Pseudomonas aeruginosa inhibitor of cysteine peptidases (PA-ICP) is a potent protein inhibitor of papain-like cysteine peptidases (CPs) identified in Pseudomonas aeruginosa, an opportunistic pathogenic bacteria that can cause severe infections in human. It belongs to the newly characterized natural CP inhibitors of the I42 family, designated the ICP family. The members of this family are present in some protozoa and bacterial pathogens. They can
inhibit both parasite and mammalian CPs with high affinity and specificity. Whether the main biological function of the proteins in the pathogens is to regulate the hydrolytic
activity of the organisms’ endogenous CPs or exogenous CPs so as to facilitate the
pathogens’ invasion or survival is still under investigation. Although Pseudomonas
aeruginosa contains a CP inhibitor, no CP genes are found in its genome, suggesting that
the targets of PA-ICP may be exogenous. This hypothesis is supported by the presence of a
putative secretion signal peptide at the N-terminus of PA-ICP which may be involved in
exporting the protein to target exogenous CPs.
In order to shed light on the biological function and inhibitory specificity of PA-ICP, the
structure and backbone dynamics of this protein were characterised using NMR
spectroscopy. In this project, the inhibitory activity of PA-ICP to a range of mammalian
model CPs was also studied. Like its previously studied homologs, PA-ICP adopts an
immunoglobulin fold comprised of seven β-strands. Three highly conserved sequence
motifs located in mobile loop regions form the CP binding site. The inhibitor exhibits higher
affinity toward the mammalian CP cathepsin L than cathepsins H and B. Homology
modelling of the PA-ICP-cathspin L interaction based on the crystal structure of the
chgasin-cathpsin L complex shows that PA-ICP may inhibit the peptidases by blocking
the enzyme’s active site and that the interactions between chagasin and CPs may be
conserved in PA-ICP-peptidase complexes. The specificity of the inhibitors may be
determined by the relative flexibility of the loops bearing the binding site motifs and the
electrostatic properties of certain residues near the binding sites
