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 Dr. Josť de Ondarza 
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G-proteins

The term "G-protein" refers to any protein which binds to (and subsequently hydrolyzes) the guanine nucleotide GTP. The superfamily of G-protein includes translation elongation factors (such as EF-1), small G-proteins (such as ras-21), and the heterotrimeric G-proteins (such as transducin). The heterotrimeric G-proteins  are primarily involved in signal transduction and linked to 7-transmembrane-domain (7-TMD) receptors, whereas the small G-proteins (often referred to as "GTP-binding proteins") play roles in a diversity of events, including exocytosis, cell-cycle regulation, transport, differentiation and signal transduction. This overview will focus on the heterotrimeric G-proteins (henceforth called "G-proteins").


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Heterotrimeric G-proteins, as the name implies, consist of three different subunits, designated alpha, beta, and gamma. The alpha subunits (blue) typically measure 39 - 52 kD in size and contain the GTPase (binding and hydrolysis of GTP) activity of the heterotrimer. The beta (teal; ~35 - 38 kD) and gamma (green; ~ 5 - 8 kD) subunits associate tightly with each other. GTP is shown in yellow. The entire complex is often anchored into the plasma membrane by means of covalent modifications of the alpha and/or gamma subunits.

Heterotrimeric G-proteins are activated by the interaction of the heterotrimer with a ligand-bound 7-TMD receptor. The immediate result of this interaction is displacement of GDP (the hydrolyzed form of GTP) from the alpha subunit by a molecule of GTP. Subsequently, the heterotrimer dissociates into an activated alpha subunit (with GTP) and the beta-gamma complex. Depending on the type of G-protein and cell involved, either the alpha subunit, the beta-gamma complex, or both may in turn bind to cellular target sites, thus passing the signal on to a diversity of intracellular effectors. Such effectors include protein kinases (e.g. PK-C), ion channels (e.g. Potassium channels), enzymes (e.g. adenylyl cyclase), and cytoskeletal proteins (e.g. tubulin). A sample signaling cascade can be viewed in this demonstration generated by Larry Keeley at Texas A&M.

 

This page was last edited on 07/24/01 .

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