<Li> Their production / release and destruction can be localized, enabling the cell to limit space and time of signal activity . </Li> <P> There are several different secondary messenger systems (cAMP system, phosphoinositol system, and arachidonic acid system), but they all are quite similar in overall mechanism, although the substances involved and overall effects can vary . </P> <P> In most cases, a ligand binds to a membrane - spanning receptor protein molecule . The binding of a ligand to the receptor causes a conformation change in the receptor . This conformation change can affect the activity of the receptor and result in the production of active second messengers . </P> <P> In the case of G protein - coupled receptors, the conformation change exposes a binding site for a G - protein . The G - protein (named for the GDP and GTP molecules that bind to it) is bound to the inner membrane of the cell and consists of three subunits: alpha, beta and gamma . The G - protein is known as the "transducer ." </P>

The secondary messenger system of g-protein coupled cholinoceptors includes guanylate cyclase