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Updated: Nov 16, 2025
Biochemical and Structural Characterization of the Carbohydrate Transport Substrate-binding-protein SP0092
Published on: October 2, 2017
Sleuthing biochemical evidence to elucidate unassigned electron density in a CBL-SLAP2 crystal complex
Leanne E Wybenga-Groot1, C Jane McGlade2
1SPARC BioCentre, The Hospital for Sick Children, 686 Bay Street, Toronto, Ontario M5G 0A4, Canada.
Abstract:
The Src-like adaptor proteins (SLAP/SLAP2) bind to CBL E3 ubiquitin ligase to downregulate antigen, cytokine and tyrosine kinase receptor signalling. In contrast to the phosphotyrosine-dependent binding of CBL substrates through its tyrosine kinase-binding domain (TKBD), CBL TKBD associates with the C-terminal tail of SLAP2 in a phospho-independent manner. To understand the distinct nature of this interaction, a purification protocol for SLAP2 in complex with CBL TKBD was established and the complex was crystallized. However, determination of the complex crystal structure was hindered by the apparent degradation of SLAP2 during the crystallization process, such that only the CBL TKBD residues could initially be modelled. Close examination of the CBL TKBD structure revealed a unique dimer interface that included two short segments of electron density of unknown origin. To elucidate which residues of SLAP2 to model into this unassigned density, a co-expression system was generated to test SLAP2 deletion mutants and define the minimal SLAP2 binding region. SLAP2 degradation products were also analysed by mass spectrometry. The model-building and map-generation features of the Phenix software package were employed, leading to successful modelling of the C-terminal tail of SLAP2 into the unassigned electron-density segments.
Insights
Src-like adaptor proteins (SLAP/SLAP2) regulate receptor signaling by binding CBL E3 ubiquitin ligase. Researchers elucidated SLAP2's C-terminal tail binding to CBL TKBD, overcoming degradation challenges for structural analysis.
Area of Science:
- Molecular Biology
- Structural Biology
- Biochemistry
Background:
- Src-like adaptor proteins (SLAP/SLAP2) are key negative regulators of receptor signaling pathways.
- SLAP/SLAP2 interact with CBL E3 ubiquitin ligase to mediate receptor downregulation.
- CBL substrates typically bind CBL via phosphotyrosine-dependent interactions, but SLAP2 binds CBL TKBD independently of phosphotyrosine.
Purpose of the Study:
- To understand the unique phospho-independent interaction between SLAP2 and CBL TKBD.
- To determine the structural basis of SLAP2 binding to CBL TKBD.
- To identify the minimal SLAP2 region responsible for CBL TKBD interaction.
Main Methods:
- Established a purification protocol for the SLAP2-CBL TKBD complex.
- Co-expressed SLAP2 deletion mutants with CBL TKBD to define the binding region.
- Analyzed SLAP2 degradation products using mass spectrometry.
- Utilized Phenix software for model building and map generation to solve the crystal structure.
Main Results:
- Successfully crystallized the SLAP2-CBL TKBD complex.
- Identified a unique CBL TKBD dimer interface with unassigned electron density.
- Defined the minimal SLAP2 binding region as its C-terminal tail.
- Modeled the SLAP2 C-terminal tail into the unassigned electron density of the CBL TKBD structure.
Conclusions:
- The C-terminal tail of SLAP2 mediates a phospho-independent interaction with the CBL TKBD dimer.
- Structural insights into this interaction provide a basis for understanding receptor signaling regulation.
- The study successfully overcame challenges related to protein degradation during structural determination.
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