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ErbB-3 and ErbB-4 function as the respective low and high affinity receptors of all Neu differentiation
E Tzahar1, G Levkowitz, D Karunagaran
1Department of Chemical Immunology, Weizmann Institute of Science, Rehovot, Israel.
Abstract:
Neu differentiation factor (NDF or heregulin) elevates tyrosine phosphorylation of the ErbB-2 receptor tyrosine kinase, and it was, therefore, thought to function as a ligand of this receptor. However, several lines of evidence raised the possibility that the interaction between NDF and ErbB-2 involves another molecule, which belongs to the family of epidermal growth factor receptors. To address this question we constructed soluble chimeric proteins between alkaline phosphatase and the extracellular domains of ErbB-2 and either ErbB-3 or ErbB-4, two newly recognized members of the epidermal growth factor receptor family. Using the soluble proteins we found that beta isoforms of NDF specifically bind to the ErbB-3 and ErbB-4 receptors but not to the soluble ErbB-2 protein. When ectopically expressed in monkey fibroblasts, the full-length ErbB-3 and ErbB-4 receptors conferred specific binding to NDF. In these cells ErbB-3 displayed lower ligand binding affinity than ErbB-4, but like the latter receptor it preferred to bind the beta isoform over the alpha class of NDFs. These results indicate that both ErbB-3 and ErbB-4 function as physiological receptors of all NDF isoforms and suggest that a still unknown ligand of ErbB-2 exists.
Insights
Neu differentiation factor (NDF) binds ErbB-3 and ErbB-4 receptors, not ErbB-2. This suggests NDF acts via ErbB-3/ErbB-4, and ErbB-2 has a different, undiscovered ligand.
Area of Science:
- Cell Biology
- Molecular Biology
- Receptor Tyrosine Kinases
Background:
- Neu differentiation factor (NDF), also known as heregulin, activates the ErbB-2 receptor tyrosine kinase.
- Evidence suggested NDF's interaction with ErbB-2 might involve other epidermal growth factor receptor (EGFR) family members.
Purpose of the Study:
- To investigate the direct binding interactions of NDF with members of the EGFR family.
- To determine if ErbB-3 and ErbB-4 act as direct receptors for NDF.
Main Methods:
- Constructed soluble chimeric proteins of alkaline phosphatase fused to extracellular domains of ErbB-2, ErbB-3, and ErbB-4.
- Assessed NDF binding to these soluble receptors and to full-length receptors ectopically expressed in monkey fibroblasts.
Main Results:
- NDF's beta isoforms specifically bound to ErbB-3 and ErbB-4, but not to soluble ErbB-2.
- Ectopic expression confirmed ErbB-3 and ErbB-4 confer specific NDF binding.
- ErbB-3 showed lower ligand affinity than ErbB-4, with both preferring NDF beta isoforms.
Conclusions:
- ErbB-3 and ErbB-4 function as physiological receptors for all NDF isoforms.
- These findings imply the existence of an as-yet-unidentified ligand for the ErbB-2 receptor.