A guanosine nucleotide dissociation inhibitor (GDI) binds to the GDP-bound form of Rho and Rab small GTPases and not only prevents exchange (maintaining the small GTPase in an off-state), but also prevents the small GTPase from localizing at the membrane, which is their place of action. This inhibition can be removed by the action of a GDI displacement factor.[1]
It also inhibits cdc42 by binding to its tail and preventing its insertion into membranes; hence it cannot trigger WASPs and cannot lead to nucleation of F-actin.
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