Functional Analysis of the Cucumisin Propeptide as a Potent Inhibitor of Its Mature Enzyme

Cucumisin is a subtilisin-like serine protease (subtilase) that is found in the juice of melon fruits (Cucumis melo L.). It is synthesized as a preproprotein consisting of a signal peptide, NH₂-terminal propeptide, and 67-kDa protease domain. We investigated the role of this propeptide (88 residues) in the cucumisin precursor. Complementary DNAs encoding the propeptides of cucumisin, two other plant subtilases (Arabidopsis ARA12 and rice RSP1), and bacterial subtilisin E were expressed in Escherichia coli independently of their mature enzymes. The cucumisin propeptide strongly inhibited cucumisin in a competitive manner with a K_i value of 6.2 ± 0.55 nm. Interestingly, cucumisin was also strongly inhibited by ARA12 and RSP1 propeptides but not by the subtilisin E propeptide. In contrast, the propeptides of cucumisin, ARA12, and RSP1 did not inhibit subtilisin. Deletion analysis clearly showed that two hydrophobic regions, Asn³²–Met³⁸ and Gly⁹⁷–Leu¹⁰³, in the cucumisin propeptide were important for its inhibitory activity. Site-directed mutagenesis also confirmed the role of a Val³⁶-centerd hydrophobic cluster within the Asn³²–Met³⁸ region in cucumisin inhibition. Circular dichroism spectroscopy revealed that the cucumisin propeptide had a secondary structure without a cognate protease domain and that the thermal unfolding of the propeptide at 90 °C was only partial and reversible. A tripeptide, Ile³⁵-Val³⁶-Tyr³⁷, in the Asn³²–Met³⁸ region was thought to contribute toward the formation of a proper secondary structure necessary for cucumisin inhibition. This is the first report on the function and structural information of the propeptide of a plant serine protease.