Phosphoinositide-dependent kinase 1: master growth regulator

Multi-cellular organisms depend on both intercellular and intracellular signaling pathways to respond appropriately to changes in their environment. Growth factors and hormones, circulating in the blood, bind to receptors on the surface of cells to instruct them on how to respond to specific cues. Insulin is one of those cues: released from the pancreas in response to elevated blood glucose levels during digestion of food, insulin binds to receptors on the surface of muscle and liver cells, triggering a cascade of biochemical reactions that results in the uptake of glucose from the blood and its storage as either glycogen or fat. A key component in this cascade of biochemical reactions is an enzyme called phosphoinositide-dependent kinase 1 (PDK1). Without PDK1, mice are not viable; with reduced levels of PDK1, mice are smaller and exhibit impaired glucose tolerance. PDK1 is a protein kinase an enzyme that transfers phosphate from the cells energy source, ATP, to other proteins that helps transduce the signal from insulin, on the surface of the cell, to effector proteins inside the cell that promote glucose uptake and storage. We recently showed that PDK1 is acutely regulated by a membrane lipid called PIP3 that is generated in response to insulin binding to its receptor. The precise mechanism, however, by which it is activated at the cell membrane by PIP3 is not known. A key step, however, is its auto-activation by a biochemical reaction called trans-autophosphorylation. In this reaction, two molecules of PDK1 phosphorylate each other to drive downstream signaling. This proposal will combine high-resolution structural biology techniques with membrane and protein biochemistry to elucidate how PDK1 is activated by PIP 3 and, conversely, how it is maintained in an inactive state in the absence of insulin. We expect to learn key details of the autophosphorylation reaction, which is shared by ~20% of the approximately 500 human protein kinases and for which we still lack a mechanistic understanding. The signaling pathway in which PDK1 is embedded is hyperactivated in the majority of human cancers, making the inhibition of PDK1, along with upstream and downstream components of the pathway, an important goal of clinical drug development. This proposal is expected to shed light on the inactive conformation of PDK1, potentially leading to new avenues for therapeutic intervention. Furthermore, PDK1 has been reported to activate 23 substrate kinases that transduce cellular information in a diverse array of signaling pathways, observations which have led to its moniker as a master kinase. This work, therefore, is expected to have implications for the regulation of the many physiological processes in the cell that have been reported to depend on PDK1.