Scheme 7-2. Stereotype of 7TMR protein structure. Seven regions are hydrophobic and insert stably in the cell membrane. The intracellular domain is involved in signal transduc-tion. Signal may be received by the extracellular domain(s) or in some instances occurs in the intracellular or membane domains. Specific amino acid variants in the appropriate domains affect the nature of the ligand (or light energy frequency) that triggers a response, and hence is how divergence and specialization in function is brought about.

Intracellular workings: G proteins are heterotrimers, proteins with three subunits: a, b, and g. The structure of the a-subunit usually defines the type of G protein. The a-subunit binds a guanine nucleotide (thus giving this class of proteins their name). When the guanine is in the form of GDP (guanine nucleotide di-phosphate, rather than tri-), the a-subunit binds with the bg-subunits. When the nucleotide becomes a GTP (guanine nucleotide triphosphate), which is induced by the receptor-G protein interaction, the G protein is activated, and the a-subunit is freed from the complex (Lewin 2000).

What the a-subunit does next depends on the class of G protein-coupled receptor; despite the large number of GPCRs, there are only a few basic ways in which they affect signal transduction. Some GPCRs regulate the activity of enzymes, and some affect the concentration of cyclic nucleotides or calcium inside the cell, with the effect of activating or inactivating ion channels. Others directly activate or inactivate ion channels, but the effect of G protein activity involved in sensory transduction is to alter the ion permeability, and thus the excitability, of the membrane of the target cell.

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