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Ubiquitin ligase activity and tyrosine phosphorylation underlie suppression of growth factor signaling by c-Cbl/Sli-1
G Levkowitz1, H Waterman, S A Ettenberg
1Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
Abstract:
Receptor desensitization is accomplished by accelerated endocytosis and degradation of ligand-receptor complexes. An in vitro reconstituted system indicates that Cbl adaptor proteins directly control downregulation of the receptor for the epidermal growth factor (EGFR) by recruiting ubiquitin-activating and -conjugating enzymes. We infer a sequential process initiated by autophosphorylation of EGFR at a previously identified lysosome-targeting motif that subsequently recruits Cbl. This is followed by tyrosine phosphorylation of c-Cbl at a site flanking its RING finger, which enables receptor ubiquitination and degradation. Whereas all three members of the Cbl family can enhance ubiquitination, two oncogenic Cbl variants, whose RING fingers are defective and phosphorylation sites are missing, are unable to desensitize EGFR. Our study identifies Cbl proteins as components of the ubiquitin ligation machinery and implies that they similarly suppress many other signaling pathways.
Insights
Cbl proteins are key regulators of epidermal growth factor receptor (EGFR) desensitization by controlling its ubiquitination and degradation. This study reveals Cbl
Area of Science:
- Cellular signaling
- Protein ubiquitination
- Receptor biology
Background:
- Receptor desensitization involves accelerated endocytosis and degradation of ligand-receptor complexes.
- The epidermal growth factor receptor (EGFR) is a key mediator of cell growth and survival.
- Cbl adaptor proteins are known to be involved in protein trafficking and signaling regulation.
Purpose of the Study:
- To elucidate the role of Cbl adaptor proteins in the downregulation of the epidermal growth factor receptor (EGFR).
- To investigate the molecular mechanisms by which Cbl proteins mediate EGFR desensitization.
- To identify the functional domains of Cbl proteins critical for EGFR ubiquitination and degradation.
Main Methods:
- Utilized an in vitro reconstituted system to study Cbl-mediated EGFR regulation.
- Investigated protein-protein interactions and post-translational modifications (e.g., phosphorylation, ubiquitination).
- Employed oncogenic Cbl variants with defective functional domains to assess their impact on EGFR desensitization.
Main Results:
- Cbl adaptor proteins directly control EGFR downregulation by recruiting ubiquitin-activating and -conjugating enzymes.
- EGFR autophosphorylation at a lysosome-targeting motif initiates Cbl recruitment, followed by c-Cbl tyrosine phosphorylation.
- This sequential process leads to EGFR ubiquitination and subsequent degradation, with all three Cbl family members enhancing ubiquitination.
Conclusions:
- Cbl proteins are essential components of the ubiquitin ligation machinery that mediate EGFR desensitization.
- Specific functional domains within Cbl proteins, including the RING finger and phosphorylation sites, are critical for this process.
- The findings imply that Cbl proteins similarly regulate other signaling pathways by controlling receptor ubiquitination and degradation.