Signal Transduction by the Adenylate Cyclase System
Regulation of cellular functions by extracellular hydrophilic signal molecules such as neurotransmitters, peptide hormones and locally acting hormonal factors requires efficient mechanisms for transmembrane signalling. By these mechanisms, the primary message, i.e., presence of a hormone or neurotransmitter at the outer surface of the plasma membrane, is translated into one or more second messages inside the cell. Formation of the intracellular messages may be induced by the neurotransmitter-occupied receptors themselves, e.g., the nicotinic acetylcholine and GABAA receptors, being channels for small cations and anions, respectively. In addition, some polypeptide hormone receptors, e.g., those of insulin and epidermal growth factor, are protein kinases themselves, causing autophosphorylation of the receptors and phosphorylation of other cellular substrates. This type of agonist-activated receptors is thought to regulate cellular functions, at least in part, by these phosphorylation reactions. The overwhelming majority, however, of plasma membrane-located receptors for hormones and neurotransmitters induces formation of intracellular messages by activating multi-component signal transduction systems, apparently located within the lipid bilayer of the plasma membrane. Out of the signalling systems studied so far, the hormone-sensitive adenylate cyclase system, which is responsible for the control of intracellular levels of cyclic AMP, is one of the best characterized examples of transmembrane signal transduction systems.
KeywordsAdenylate Cyclase Cholera Toxin Guanine Nucleotide Pertussis Toxin Signal Transduction System
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