How IGF-1 activates its receptor

Jennifer M Kavran,Alexander Ramek,David E Shaw,Daniel J Leahy,Patrick O Byrne,Yibing Shan,Jacqueline M Mccabe,Kalina Hristova,Zhihong Wang,Mary Katherine Connacher,Sarvenaz Sarabipour,Philip A Cole

doi:10.7554/elife.03772

Abstract

The type I insulin-like growth factor receptor (IGF1R) is involved in growth and survival of normal and neoplastic cells. A ligand-dependent conformational change is thought to regulate IGF1R activity, but the nature of this change is unclear. We point out an underappreciated dimer in the crystal structure of the related Insulin Receptor (IR) with Insulin bound that allows direct comparison with unliganded IR and suggests a mechanism by which ligand regulates IR/IGF1R activity. We test this mechanism in a series of biochemical and biophysical assays and find the IGF1R ectodomain maintains an autoinhibited state in which the TMs are held apart. Ligand binding releases this constraint, allowing TM association and unleashing an intrinsic propensity of the intracellular regions to autophosphorylate. Enzymatic studies of full-length and kinase-containing fragments show phosphorylated IGF1R is fully active independent of ligand and the extracellular-TM regions. The key step triggered by ligand binding is thus autophosphorylation.

Highlights

The insulin and type-1 insulin-like growth factor receptors (IR and IGF1R) are homologous receptor tyrosine kinases (RTKs) that regulate cell metabolism, growth, and differentiation in a variety of mammalian tissues (De Meyts, 2004; Siddle, 2011, 2012)
The reported dimer of the liganded Insulin Receptor (IR) fragment is mediated only by reciprocal interactions between L1 from one subunit and αCT from the other. As this dimer neither resembles the inverted ‘V’ conformation of the unliganded IR ECD nor is compatible with the inter-subunit disulfide bond between Fn1 domains, it is likely a nonphysiological artifact arising from the deletion of Fn2-insert domain (ID)-Fn3 domains and fusion of αCT to Fn1 (Menting et al, 2013)
We examined all IR interactions present in the crystal and identified an alternative dimer (Figure 2B), which had previously been noted as a part of a dimer of dimers (Menting et al, 2013), that is compatible with the disulfide bond between Fn1 domains (Schaffer and Ljungqvist, 1992) and preserves the inter-subunit contacts between opposing L2-Fn1 domains (L2-Fn1:L2′-Fn1′) present at the apex of the inverted ‘V’ in the unliganded IR structure (Figure 2)

Summary

Introduction

The insulin and type-1 insulin-like growth factor receptors (IR and IGF1R) are homologous receptor tyrosine kinases (RTKs) that regulate cell metabolism, growth, and differentiation in a variety of mammalian tissues (De Meyts, 2004; Siddle, 2011, 2012). Kavran et al confirmed that when a molecule of insulin-like growth factor binds to the extracellular domain of the IGF receptor, it causes a shape change that moves the parts of the receptor that span the cell membrane closer together This results in the regions of the receptor proteins located inside the cell adding chemical tags, called phosphate groups, to one another— which activates the receptor. An inter-subunit interaction that stabilizes the separation of the Fn2–3 ‘legs’ of the ECD is disrupted by ligand binding, suggesting that TM separation may be important for maintaining the receptor in an inactive state We tested this model in a series of biochemical and biophysical assays and show that the IGF1R ECD autoinhibits activity by holding the TMs apart in the absence of ligand. The key step regulated by ligand binding is autophosphorylation and not kinase activity per se, and the role of the IR/IGF1R ECD is to inhibit activity in the absence of ligand rather than promote activity in the presence of ligand

Results

Discussion

Materials and methods

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