Platelet-activating factor , binding structure

The insulin receptor is the prototype for a number of receptor enzymes with a similar structure and receptor Tyr kinase activity. The receptors for epidermal growth factor and platelet-derived growth factor, for example, have structural and sequence similarities to the insulin receptor, and both have a protein Tyr kinase activity that phosphorylates IRS-1. Many of these receptors dimerize after binding ligand the insulin receptor is already a dimer before insulin binds. The binding of adaptor proteins such as Grb2 to (P) Tyr residues is a common mechanism for promoting protein-protein interactions, a subject to which we return in Section 12.5. 

The term selectin was introduced to describe three adhesion molecules whose function and expression were highly selective and which possessed a terminal lectin domain. The nomenclature for each molecule relates to the cell on which they were first described E-selectin (endothelium), L-selectin (lymphocyte), and P-selectin (platelet). All three share similar structural features (1) an extracellular amino terminal carbohydrate-binding (i.e., lectin-like) domain that requires Ca2+ for activation (2) an epidermal growth factor-like domain and (3) repeated domains with homologies to complement-regulatoiy proteins. 

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