Speaker:Lawrence J. Mandarino,
Director, Center for Metabolic Biology Professor and Chair,
Department of Kinesiology

Title:Regulation of Protein Function by Multiple Phosphorylation Events - Insulin Receptor Substrate-1 (IRS-1) as a Model

Abstract:The functions of many proteins are regulated by post-translational modifications. Phosphorylation of proteins on serine or threonine amino acid residues is a well-known modifier of protein function. Some proteins, those that have only one or two such phosphorylation sites, are amenable to functional mutation analysis that can define the effect of phosphorylation on the activity of the protein. The regulation of other proteins, those which have many phosphorylation sites, is more difficulty to study. IRS-1 is a central protein in the insulin signaling cascade, and defects in the ability of IRS-1 to transduce the insulin signal in muscle cells are closely related to the development of insulin resistance. Insulin resistance, in turn, is the first abnormality apparent in individuals who suffer from obesity, type 2 diabetes mellitus, and cardiovascular disease. These diseases account for about 40% of health care costs in the United States. Human IRS-1 is a 1242 amino acid protein that contains approximately 240 potential serine or threonine phosphorylation sites, of which over 50 have been identified as being actually phosphorylated in cells or in vitro. Some of these sites have a known effect on the function of IRS-1, but the function of most of these sites is unknown. Furthermore, the interactions among these sites are rarely studied, and little attention at all is given to whether the overall pattern of phosphorylation itself regulates IRS-1 function. We have developed new mass spectrometry techniques that can provide relative phosphorylation state analysis of IRS-1 isolated from small, needle biopsies of human muscle, allowing access for the first time to a global phosphorylation analysis of this protein in vivo in healthy subjects and insulin resistant patients. Our goal is to understand how phosphorylation of this molecule predicts insulin action in vivo in humans.