Putting G protein-coupled receptor-mediated activation of phospholipase C in the limelight

Phospholipase C (PLC) activation by cell surface receptors has been recognized as a fundamental early transmembrane signaling event that triggers a wide variety of cellular responses. These range from egg fertilization through immune cell activation, hormone secretion, and synaptic transmission to invertebrate photoreception. In each case, ligand binding to cell surface receptors initiates a chain of similar molecular events that involve heterotrimeric G nucleotide-binding proteins and PLC enzymes, ultimately leading to the hydrolysis of the plasma membrane regulatory lipid, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5) P(2)). Each of the individual molecular steps along this cascade has been scrutinized separately in populations of cells or purified membrane preparations, or using reconstituted recombinant proteins by in vitro biochemical analysis, providing invaluable information about their inner molecular workings. However, recent progress in fluorescence technology has now allowed detection and kinetic analysis of each of these biochemical events in single living cells. For the first time, these molecular steps have been put in a sequence after a thorough systematic analysis of each of them with true rate constants measured in intact cells. This accomplishment, combined with mathematical modeling, has created a novel framework in which the individual molecular steps could be analyzed and predictions be tested about their regulatory features. These new developments will help us better understand the question of what made this pathway such a suitable instrument for the detection of a plethora of signal modalities in eukaryotic cells.