Postulating two functions of a receptor, ligand binding and message propagation (i.e., signaling), suggests the existence of functional domains within the receptor: a ligand-binding domain and an effector domain. The structure and function of these domains often can be deduced from highresolution structures of receptor proteins and by analysis of the behavior of intentionally mutated receptors.
The regulatory actions of a receptor may be exerted directly on its cellular target(s), effector protein(s), or may be conveyed by intermediary cellular signaling molecules called transducers. The receptor, its cellular target, and any intermediary molecules are referred to as a receptor-effector system or signal-transduction pathway. Frequently, the proximal cellular effector protein is not the ultimate physiological target but rather is an enzyme or transport protein that creates, moves, or degrades a small metabolite (e.g., a cyclic nucleotide or inositol trisphosphate) or ion (e.g., Ca2+) known as a second messenger. Second messengers can diffuse in the proximity of their binding sites and convey information to a variety of targets, which can respond simultaneously to the output of a single receptor binding a single agonist molecule. Even though these second messengers originally were thought of as freely diffusible molecules within the cell, their diffusion and their intracellular actions are constrained by compartmentation—selective localization of receptor-transducer-effector-signal termination complexes—established via protein-lipid and protein-protein interactions.
Receptors and their associated effector and transducer proteins also act as integrators of information as they coordinate signals from multiple ligands with each other and with the metabolic activities of the cell. An important property of physiological receptors that also makes them excellent targets for drugs is that they act catalytically. The catalytic nature of receptors is obvious when the receptor itself is an enzyme, but all known physiological receptors are formally catalysts. When, for example, a single agonist molecule binds to a receptor that is an ion channel, hundreds of thousands to millions of ions flow through the channel every second. Similarly, a single steroid hormone molecule binds to its receptor and initiates the transcription of many copies of specific mRNAs, which, in turn, can give rise to multiple copies of a single protein.
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