Bifunctional Probes of Cathepsin Protease Activity and pH Reveal Alterations in Endolysosomal pH during Bacterial Infection.

Cysteine cathepsins are lysosomal proteases involved in regulation of both normal cellular processes and disease. Biochemical studies with peptide substrates indicate that cathepsins have optimal activity at acidic pH and highly attenuated activity at neutral pH. In contrast, there is mounting evidence that cathepsins have biological roles in environments that have non-acidic pH. To further define the specific pH environments where cathepsins act, we designed bifunctional activity-based probes (ABPs) that allow simultaneous analysis of cathepsin protease activity and pH. We use these probes to analyze the steady-state environment of cathepsin activity in macrophages and to measure dynamic changes in activity and pH upon stimulation. We show that Salmonella typhimurium induces a change in lysosomal pH that ultimately impairs cathepsin activity in both infected cells and a fraction of bystander cells, highlighting a mechanism by which Salmonella can simultaneously flourish within host cells and alter the behavior of nearby uninfected cells.