DELETION OF ASN(281) IN THE ALPHA-SUBUNIT OF THE HUMAN INSULIN-RECEPTOR CAUSES CONSTITUTIVE ACTIVATION OF THE RECEPTOR AND INSULIN DESENSITIZATION

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 all ele carried a major deletion, including exons 10-13, which shifted the reading frame and introduced a premature chain termination codon in t he IR sequence. This allele was expressed at a very low level in cultu red fibroblasts (<10% of total IR messenger ribonucleic acid content) and encoded a truncated protein lacking transmembrane and tyrosine kin ase domains. The maternal IR allele was deleted of 3 bp in exon 3, cau sing the loss of Asn(281) in the alpha-subunit. This allele generated levels of IR messenger ribonucleic acid and cell surface receptors sim ilar 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 ac tivity. As a result of this IR-preactivated state, the cells were dese nsitized to insulin stimulation of glycogen and DNA syntheses. These f indings strongly suggest that Asn(281) of the IR alpha-subunit plays a critical role in the inhibitory constraint exerted by the extracellul ar alpha-subunit over the intracellular kinase activity.