Signal Transduction

size. GHRH binds to 7TM receptors on the somatotrophs and the signals are transduced through Gs. In some patients presenting with the symptoms of giantism or acromegaly, it is found that R201 of the s in the somatotrophs has been replaced by cysteine or histidine. The consequent and persistent activation of adenylyl cyclase and the elevated level of cAMP causes hypersecretion of growth hormone and gross enlargement of the gland.41 Another somatic mutation of s in thyroid cells causes excess secretion of thyroid hormone. Mutations in the equivalent position (R179: see Table 4.5, page 103), suppressing the GTPase activity of Gi2 are associated with adrenocortical and ovarian tumours.42

ADP-ribosylation of -subunits

A normal physiological (i.e. not toxin-induced) cycle of ADP-ribosylation and de-ribosylation may control the activity of -subunits and constitute another arm of control by 7TM receptors. When cells are incubated with radiolabelled adenine (as a metabolic precursor of NAD ), the most heavily labelled protein is the -subunit of heterotrimeric G proteins. The extent of

labelling under basal conditions, about 0.2% of the total pool of -subunits in CHO cells, depends on the ratio of activities of the transferase (present in the plasma membrane) and the hydrolase (present in the cytosol). The point of attachment of the ADP-ribosyl group is an arginine present in the N-terminal segment of the -subunit.43 This portion of the -subunit provides a contact surface that abuts -subunits and also its effectors including adenylyl cyclase, phospholipase C 2 and the muscarinic K channel.

As pointed out earlier, the Ca2 -calmodulin sensitive AC1 is normally subject to inhibition by -subunits. However, ADP-ribosylated -subunits are without effect.43 Likewise, activation of phosphatidylinositol-3-kinase by-subunits (see page 547) fails when they have been modified by ADPribosylation. If -subunits were to become ADP-ribosylated as part of a normal process regulated by agonist intervention at receptors, it would have the effect of maintaining the effector enzyme in an activated state. A number of ligands (e.g. cholecystokinin, UTP, and thrombin) that are active at receptors that activate phospholipase C through Gq, increase the extent of

-subunit ADP ribosylation.44 Similarly, activation of the serotonin receptor that couples through a Gi- or Go-like system to induce activation of the MAP kinase pathway (Chapter 12), also causes ADP-ribosylation of -subunits.

ADP-ribosylation and deribosylation: a general mechanism of cell control

In addition to G-protein -subunits, the cycle of ADP-ribosylation and deribosylation, catalysed by endogenous intracellular enzymes, also regulates the activities of other systems. These include mitochondrial glutamate dehydrogenase and the chaperone BiP, resident in the endoplasmic reticulum.

146