Nature-derived peptides are a valuable source for bioactive molecules. For instance, tropical Psychotria plants are rich in cyclic cystine knot peptides, called cyclotides. We previously isolated the peptide psysol 2 from Psychotria solitudinum and tested human prolyl oligopeptidase (POP) inhibition for the peptide [1]. Novel peptide inhibitors are valuable as research tool or as therapeutics in POP related diseases. To date, we lack information on their pharmacology for POP, such as the mode of inhibition as well as possible effects on the POP protein network.

The prototype psysol 2 was investigated for its POP inhibitory mechanism. A mixed type non-competitive inhibitory mode was identified, i.e. the peptide interacts with both ,the protease and the substrate-enzyme complex, at different affinities. A Yonetani-Theorell cross-competition analysis revealed that psysol 2 binds simultanously and nonexclusively to the protein as compared to a product-like transition state inhibitor.

POP is a constitutively expressed protein in lymphocytes and T-cell receptor activation affected the expression levels and enzyme activities. We tested psysol 2 and Kyp-2047, a selective protease inhibitor, on stimulated T-lymphocytes and demonstrated anti-proliferative activity. We explored the effect of psysol 2 on the POP-PP2A (protein phosphatase 2A) axis and a possible role in T-cell proliferation. The phosphorylation (p-Tyr307) as well as methylation (met-Leu309) status of PP2A catalytic subunit C changed by treating CD3+ cells with the peptide. These results indicated that PP2A activity increases with psysol 2 concentration and this may reduce TCR dependent proliferation signals in activated T-lymphocytes.

This study highlights cyclotides as first-in-class non-standard mode peptide inhibitors of human POP with an unique mechanism that exerts effects on the protease activity and protein-protein interactions. By utilizing the antiproliferative psysol 2 further insights on possible mechanisms were identified, which are certainly useful in the current (pre)-clinical developmental work on cyclotide-based therapeutics for T-cell associated diseases.