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G protein

G proteins, short for guanine nucleotide 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 guanosine diphosphate (GDP) for guanosine triphosphate (GTP) as a general molecular "switch" function to regulate cell processes.

G proteins belong to the larger grouping of GTPases.

History

Alfred Gilman and Martin Rodbell were awarded the Nobel Prize in Physiology or Medicine in 1994 for their discovery of and research on G proteins.

Function

G proteins are perhaps the most important signal transducing molecules in cells. In fact, diseases such as diabetes 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 preventive approaches. G proteins are also central in our urge to urinate and defecate as the second messengers they transduce (stimulate) various receptors in our lower intestine and bladder wall.

Large and small G proteins

• "G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins. These proteins are activated by G protein-coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits.
• There are also "small" G proteins or small GTPases like Ras that are monomeric, but also bind GTP and GDP and are involved in signal transduction.

Receptor-activated G proteins

Receptor activated G proteins are bound to the inside surface of the cell membrane. They consist of the G_α and the tightly associated G_βγ subunits. When a ligand activates the G protein-coupled receptor, it induces a conformation change in the receptor (a change in shape) that allows the G protein to now bind to the receptor. The G protein then releases its bound GDP from the G_α subunit, and binds a new molecule of GTP. This exchange triggers the dissociation of the G_α subunit, the G_βγ dimer, and the receptor. Both, G_α-GTP and G_βγ, can then activate different signalling cascades (or second messenger pathways) and effector proteins, while the receptor is able to activate the next G protein. The G_α subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to reassociate with G_βγ and starting a new cycle.

A well characterized example of a G protein-triggered signalling cascade is the cAMP pathway. The enzyme adenylate cyclase is activated by G_αs-GTP and synthesizes the second messenger cyclic adenosine monophosphate (cAMP) from ATP. Second messengers then interact with other proteins downstream to cause a change in cell behavior.

Lipidation

Many G proteins are modified by having specific lipids attached to them, i.e. they are lipidated.

• Trimeric G proteins may be myristolated, palmitoylated, or prenylated.
• Small G proteins may be prenylated.

References

• Eric R. Kandel, James H. Schwartz, Thomas M. Jessell, Principles of neural science, McGraw-Hill, New York, 2000, No. ISBN 0-8385-7701-6.
• Lodish et al. 2000. Molecular Cell Biology 4th ed. W.H. Freeman and Company, New York.
• Voet, Donald and Judith G. Voet. 1995. Biochemistry 2nd ed. John Wilely & Sons, New York.
• Gilman A, G proteins: transducers of receptor-generated signals., Annu Rev Biochem, Vol. 56, pp. 615-49, 1987, No. PMID 3113327.

External link

Contents

• History
• Function
• Large and small G proteins
• Receptor-activated G proteins
• Lipidation
• References
• External link