RECEPTORS AND SIGNAL TRANSMISSION
A wide variety of molecules are used in signaling, but in all cases the recipient cell needs a receptor . Usually the receptor is a protein situated in the cytoplasmic membrane with the site for binding the messenger facing the outside. When a signal molecule appears, it binds to the receptor causing a conformational change. The receptor then passes the signal on to other proteins, known generally as signal transmission proteins . Both receptors and signal transmission proteins often dimerize or dissociate during the process of signal transmission. There are three main types of signal transmissions.
(a) Phosphotransferase systems . Activation of the receptor causes the phosphorylation of signal transmission proteins ( Fig. 19.3 ). Protein kinases transfer phosphate groups from ATP to other proteins. The activated receptor may act as a protein kinase itself, or it may bind to and activate a separateprotein kinase. Often several proteins take part in aphosphotransfer cascade that allows the signal to be amplified or modulated in a variety of ways.
(b) Second messengers . Activation of the receptor results in synthesis of a small intracellular signal molecule known as the second messenger ( Fig. 19.4 ). This may directly activate or inhibit various enzymes and may also enter the nucleus and affect gene expression. Often a GTP-binding G protein links the receptor to the enzyme that makes the second messenger.
(c) Ion channel activation. In this case, the receptor itself acts as an ion channel ( Fig. 19.5 ). On receiving the external message it either opens or closes. The altered movement of ions through the channel then mediates further signaling.