The basis of ErbB4 ligand specificity
The Epidermal Growth Factor (EGF) family of growth factors regulates signaling by the ErbB family of type I receptor tyrosine kinases, whose members include the Epidermal Growth Factor Receptor (EGFR/ErbB1), ErbB2/HER2/Neu, ErbB3/HER3, and ErbB4/HER4. There is much that remains to be known about exactly how binding of EGF family growth factors regulates ErbB receptor signaling, and much remains to be known about the identity of molecular determinants that regulate this binding and stimulation of subsequent receptor signaling. Therefore the purpose of this study was to characterize the molecular determinants that regulate specific signaling stimulated by interactions between members of the Neuregulin (NRG) sub-family of EGF family growth factors and their receptors (ErbB3 and ErbB4). The NRGs are classified as ErbB3 and ErbB4 ligands, but there has been no systematic functional comparison of NRG3, NRG4, and the two splicing isoforms of NRG2 (NRG2α and NRG2β). Here, we report that NRG2α and NRG2β are ErbB3 agonists and stimulate ErbB3 coupling to biological responses, but NRG3 and NRG4 do not appear to be ErbB3 agonists and do not stimulate ErbB3 coupling to biological responses. NRG3 and NRG4 are both potent stimuli of ErbB4 tyrosine phosphorylation, but only NRG3 stimulates a modest amount of ErbB4 coupling to biological responses. NRG2β is a potent stimulus of ErbB4 tyrosine phosphorylation and stimulates ErbB4 coupling to biological responses. NRG2α is not a potent stimulus of ErbB4 tyrosine phosphorylation and does not stimulate ErbB4 coupling to biological responses. We hypothesized that five carboxyl-terminal amino acids in NRG2β regulate NRG2β stimulation of ErbB4 tyrosine phosphorylation and ErbB4 coupling to biological responses. We report that Gln43, Met47, Asn49, and Phe50 of NRG2β regulate NRG2β efficacy, whereas Phe45 of NRG2β regulates NRG2β potency. Molecular modeling of the NRG2β-ErbB4 interface suggests that Phe45 of ErbB4 interacts with Leu437 and Lys438 of ErbB4. Therefore we hypothesized that Phe45 regulates NRG2β potency by regulating the affinity of NRG2β for ErbB4 through interactions with amino acids Leu437 and Lys438 of ErbB4. We report that Phe45 does regulate the affinity of NRG2β for ErbB4 and it may do so by interacting with hydrophobic amino acids in ErbB4.
Riese, Purdue University.
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