GTPases form a family of proteins that are often involved when cells need to control complex processes. They all share the ability to hydrolyze the nucleotide GTP but otherwise differ markedly in the processes they control and their mode of operation. We have already met one GTPase: the protein EF-tu, a component of the protein synthesis mechanism (page 172). Once a GTPase has hydrolyzed GTP to GDP it adopts an inactive shape and is unable to activate its target process (Fig. 10.3). In contrast, if the protein expels the GDP and binds a molecule of GTP, it then adopts its active form. The cycle between the GDP-bound and GTP-bound state is regulated by effector proteins. GTPase activating proteins or GAPs speed up the rate at which GTPases hydrolyze GTP, and hence the rate at which they inactivate, while guanine nucleotide exchange factors, or GEFs, assist in the exchange of GDP for GTP and therefore help GTPases adopt their active configuration.
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