Human Skeletal Muscle Insulin Receptor Substrate-1: Characterization of the cDNA, Gene, and Chromosomal Localization

Insulin receptor substrate-1 is a major substrate of insulin receptor Tyr kinase. We have now cloned the IRS-1 cDNA from human skeletal muscle, one of the most important target tissues of insulin action, localized and cloned the human IRS-1 gene, and studied the expression of the protein in Chinese hamster ovary cells. Human IRS-1 cDNA encodes a 1242 amino acid sequence that is 88% identical with rat liver IRS-1. The 14 potential Tyr phosphorylation sites include 6 Tyr-Met-X-Met motifs and 3 Tyr-X-X-Met motifs that are completely conserved in human IRS-1. Human IRS-1 has >50 possible Ser/Thr phosphorylation sites and one potential ATP-binding site close to the NH₂-terminal. The human IRS-1 gene contains the entire 5ʹ-untranslated region and protein coding region in a single exon and was localized on chromosome 2 q36–37 by in situ hybridization. By Northern blot analysis, IRS-1 mRNA is rare and consists of two species of 6.9 and 6 kilobase. By using quantitative polymerase chain reaction after reverse transcription of total RNA from human fetal tissues, IRS-1 mRNA could be identified in all tissues. When human IRS-1 cDNA was expressed in Chinese hamster ovary cells, the protein migrated between 170,000–180,000 M_r in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was rapidly Tyr phosphorylated upon insulin stimulation. Thus, IRS-1 is widely expressed and highly conserved across species and tissues. Compared with rat protein, human IRS-1 contains more potential Ser/Thr phosphorylation sites and only one nucleotide binding site. The entire protein coding sequence is contained within a single exon.