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Pharmacology Guide

Molecular basis of drug action: G-proteins

The name is short for guanine nucleotide binding protein. They are a class of membrane proteins which link the activation of a 7-helical receptor to the subsequent activation of its second messenger. They are heterotrimeric proteins, i.e. they have three different subunits (an alpha- subunit, and the beta- and gamma- subunits which are tightly bound to each other). 

More than 100 examples of at least 20 different types of G-protein have so far been cloned, of which many can be grouped into categories depending on which second messenger they predominantly control. Thus, Gs frequently activates adenylyl cyclase, whereas Gi inhibits it; Gq mediates the stimulation of phospholipase C and hence phosphoinositide turnover. However, these distinctions are not absolute: many G proteins do not necessarily couple to the same second messenger in different cells, and they may even couple to more than one in a single cell. 

Gs protein is irreversibly activated by cholera toxin. Subtypes of Gi protein have been identified, some of which are irreversibly inactivated by pertussis toxin. In both instances by ADP-ribosylation of the alpha-subunit. 

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