ABSTRACT
A family of membrane PROTEINS that play a critical role in transmembrane signalling in the nervous system. G proteins, of which there are several forms, are named for their ability to bind small molecules called GUANINE NUCLEOTIDES, such as GUANINE TRIPHOSPHATE (GTP). In most synaptic transmission, two events occur when an external signal (such as a neurotransmitter or sensory stimulus) has an effect on cellular function. First, the external signal binds to its receptor on the outside of the neuronal membrane. When this occurs, GTP binds to the G protein on the internal surface of the membrane, which is linked to the RECEPTOR, thus making it active. Activation of the G protein regulates the levels of SECOND MESSENGERS, which in turn control the cascade of intracellular events ultimately leading to the cell's physiological response. There are two main types of G proteins: stimulatory G proteins (Gs), which activate transduction and inhibitory G proteins (Gi), which inhibit transduction. The G protein can also be directly coupled to an ION CHANNEL; in this case the activational state of the G protein regulates the permeability of certain ion channels. Although G proteins play many roles in cellular function, among their best known roles is controlling the intracellular levels of the second messenger CYCLIC AMP.
