mechanisms specifically targeting only those that need eliminating while sparing essential cellular processes. Specificity is achieved by E3 ligases, which conditionally bind their ubiquitylation substrates in response to cellular signals. We have been focusing on TDO2 catalyzing the first and rate-limiting step in the kynurenine pathway contributing to the biosynthesis of the central cofactor and substrate NAD+. TDO2 is tightly regulated by metabolic conditions, not only in terms of its enzymatic activity but also through TDO2 stability being determined by the levels of its substrate Trp. We have applied genome-wide CRISPRi screening to discover a two-step pathway that recapitulates Trp-dependent ubiquitylation of TDO2. By implementing cryo-EM, biochemical and cellular assays, we have identified how Trp binding safeguards TDO2 degradation in a glue-like mechanism. I will delineate the molecular basis for metabolite sensing by an E3 ligase pathway allowing the regulation of physiological Trp levels.