A CK2-FBXW11 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, mechanistic knowledge of how endogenous ligands gate UPS decisions remains rudimentary. Here, we define control of UPS access to Tryptophan-2,3-dioxygenase (TDO2), which converts the essential amino acid tryptophan (Trp) to N-formylkynurenine. When Trp concentrations are limiting, TDO2 is degraded to avert tryptophanemia. Using CRISPRi screening and biochemistry, we identify a CK2-FBXW11 kinase-E3 ligase cascade that generates and recognizes tandem TDO2 phosphodegrons when not protected by Trp. Trp binding to an exosite safeguards TDO2 from phosphorylation-dependent ubiquitylation. Effects of Trp analogs on CK2-FBXW11-dependent ubiquitylation indicated that the indole, amino, and carboxylate groups are necessary for substrate shielding. Cryo-EM reveals how these moieties order a region proximal to the phosphodegrons; without Trp, this segment is flexible, enabling phosphorylation-coupled ubiquitylation. Overall, our data uncovered an endogenous small molecule allosterically stabilizing its own metabolizing enzyme through protection from a phosphorylation-ubiquitylation cascade. ### Competing Interest Statement B.A.S. is a member of the scientific advisory boards of Proxygen and Lyterian. The other authors declare no competing interests. Max Planck Society, https://ror.org/01hhn8329 European Union, ERC AdvG, UPSmeetMet, 101098161 to BAS Boehringer Ingelheim Fonds, https://ror.org/00dkye506

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Structural basis for E4 enzyme Ufd2-catalyzed K48/K29 branched ubiquitin chains - Nature Chemical Biology

Tong et al. chemically trapped the structure of the E4 enzyme Ufd2, which mediates K48 branched ubiquitination on K29diUb and K29triUb, identifying Ufd2’s core region as a K29diUb binding domain and a...

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Structural basis of an EMC:Spf1 insertase-dislocase complex in the eukaryotic endoplasmic reticulum

Most eukaryotic membrane proteins are inserted into the membrane at the endoplasmic reticulum (ER). This essential but error-prone process relies on molecular quality control machineries to prevent mi...

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The EMC acts as a chaperone for membrane proteins - Nature Communications

Membrane proteins are essential for any cell but difficult to fold. Here, the authors show that the EMC acts as a chaperone for membrane proteins. They dissect client recognition and provide a molecul...

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TRIP12 structures reveal HECT E3 formation of K29 linkages and branched ubiquitin chains

Nature Structural & Molecular Biology, Published online: 26 May 2025; doi:10.1038/s41594-025-01561-1Using biochemistry, chemical biology, and cryo-EM, Maiwald et al. elucidate how TRIP12 forms K29 linkages and K29/K48-linked branched ubiquitin chains, revealing a mechanism for polyubiquitylation shared by some HECT E3s.

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TRIP12 structures reveal HECT E3 formation of K29 linkages and branched ubiquitin chains - Nature Structural & Molecular Biology

Using biochemistry, chemical biology, and cryo-EM, Maiwald et al. elucidate how TRIP12 forms K29 linkages and K29/K48-linked branched ubiquitin chains, revealing a mechanism for polyubiquitylation sha...

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UbiREAD deciphers proteasomal degradation code of homotypic and branched K48 and K63 ubiquitin chains

Ubiquitin chains determine the fates of their modified proteins, including proteasomal degradation. Kiss et al. present UbiREAD, a technology to monitor cellular degradation and deubiquitination at hi...

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