G-protein

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

1. 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.
2. The Rho/Rac family mediates interactions between the cytoskeleton and cell membrane.
3. The Ras family, regulates growth by transmitting signals from the cell membrane to the nucleus.
4. 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.

References

Ganong WF. Review of medical physiology. 22nd ed. New York: Lange Medical Books/McGraw-Hill; 2005.