You are carrying out experiments to examine insulin-stimulated glucose uptake in isolated skeletal muscle cells. You know that insulin first binds to its receptor on the plasma membrane of these cells triggering an increase in phosphatidylinositol-3 kinase (PI3K) activity. The net effect of increased PI3K activity is insertion of subcellular membranes containing GLUT4 protein into the plasma membrane. Which of the following is the most likely first intracellular step in this signaling cascade?
A) activation of PI3K
B) activation of phospholipase Cb (PLCb)
C) autophosphorylation of the receptor tyrosine kinase
D) phosphorylation of insulin receptor substrate 1 (IRS1)
E) phosphorylation of mitogen-activated protein kinase (MAPK)

Question

Quizplus · Accepted Answer

The first intracellular step after insulin binds to its receptor is the autophosphorylation of the receptor tyrosine kinase.  All of the postreceptor responses initiated by insulin binding to its receptor are mediated as a consequence of the activation of several signal transduction pathways. These signal transduction cascades involve the activation of several different kinases, such as PI3K, that phosphorylate their substrates in response to activation from upstream factors in the insulin signaling cascade. In order for these events to occur, the intrinsic tyrosine kinase activity of the insulin receptor must be activated which results in autophosphorylation of the receptor. The tyrosine phosphorylated receptor then serves as a docking site for proteins that, in turn, activate PI3K activity.

You Are Carrying Out Experiments to Examine Insulin-Stimulated Glucose Uptake