G Proteins

G proteins are so-called because they bind the guanine nucleotides GDP and GTP. They are heterotrimers (i.e., made of three different subunits) associated with The three subunits are:

How They Work

Activated Gα is a GTPase so it quickly converts its GTP to GDP. This conversion, coupled with the return of the Gβ and Gγ subunits, restores the G protein to its inactive state. [Link to additional mechanisms that aid in turning GPCRs off.]

Some Types of Gα Subunits


This type stimulates (s = "stimulatory") adenylyl cyclase. It is the one depicted here. It is associated with the receptors for many hormones such as:

s is the target of the toxin liberated by Vibrio cholerae, the bacterium that causes cholera. Binding of cholera toxin to Gαs keeps it turned "on". The resulting continuous high levels of cAMP causes a massive loss of salts from the cells of the intestinal epithelium. Massive amounts of water follow by osmosis causing a diarrhea that can be fatal if the salts and water are not quickly replaced.


This activates phospholipase C (PLC) which generates the second messengers: q is found in G proteins coupled to receptors for


This inhibits (i = "inhibitory") adenylyl cyclase lowering the level of cAMP in the cell. Gai is activated by the receptor for somatostatin.


The "t" is for transducin, the molecule responsible for generating a signal in the rods of the retina in response to light. Gαt triggers the breakdown of cyclic GMP (cGMP).
Link to discussion of the molecular events in vision.

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7 February 2007