G proteins are named for their ability to bind and hydrolyze the guanine nucleotide GTP. In the widest sense, the superfamily of guanine nucleotide-binding proteins comprises two structurally distinct classes, the monomeric GTP-binding proteins (also called monomeric GTPases) which are involved in a variety of cellular processes and the heterotrimeric GTP-binding proteins which are primarily involved in transmembrane signal transduction by coupling membranous receptors to various effector molecules. Traditionally, the term “G protein” is only applied to the latter group, the heterotrimeric GTP-binding proteins.
Cellular functions in a living organism are regulated and coordinated by a huge variety of extracellular signals including hormones, growth factors, paracrine factors, neurotransmitters, or sensory stimuli. Many of these signals are received by cells...