▲ FIGURE 3-29 Cycling of GTPase switch proteins between the active and inactive forms. Conversion of the active into the inactive form by hydrolysis of the bound GTP is accelerated by GAPs (GTPase-accelerating proteins) and RGSs (regulators of G protein-signaling) and inhibited by GDIs (guanine nucleotide dissociation inhibitors). Reactivation is promoted by GEFs (guanine nucleotide-exchange factors).

of the GTPase superfamily function in protein synthesis, the transport of proteins between the nucleus and the cytoplasm, the formation of coated vesicles and their fusion with target membranes, and rearrangements of the actin cytoskeleton.

All the GTPase switch proteins exist in two forms (Figure 3-29): (1) an active ("on") form with bound GTP (guanosine triphosphate) that modulates the activity of specific target proteins and (2) an inactive ("off") form with bound GDP (guanosine diphosphate). The GTPase activity of these switch proteins hydrolyzes bound GTP to GDP slowly, yielding the inactive form. The subsequent exchange of GDP with GTP to regenerate the active form occurs even more slowly. Activation is temporary and is enhanced or depressed by other proteins acting as allosteric regulators of the switch protein. We examine the role of various GTPase switch proteins in regulating intracellular signaling and other processes in several later chapters.

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