[The insulin receptor: mechanism of activation and message transmission]

The insulin receptor is a heterotetrameric glycoprotein composed of two 130 kD extracellular alpha subunits and two 95 kD membrane-spanning beta subunits. The insulin receptor functions as an allosteric enzyme which undergoes conformational changes when its alpha subunit binds insulin, resulting in activation and autophosphorylation of the tyrosine kinase contained in the beta subunit. This receptor activation is due to intermolecular reactions responsible for amplification of the hormone-induced response at the receptor level. Activation of the receptor tyrosine kinase initiates a cascade of phosphorylation/dephosphorylation reactions and enzyme activation/deactivation reactions. Insulin causes very rapid activation of the enzymes MAP kinase (Microtubule Associated Protein kinase) and phosphatidylinositol-3 kinase, which may act as key links between the insulin receptor and the cell effectors responsible for hormone-induced responses.