Site-Directed Spin Label EPR Studies of the Structure and Membrane Interactions of the Bacterial Phospholipase ExoU

Site-directed spin labeling (SDSL) has been invaluable in the analysis of protein structure and dynamics and has been particularly useful in the study of membrane proteins. ExoU, an important virulence factor in Pseudomonas aeruginosa infections, is a bacterial phospholipase A2 that functions at the membrane-aqueous interface. Using the SDSL methodology developed in the Hubbell lab, we find that the region surrounding the catalytic site of ExoU is buried within the tertiary structure of the protein in the soluble, apoenzyme state, but shows a significant increase in dynamics upon membrane binding and activation by ubiquitin. Continuous wave (CW) power saturation EPR studies show that the conserved serine hydrolase motif of ExoU localizes to the membrane surface in the active, holoenzyme state. SDSL studies on the C-terminal four-helix bundle (4HB) domain of ExoU similarly show a co-operative effect of ubiquitin binding and membrane association. CW power saturation studies of the 4HB domain indicate that two interhelical loops intercalate into the lipid bilayer upon formation of the holoenzyme state, anchoring ExoU at the membrane surface. Together these studies establish the orientation and localization of ExoU at the membrane surface and illustrate the power of SDSL as applied to peripheral membrane proteins.