Pathogens manipulate host GTPases. (A) Positive regulation of host GTPases. The S. Typhimurium effector SopE binds and activates Rho/Rac/Cdc42 GTPases, thereby stimulating assembly of an actin-based network at the plasma membrane. SopE acts as a GEF and catalyzes the exchange of GTP for GDP. Similarly, the L. pneumophila effector DrrA functions as a GEF for the small GTPase Rab1 (a key GTPase involved in ER to Golgi and intra-Golgi vesicular transport) and activates it on the LCV. (B) Negative regulation of host GTPases. Bacteria have evolved several ways to negatively regulate GTPases. First, bacteria encode GAPs, such as SptP and LepB, to catalyze the hydrolysis of GTP to GDP to inactivate GTPases. Second, bacteria encode enzymes, such as VopS, IbpA, and DrrA, that inhibit downstream signaling of GTPases by posttranslationally modifying them with AMP. AMPylated Rac failed to interact with its effector p21-activated kinase (PAK). Similarly, AMPylated Rab1 was unable to bind to its effector MICAL-3 (microtubule-associated monoxygenase, calponin, and LIM domain containing 3). Recently, the L. pneumophila effector SidD has been shown to act on Rab1 to remove this AMP modification. Third, a novel modification on Rab1 and Rab35 was reported whereby the L. pneumophila effector AnkX modified the class II switch region of the Rabs with a phosphocholine moiety. This modification was observed in Rabs bound to both GDP and GTP. Phosphocholination of Rab35-GDP prevented its binding to its GEF connecdenn (CD). Finally, effectors such as YopT function as proteases to cleave Rho GTPases to inhibit their function.