Clinical Implications of Guanine Nucleotide–Binding Proteins as Receptor–Effector Couplers

IT has long been known that some cellular proteins bind guanine nucleotides such as guanosine tri-phosphate (GTP) with high affinity. These binding proteins are called G proteins and have been linked to such critical intracellular functions as protein synthesis.¹ In the past decade three new G proteins have been discovered that reside in the plasma membrane and transmit information from the outside to the inside of the cell. Two of these new G proteins form part of the adenylate cyclase system, a membrane-bound enzyme complex found in virtually every cell, and the third G protein is found in the disk.