Abstract 2548: Substrate binding, not phosphorylation, activates CBL poly-ubiquitination activity
Authors: Zientara-Rytter Kasia , Baron Veronique , Shashkin Pavel , Zhu Henry 
Published: April 2023

Abstract
Casitas B-lineage lymphoma (CBL) is a family of RING-type E3 ligases that target proteins for degradation via the proteasomal pathway. CBL-b specifically functions as a negative effector of T cell activation by down regulating the T cell receptor, whereas c-CBL interacts with various receptor tyrosine kinases involved in cell signaling and activation, targeting them for degradation. Therefore, the CBL family regulates the biology of immune cells and targeting CBL-b or c-CBL is a potential therapeutic strategy for the treatment of cancer, infection, or autoimmune diseases. In this study, we used TR-FRET (Time-Resolved Fluorescence Resonance Energy Transfer) to uncover new mechanistic aspects of CBL activation. Our assays were designed and optimized using purified components (E1/E2/c-CBL or CBL-b E3 ligase and kinase substrate Tyro3 or Axl) to recreate the polyubiquitination cascade in vitro. The assays use a Europium cryptate-labeled Ubiquitin (donor) and a Cy5-labeled Ubiquitin (acceptor) to complete the TR-FRET pairing. Since both the TR-FRET donor and acceptor are incorporated into poly-ubiquitin chains formed on the E3 ligase, the technique measures only poly-ubiquitination and not mono-ubiquitination. This FRET-based assay is homogeneous, making it especially suitable for screening applications and real-time kinetics. Validation of the assay indicated that the presence of a kinase substrate (Tyro3 or Axl) activates CBL-b and c-CBL, which are known to display a similar mechanism of activation. Importantly, our assay design allowed us to show that known CBL-B inhibitor CBL-B-IN-1 is not strictly selective toward CBL-B but also affects c-CBL E3 ligase activity to similar extend since the IC50 of the compound for c-CBL and CBL-B differ by only 2-fold. Finally, it has been proposed that CBL-b phosphorylation at Y106, Y133, and Y363 by Tyro3 kinase is required for its activation. Indeed, in our assay, we observed that CBL-b mutation at Y363F decreases TYRO3 ubiquitination drastically. Surprisingly, phospho-mimetic mutation Y363E did not restore CBL-b-mediated ubiquitination of Tyro3, indicating that substrate binding close to Y363 is the predominant requirement for CBL activation. The weak E3 ligase activity of CBL-b Y363E mutant was insensitive to CBL-B-IN-1 treatment.In conclusion, our data indicates that no significant difference in the parameters of substrate ubiquitination exists between CBL-B and c-CBL, and that ubiquitination of receptor tyrosine kinase Tyro3 by either c-CBL or CBL-B depends on the kinase binding to CBL but not on the phosphorylation of CBL.
Citation Format: Kasia Zientara-Rytter, Veronique Baron, Pavel Shashkin, Henry Zhu. Substrate binding, not phosphorylation, activates CBL poly-ubiquitination activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2548.