Deletion of Asn281 in the α-Subunit of the Human Insulin Receptor Causes Constitutive Activation of the Receptor and Insulin Desensitization

Abstract

We studied the structure and function of the insulin receptor (IR) in two sisters with leprechaunism. The patients had inherited alterations in the IR gene and were compound heterozygotes. Their paternal IR allele carried a major deletion, including exons 10-13, which shifted the reading frame and introduced a premature chain termination codon in the IR sequence. This allele was expressed at a very low level in cultured fibroblasts (<10% of total IR messenger ribonucleic acid content) and encoded a truncated protein lacking transmembrane and tyrosine kinase domains. The maternal IR allele was deleted of 3 bp in exon 3, causing the loss of Asn281 in the α-subunit. This allele generated levels of IR messenger ribonucleic acid and cell surface receptors similar to those seen in control fibroblasts. However, IRs from patients' cells had impaired insulin binding and exhibited in vivo and in vitro constitutive activation of autophosphorylation and tyrosine kinase activity. As a result of this IR-preactivated state, the cells were desensitized to insulin stimulation of glycogen and DNA syntheses. These findings strongly suggest that Asn281 of the IR α-subunit plays a critical role in the inhibitory constraint exerted by the extracellular α-subunit over the intracellular kinase activity.