Research ArticleBiochemistry

Substrate binding allosterically relieves autoinhibition of the pseudokinase TRIB1

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Sci. Signal.  25 Sep 2018:
Vol. 11, Issue 549, eaau0597
DOI: 10.1126/scisignal.aau0597

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Targeting pseudokinases with kinase inhibitors

Pseudokinases are structurally similar to kinases but lack catalytic activity; instead, pseudokinases typically function as scaffolds, often promoting the degradation of substrate proteins by bringing them into close proximity with ubiquitin ligases. Two studies explored the structures and protein interactions of the pseudokinases TRIB1 (Jamieson et al.) and TRIB2 (Foulkes et al.). Their findings reveal new insights into the structural regulation of TRIB proteins and show that these proteins, which are implicated in leukemia and other cancers, can bind to clinically approved kinase inhibitors. Binding by these drugs caused structural changes in the TRIB proteins that deprotected them from degradation upon interacting with ubiquitin ligases, indicating that these drugs might be repurposed to block the function of TRIBs in cancer patients.

Abstract

The Tribbles family of pseudokinases recruits substrates to the ubiquitin ligase COP1 to facilitate ubiquitylation. CCAAT/enhancer-binding protein (C/EBP) family transcription factors are crucial Tribbles substrates in adipocyte and myeloid cell development. We found that the TRIB1 pseudokinase was able to recruit various C/EBP family members and that the binding of C/EBPβ was attenuated by phosphorylation. To explain the mechanism of C/EBP recruitment, we solved the crystal structure of TRIB1 in complex with C/EBPα, which revealed that TRIB1 underwent a substantial conformational change relative to its substrate-free structure and bound C/EBPα in a pseudosubstrate-like manner. Crystallographic analysis and molecular dynamics and subsequent biochemical assays showed that C/EBP binding triggered allosteric changes that link substrate recruitment to COP1 binding. These findings offer a view of pseudokinase regulation with striking parallels to bona fide kinase regulation—by means of the activation loop and αC helix—and raise the possibility of small molecules targeting either the activation “loop-in” or “loop-out” conformations of Tribbles pseudokinases.

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