G proteins, short for guanine nucleotide binding proteins, are GTPases that are involved in second messenger cascades.
The G protein signaling mechanism uses the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP). The GTP-protein complex brings about the effect, and is converted back to GDP by the inherent GTPase activity of the protein restoring the resting state.
Small G proteins
- The Rab family of these proteins regulate the rate of vesicle traffic between the endoplasmic reticulum, the Golgi apparatus, lysosomes, endosomes, and the cell membrane.
- The Rho/Rac family mediates interactions between the cytoskeleton and cell membrane.
- The Ras family, regulates growth by transmitting signals from the cell membrane to the nucleus.
- The members of these three families are related to the product of the ras proto-oncogene.
Heterotrimeric G proteins
These larger G proteins couple cell surface receptors to catalytic units that catalyze the intracellular formation of second messengers or couple the receptors directly to ion channels. These G proteins are made up of three subunits designated α, β, and γ. The α subunit is bound to GDP, which it converts to GTP when a ligand binds. The α subunit then separates from the combined β and γ and acts as an effectors. The βγ units also activates a variety of effectors.
Heterotrimeric G proteins relay signals from over 1000 receptors, and their effectors in the cells include ion channels and enzymes.
Ganong WF. Review of medical physiology. 22nd ed. New York: Lange Medical Books/McGraw-Hill; 2005.