Elucidation of a pH-folding switch in the Pseudomonas syringae effector protein AvrPto

Pathogenic bacteria have developed extraordinary strategies for invading host cells. The highly conserved type III secretion system (T3SS) provides a regulated conduit between the bacterial and host cytoplasm for delivery of a specific set of bacterial effector proteins that serve to disrupt host signaling and metabolism for the benefit of the bacterium. Remarkably, the inner diameter of the T3SS apparatus requires that effector proteins pass through in at least a partially unfolded form. AvrPto, an effector protein of the plant pathogen Pseudomonas syringae, adopts a helical bundle fold of low stability (Delta G(F -> U) = 2 kcal/mol at pH 7, 26.6 degrees C) and offers a model system for chaperone-independent secretion. P. syringae effector proteins encounter a pH gradient as they translocate from the bacterial cytoplasm (mildly acidic) into the host cell (neutral). Here, we demonstrate that AvrPto possesses a pH-sensitive folding switch controlled by conserved residue H87 that operates precisely in the pH range expected between the bacterial and host cytoplasm environments. These results provide a mechanism for how a bacterial effector protein employs an intrinsic pH sensor to unfold for translocation via the T3SS and refold once in the host cytoplasm and provide fundamental insights for developing strategies for delivery of engineered therapeutic proteins to target tissues.