Bacterial pathogens secrete effector proteins that take over the host cellular machinery to create conditions conducive to survival and proliferation. In a screen for effector proteins that alter vesicular trafficking through the general secretory pathway, Selyunin et al. found that expression of the type III effector EspG encoded by the enterohemorrhagic Escherichia coli (EHEC) O157:H7 inhibited exocytosis of ectopically expressed human growth hormone from HeLa cells. Fluorescently tagged EspG localized to the Golgi and induced fragmentation of this organelle. A yeast two-hybrid screen revealed that EspG interacted with the GTPases ARF1 and ARF6 (which are involved in vesicle transport) and p21-activated kinases (PAKs) 1, 2, and 3 (which are effectors for the GTPases Cdc42 and Rac1). In vitro binding assays indicated that EspG interacted with the ARF GTPase domain and with the Iα3 helix in the autoinhibitory domain of PAKs. In the crystal structure of EspG in complex with the ARF6 GTPase domain, EspG selectively engaged the switch I loop of GTP-loaded (but not GDP-bound) forms of ARF6, which enabled EspG to block access of ARF-GAP (GTPase-activating protein) to ARF6 and prevent GAP-stimulated GTP hydrolysis. The crystal structure of EspG in complex with the PAK2 Iα3 helix compared with that of the Iα3 helix in the autoinhibited PAK1 homodimer suggested that EspG binding disrupted the interaction of the kinase and autoinhibitory domains to allosterically activate PAK, a finding that was confirmed by kinase activity assays. The binding sites for ARF6 and PAK2 on EspG were distinct and nonoverlapping. ARF1GTP recruited EspG, which then recruited PAK2 to the membrane surface of Golgi mimetic liposomes. In HEK293A cells, transfected ARF1 and EspG colocalized at the Golgi, and a construct encoding fluorescently tagged PAK2 Iα3 helix colocalized with wild-type EspG but not with an EspG mutant unable to activate PAK2. The authors suggest that the artificial scaffold created by EspG enables the bacterium to subvert the ARF and PAK signaling pathways without interference from endogenous regulatory systems.
A. S. Selyunin, S. E. Sutton, B. A. Weigele, L. E. Reddick, R. C. Orchard, S. M. Bresson, D. R. Tomchick, N. M. Alto, The assembly of a GTPase-kinase signalling complex by a bacterial catalytic scaffold. Nature 469, 107–111 (2011). [PubMed]