The X-ray structure of the insulin receptor ectodomain complexed with four antibody fragments
The insulin receptor (IR) is a member of the tyrosine kinase receptor superfamily. Its major roles in higher organisms are the regulation of lipid, protein and carbohydrate metabolism including the maintenance of glucose homeostasis whereas the closely related insulin-like growth factor receptor (IGF-1R) is involved in normal growth and development. Of the two insulin receptor isoforms IR-A and IR-B, the former acts as a high-affinity receptor for insulin-like growth factor-II (IGF-II) and has been implicated, along with IGF-1R, in malignant transformation. There is a third member of the the IR family, the orphan insulin receptor related receptor (IRR). IR family members which are all homodimers, can also function as hybrid heterodimers.
Despite the fact that the atomic structure of insulin has been known for forty years and the primary sequence of its receptor for twenty-two years, detail at the molecular level of the interaction between insulin and its receptor is remarkably limited, as is detail about the way in which ligand binding effects signal transduction. A breakthrough occurred during 2006 with the determination by Ward, Lawrence and co-workers of the three-dimensional structure of the entire IR ectodomain in its apo state at a resolution of 3.8 Å. This structure provided the first insight into the likely structure of the intact insulin binding site, suggesting it to be formed by a juxtaposition of the first leucine-rich repeat domain of one monomer with the first fibronectin domain of the alternate monomer. The crystallographic electron density maps also indicated the possible location the C-terminus of the receptor ï¡-chain, a segment (CT) which is known to be essential for ligand binding. The methodological key to the structure of the receptor was to crystallise it in combination with four Fabs molecules (Fig. 1a), which significantly decreased the need for crystal contacts to involve the receptor's N-linked glycans.
We are now seeking to determine the structure of the ligand-bound insulin receptor in both high- and low-affinity states and to determine the nature of the structural transitions that occur upon ligand binding.