The Assembly of Pathogenic Signaling Circuits by a Family of Bacterial Secreted Effector Proteins
Orchard, Robert Charles 1987-
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Bacterial type III secreted effector proteins facilitate Gram-negative bacterial replication, dissemination, and immune evasion in the infected host organism. While much attention has been focused on the cell inhibitory mechanisms of these virulence factors, there is emerging evidence that bacterial effectors exert direct control over host cellular behavior by assembling new signaling circuits from pre-existing regulatory modules. However, these mechanisms are poorly understood. In this work, we utilize the WxxxE family of effector proteins as a model system to understand how pathogens rewire host signaling cascades. These effectors share a core catalytic domain that functions as a guanine-nucleotide exchange factor (GEF) for Rho family GTPases. Using a structure to function approach, we uncover a GEF-GTPase pairing mechanism important for signaling fidelity and pathogenic diversity. Guided by these structural insights, we next wanted to know how E. coli, an extracellular pathogen, induces the polarization of host actin molecules. By using synthetic derivatives of the enteropathogenic E. coli GEF Map, we discover that Cdc42 GTPase activity cycles are controlled in space and time by Map’s interaction with F-actin. Mathematical modeling reveals how actin dynamics coupled to a Map-dependent positive feedback loop spontaneously polarizes Cdc42. By reconstituting the system, we further show how cells polarize in response to an extracellular spatial cue. These results demonstrate how pathogens gain systems level control over host signaling networks and suggest a new view of cellular polarity centered on the interaction between GEFs and F-actin. To explore alternative mechanisms that bacteria utilize to assemble circuits, we utilize yeast genetics to identify novel membrane-interactions. We identify for the first time the direct association of the Shigella GEF IpgB1 with acidic phospholipids. Surprisingly, we find that these protein-lipid interactions are not required for IpgB1’s known role in Shigella invasion. However, we do find that IpgB1’s interactions with eukaryotic membranes are essential for bacterial replication and persistence within host cells. Furthermore, we identify a pathogenic circuit that connects GTPase activity with phospholipid metabolism. In summation, our findings illustrate the complex evolutionary relationship between pathogen and host, and how investigating these interactions provide insight into endogenous signaling systems.