Phosphorylation of Ser in Insulin Receptor Substrate-1 Blocks Interactions with the Insulin Receptor and Inhibits Insulin Action*

Serine phosphorylation of insulin receptor substrate-1 (IRS-1) inhibits insulin signal transduction in a variety of cell backgrounds, which might contribute to peripheral insulin resistance. However, because of the large number of potential phosphorylation sites, the mechanism of inhibition has been difficult to determine. One serine residue located near the phosphotyrosinebinding (PTB) domain in IRS-1 (Ser in rat IRS-1 or Ser in human IRS-1) is phosphorylated via several mechanisms, including insulin-stimulated kinases or stress-activated kinases like JNK1. During a yeast trihybrid assay, phosphorylation of Ser by JNK1 disrupted the interaction between the catalytic domain of the insulin receptor and the PTB domain of IRS-1. In 32D myeloid progenitor cells, phosphorylation of Ser inhibited insulin stimulation of the phosphatidylinositol 3-kinase and MAPK cascades. These results suggest that inhibition of PTB domain function in IRS-1 by phosphorylation of Ser (Ser in human IRS-1) might be a general mechanism to regulate insulin signaling.