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G proteinG Proteins, short for Guanine Nucleophile Binding Proteins, are a family of proteins involved in second messenger cascades. They are so-called because of their signaling mechanism, which uses the exchange of guanine dinucleotide phosphate (GDP) for guanine trinucleotide phosphate (GTP) as a molecular "switch" to allow or inhibit biochemical reactions inside the cell. Alfred Gilman and Martin Rodbell were awarded the Nobel Prize in Physiology or Medicine in 1994 for their discovery and research on G proteins.
General properties
G proteins belong to the larger grouping of GTPases. "G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes loosely referred to as the "large" G proteins. These proteins are associated with G protein coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits. There are also "small" G proteins that are not "membrane-associated" nor "heterotrimeric", but are also bound to GTP/GDP and involved in signal transduction. Examples include ras and other monomeric small GTPases.
G proteins are perhaps the most important signal transducing molecules in cells. In fact, diseases such as diabetes, alcoholism, and certain forms of pituitary cancer, among many others, are thought to have some root in the malfunction of G proteins, and thus a fundamental understanding of their function, signaling pathways and protein interactions may lead to eventual treatments and possibly the creation of various preventative approaches.
Receptor-associated G proteins
Receptor associated G proteins are bound to the inside of the cell membrane. When a ligand activates a receptor, the G protein will bind to the receptor and stimulate the exchange of GTP for GDP on the G alpha subunit of the G protein. This exchange allows intracellular signalling through the dissociation of their alpha subunit from the beta and gamma subunit complex. All G proteins transduce signals to intracellular effectors such as the example of adenylate cyclases, which increase the second messenger cyclic adenosine monophosphate (cAMP). Second messengers then interact with other proteins down stream to cause a change in cell behavior. Both the alpha subunit and the beta-gamma subunit complex act as signaling elements, capable of activating downstream molecules. The α subunit will eventually hydrolize the attached GTP to GDP, releasing it from its effector and allowing it to bind with GβGγ and re-attach to the receptor.
Alpha subunits
Gα subunits consist of two domains, the GTPase domain, and the alpha-helical domain. There exist at least 20 different alpha subunits, which are separated into several main families:
- Gαs or simply Gs (stimulatory) - activates adenylate cyclase to increase cAMP synthesis
- Gαi or simply Gi (inhibitory) - inhibits adenylate cyclase
- Golf (olfactory) - couples to olfactory receptors
- Gt (transducin) - transduces visual signals in conjunction with rhodopsin in the retina
- Gq - stimulates phospholipase C
- The G12/13 family - important for regulating cytoskeleton, cell junctions and other processes related to movements
Beta-Gamma Complex
The β and γ subunits are closely bound to one another and are referred to as the beta-gamma complex. The β - γ complex is released from the α subunit when GTP binds to the α. Released from the α subunit, the β - γ complex can act as a signaling molecule itself, by activating other second messengers or by gating ion channels directly. For example, the β - γ complex attached to histamine receptors can activate phospholipase A2. Beta-gamma complexes attached to muscarinic acetylcholine receptors, on the other hand, directly open G protein coupled inward rectifying potassium (GIRK) channels.
Sources
- Kandel, Eric, James Schwartz, and Thomas Jessel. 2000. Principles of Neural Science. 4th ed. McGraw-Hill, New York.
- Voet, Donald and Judith G. Voet. 1995. Biochemistry 2nd ed. John Wilely & Sons, New York.
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