Mechanism of Pseudomonas aeruginosa Small Protease (PASP), a Corneal Virulence Factor

Pseudomonas aeruginosa is the leading cause of contact lens-associated bacterial keratitis. Secreted bacterial proteases have a key role in keratitis, including the P. aeruginosa small protease (PASP), a proven corneal virulence factor. We investigated the mechanism of PASP and its importance to corneal toxicity.

PASP, a serine protease, was tested for activity on various substrates. The catalytic triad of PASP was sought by bioinformatic analysis and site-directed mutagenesis. All mutant constructs were expressed in a P. aeruginosa PASP-deficient strain; the resulting proteins were purified using ion-exchange, gel filtration, or affinity chromatography; and the proteolytic activity was assessed by gelatin zymography and a fluorometric assay. The purified PASP proteins with single amino acid changes were injected into rabbit corneas to determine their pathological effects.

PASP substrates were cleaved at arginine or lysine residues. Alanine substitution of PASP residues Asp-29, His-34, or Ser-47 eliminated protease activity, whereas PASP with substitution for Ser-59 (control) retained activity. Computer modeling and Western blot analysis indicated that formation of a catalytic triad required dimer formation, and zymography demonstrated the protease activity of the homodimer, but not the monomer. PASP with the Ser-47 mutation, but not with the control mutation, lacked corneal toxicity, indicating the importance of protease activity.

PASP is a secreted serine protease that can cleave proteins at arginine or lysine residues and PASP activity requires dimer or larger aggregates to create a functional active site. Most importantly, proteolytic PASP molecules demonstrated highly significant toxicity for the rabbit cornea.