Modelling of the disulphide-swapped isomer of human insulin-like growth factor-1: implications for receptor binding

Insulin-like growth factor-1 (IGF-1) is a serum protein which unexpectedly folds to yield two stable tertiary structures with different disulphide con nectivities; native IGF-1 [18-61,6-48,47-52] and IGF-1 swap [18-61,6-47, 48 -52]. Here we demonstrate in detail the biological properties of recombinan t human native IGF-1 and IGF-1 swap secreted from Saccharomyces cerevisiae, IGF-1 swap had a similar to 30 fold loss in affinity for the IGF-1 recepto r overexpressed on BHK cells compared with native IGF-1,The parallel increa se in dose required to induce negative cooperativity together with the para llel loss in mitogenicity in NIH 3T3 cells implies that disruption of the I GF-1 receptor binding interaction rather than restriction of a post-binding conformational change is responsible for the reduction in biological activ ity of IGF-1 swap. Interestingly, the affinity of IGF-I swap for the insuli n receptor was similar to 200 fold lower than that of native IGF-1 indicati ng that the binding surface complementary to the insulin receptor (or the a bility to attain it) is disturbed to a greater extent than that to the IGF- 1 receptor. A 1.0 ns high-temperature molecular dynamics study of the local energy landscape of IGF-1 swap resulted in uncoiling of the first A-region a-helix and a rearrangement in the relative orientation of the A- and B-re gions, The model of IGF-1 swap is structurally homologous to the NMR struct ure of insulin swap and CD spectra consistent with the model are presented. However, in the model of IGF-1 swap the C-region has filled the space wher e the first A-region a-helix has uncoiled and this may be hindering interac tion of Val44 with the second insulin receptor binding pocket.