Functional and structural complexity of signal transduction via G-protein-coupled receptors

A prerequisite for the maintenance of homeostasis in a living organism is fine-tuned communication between different cells. The majority of extracellular signaling molecules, such as hormones and neurotransmitters, interact with a three-protein transmembrane signaling system consisting of a receptor, a G protein, and an effector. These single components interact sequentially and reversibly. Considering that hundreds of G-protein-coupled receptors interact with a limited repertoire of G proteins, the question of coupling specificity is worth considering. G-protein-mediated signal transduction is a complex signaling network with diverging and converging transduction steps at each coupling interface. The recent realization that classical signaling pathways are intimately intertwined with growth-factor-signaling cascades adds another level of complexity. Elaborate studies have significantly enhanced our knowledge of the functional anatomy of G-protein-coupled receptors, and the concept has emerged that receptor function can be modulated with high specificity by coexpressed receptor fragments. These results may have significant clinical impact in the future.