Heterotrimeric Guanine Nucleotide Binding Proteins: Structure and Function

The ability to sense and appropriately respond to extracellular signaling molecules is one of the most important properties of an individual cell within a multicellular organism. As many signaling molecules are hydrophilic and unable to penetrate the cell membrane, cells require efficient mechanisms to recognize mediators at the outer surface of the cell and to transduce the signal into the cell. Some systems are relatively self-contained in structure like ligand-operated ion channels and receptors with an intrinsic tyrosine kinase or other enzymatic activity. Other sytems are built from multiple protein components. The vast majority of the multi-component systems are those regulated by heterotrimeric guanine nucleotide binding proteins (G proteins). The G protein-regulated signal transduction machinary is generally composed of three distinct molecular entities.