Structural basis for the conformational changes of insulin receptor induced by three different hormone ligands

Abstract

The insulin receptor (IR) is central to the regulation of glucose and lipid metabolism. Although insulin is its primary ligand, insulin-like growth factors I and II (IGF-I and IGF-II) also engage IR, albeit with reduced affinity. The structural basis of cooperative ligand binding, however, has remained poorly understood. Here, we report cryo-Electron Microscopy (cryo-EM) structures of IR in complex with insulin, IGF-I, and IGF-II, revealing that all three ligands engage the receptor at overlapping binding sites and can induce a conserved T-shaped quaternary assembly involving four ligand molecules at site 1/1′ and site 2/2′. Despite this shared overall architecture, distinct ligand-specific conformational changes are observed. Notably, IGF-I and IGF-II adopt different binding sequence at site 1 and site 2 compared to insulin, suggesting unique interaction dynamics. These structural insights highlight divergent mechanisms of ligand recognition and cooperative binding, providing a deeper understanding of hormone-induced conformational modulation of the IR.

1. Introduction

The insulin receptor (IR) is a transmembrane receptor tyrosine kinase (RTK) that plays a fundamental role in regulating glucose and lipid homeostasis in humans, particularly by mediating insulin-dependent signaling pathways that control glucose uptake, metabolism, and storage in key tissues such as muscle, fat, and liver cells [1–5]. Dysregulation of IR signaling is associated with metabolic disorders including type 2 diabetes mellitus and insulin resistance syndromes, making IR a critical therapeutic target [1,3,4].

Research suggests that the IR dimer comprises two identical protomers (protomer A and protomer B), each composed of α- and β-subunits linked by disulfide bonds and exhibiting a Λ-shaped symmetric structure [6–8]. Upon insulin binding to the extracellular domain (ECD) of IR, the signal is transmitted through the transmembrane domain to the intracellular domain, inducing phosphorylation of the tyrosine kinase domain (TK) [7–11]. Insulin binds IR at two primary interaction sites, the site 1 and site 2, each present on both protomers, resulting in a total of four potential ligand-binding sites per receptor dimer (site 1/1′ and site 2/2′). Cryo-EM studies have revealed both asymmetric (Ƭ-shaped) and symmetric (T-shaped) conformations of IR in complex with insulin, corresponding to partially and fully active signaling states, respectively [10,11].