IGF1 Binding to Integrin α6β4 Leads to Binding of the Calx-β Domain to Non-Catalytic (Allosteric) Site of IGF1R Kinase and Enhances Cell Survival

Previous studies showed that IGF1 binds to α6β4 and induces α6β4-IGF1-IGF1R complex, which leads to the IGF1R kinase activation. An IGF1 mutant defective in integrin binding was defective in signaling and ternary complex formation, and acted as an antagonist, although the mutant still bound to IGF1R, suggesting that IGF1 binding to α6β4 plays a critical role in IGF1R activation. β4 has a unique long tail (>1000 residues) and deletion of part of the β4 tail containing the calx-β domain is known to reduce cell proliferation. We thus hypothesized that calx-β is involved in IGF1 signaling. Docking simulation predicted that calx-β binds to IGF1R kinase. We discovered that isolated calx-β bound to the IGF1R kinase domain. Point mutations in the predicted calx-β binding site in IGF1R kinase inhibited calx-β binding to IGF1R kinase. Notably, the isolated calx-β domain with cell-penetrating peptide (Tat-calx-β) enhanced survival of keratinocytes and non-transformed cells without IGF1. Tat-calx-β did not enhance survival of cancer cells. Several missense mutations are clustered in the predicted IGF1R kinase binding site of the calx-β domain of β4 in genetic skin blistering disease (JEB-PA). We discovered that several JEB-PA mutants were defective in calx-β binding to the IGF1R kinase and inhibited cell survival of keratinocytes, suggesting that these mutations may suppress calx-b binding to IGF1R kinase. These findings suggest that IGF1 binding to α6β4 triggers calx-β binding to the IGF1R kinase and activates IGF1R kinase.