A Yersinia effector known as YopT and a Pseudomonas avirulence protein known as AvrPphB define a family of 19 proteins involved in bacterial pathogenesis. We show that both YopT and AvrPphB are cysteine proteases, and their proteolytic activities are dependent upon the invariant C/HID residues conserved in the entire YopT family. YopT cleaves the post-translationally modified Rho GTPases near their carboxyl termini, releasing them from the membrane. This leads to the disruption of actin cytoskeleton in host cells. The proteolytic activity of AvrPphB is essential for autoproteolytic cleavage of an AvrPphB precursor as well as for eliciting the hypersensitive response in plants. The biochemical functions of most avirulence (Avr) proteins are unknown. This proposal focuses on developing a molecular understanding of how the AvrPphB family of proteins is activated and post-translationally modified. In addition, efforts to identify the substrate(s) for AvrPphB are described. Finally, we propose to obtain the x-ray structure of AvrPphB complexed with a peptide substrate. These experiments will provide us with new insights into the molecular mechanism of pathogenesis. [unreadable] [unreadable]