Characterisation of insulin/IGF hybrid receptors: contributions of the insulin receptor L2 and Fn1 domains and the alternatively spliced exon 11 sequence to ligand binding and receptor activation

Samira Benyoucef , Katharina H Surinya , Dirk Hadaschik , Kenneth Siddle (1)

1 Clinical Biochemistry

The insulin receptor (IR) and type I IGF receptor (IGFR) are homo-dimers but the respective pro-receptors can also hetero-dimerise to form insulin/IGF hybrid receptors. There are conflicting data on the ligand affinity of hybrids, and especially on the influence of different IR isoforms. To investigate further the contribution of individual ligand binding epitopes to affinity and specificity in the IR/IGFR family, we generated hybrids incorporating both IR isoforms and IR/IGFR domain-swap chimeras, by ectopic co-expression of receptor constructs in CHO cells, and studied ligand binding using both radioligand competition and bioluminescence resonance energy transfer assays. We found that IR-A/IGFR and IR-B/IGFR hybrids bound insulin with similar and relatively low affinity, intermediate between that of homodimeric IR and homodimeric IGFR. However, both IR-A/IGFR and IR-B/IGFR hybrids bound IGF-I and IGF-II with high affinity comparable to homodimeric IGFR. Incorporation of a significant fraction of either IR-A or IR-B into hybrids resulted in abrogation of insulin- but not IGF-I-stimulated autophosphorylation. We conclude that the sequence of 12 amino acids encoded by IR exon 11 has little or no effect on ligand binding and activation of IR/IGFR hybrids, and that hybrids bind IGFs but not insulin at physiological concentrations regardless of the IR isoform they contain. To reconstitute high affinity insulin binding within a hybrid receptor, chimaeras in which the IGFR L1 or L2 domains had been replaced by equivalent IR domains were co-expressed with full length IR-A or IR-B. In the context of an IR-A/IGFR hybrid, substitution of IR residues 325-524 (L2 domain + part of first fibronectin domain) for the corresponding IGFR sequence increased the affinity for insulin by 20 fold. We conclude that the L2 and/or first fibronectin domains of IR contribute in trans with the L1 domain to create a high affinity insulin binding site within a dimeric receptor.

Mots clés: Life Sciences

Samira Benyoucef, Katharina H Surinya, Dirk Hadaschik, Kenneth Siddle. Characterisation of insulin/IGF hybrid receptors: contributions of the insulin receptor L2 and Fn1 domains and the alternatively spliced exon 11 sequence to ligand binding and receptor activation. Biochemical Journal, 2007, 403 (3), pp.603-613. ⟨10.1042/BJ20061709⟩. ⟨hal-00478697⟩