GTPase activity controls

Cholera toxin catalyzes the ADP-ribosylation of a specific arginine residue in G and Gat. This covalent modification inhibits the intrinsic GTPase activity of these a subunits and thereby freezes them in their activated, or free, state (Fig. 19-1C). By this mechanism, cholera toxin stimulates adenylyl cyclase activity and photoreceptor transduction mechanisms. The ability of cholera toxin to ADP-ribosylate G may require the presence of a distinct protein, ADP-ribosylation factor (ARF). ARF, which is itself a small G protein (Table 19-2), also is ADP-ribosylated by cholera toxin. ARF is implicated in controlling membrane vesicle trafficking (see Ch. 9). 

There are also many neurotransmitter and hormone receptors that contribute to the fine control of cAMP formation by inhibition of adenylyl cyclase. The action of inhibitory receptors is mediated by several different forms of the Gai family, specifically the Gail, Gai2, Gai3, Gao and Goa subtypes. The Ga subunits of these isoforms can inhibit the catalytic activity of adenylyl cyclase when the enzyme is activated by either Gas or forskolin. The inhibition of catalytic activity does not occur via competition with Gas but appears to occur by an interaction at a symmetric site on the AC molecule. Gai-mediated inhibition of adenylyl cyclase is most dramatic for AC5 and AC6. A few other forms of adenylyl cyclase, most notably AC1, can be inhibited by Gao but this effect is not as potent as the inhibition of AC5 and AC6 by Gai isoforms. The GTPase activity of Gai family members can be accelerated by a large family of RGS proteins (see Chapter 19). 

Figure 21.7 Control of the activity of Ras by a balance of the activities of guanine nucleotide exchange factor and GTPase. GAP is the abbreviation for GTPase-activating factor and GEF for guanine nucleotide exchange factor. Both are enzymes. Both the activities are controlled by stimuli from various cell surface receptors. Ras oncogenes are present in about 30% of all human tumours.

Under the influence of GTPase-activating proteins, the rate of GTP hydrolysis of the Ras protein may be increased up to 10 -fold. The GTPase-activating proteins control the activity state of Ras protein by drastically reducing the lifetime of the active GTP state. Due to this property, they fimction as negative regulators of the Ras protein. 

All signaling mechanisms must have this modulation feature to allow the possibility of control. For example, the Ras protein of mammalian cells is a membrane-bound GTPase. Mutations that decrease Ras s GTPase activity can contribute to uncontrolled growth (i.e., tumor formation) of mammalian cells. 

Figure 1 Ras GTPases function as regulated GDP/GTP molecular switches. Diverse extracellular signals, for example those received by membrane-bound receptors such as G-protein coupled receptors and receptor tyrosine kinases, can cause Ras GTPase activation at the plasma membrane and endomembranes. Receptor-mediated activation of Ras most commonly involves the activation of RasGEFs, which then cause transient activation of Ras. Activated Ras-GTP adopts a conformation that enhances its affinity for transient binding to and activation of downstream effectors (E).A The activated effectors then regulate distinct cytoplasmic signaling networks that control cellular proliferation, differentiation, and survival. Ras signaling is terminated by RasGAP-mediated stimulation of hydrolysis of bound GTP to GDP, which precludes further Ras-effector interaction. Tumor-associated Ras mutant proteins are insensitive to GAP stimulation.

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