Given the myriad extracellular signals impinging on cells, the number of mechanisms used to convey information to them is surprisingly limited. A common feature is a signal discriminator, better known as a receptor. Often, the receptor is also an intracellular signal generator. Binding of an activating signal to the receptor triggers its intrinsic signal-generating capacity. An exception to this pattern is a large class of cell-surface receptors that are coupled to distinct intracellular signal generators by molecular go-betweens, called G proteins. G proteins are products of a large gene family. They specifically bind guanine nucleotides and hydrolyze the last phosphate.
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