ABSTRACT
This chapter explains basic mode of action of the heptahelical receptor type with the help of a few examples. Heptahelical receptors represent one of the most successful models of evolution. In animal cells, receptor proteins anchored in the cell membrane by seven transmembrane domains represent the most abundant receptor type encoded by one of the largest gene families. Although muscarine was found to be an acetylcholine agonist and the binding sites turned out to be acetylcholine receptors, the name muscarinic receptors had become firmly established. In mammals various isoforms of protease-activated receptors have been found. The brassinosteroid receptor is coupled to a protein kinase rather than to a G-protein and the mechanism of signal transduction remains a contentious issue. Some G-protein-coupled receptors (GPCRs) induce the release of the intracellular messenger ceramide along a G-protein-independent signaling pathway. Adaptation of the signal-processing apparatus takes place primarily at the receptor according to a mechanistic scheme that probably holds true for most GPCRs.
