InterPro domain: IPR000737

The squash inhibitors form one of a number of serine proteinase inhibitor families. They belong to MEROPS inhibitor family I7, clan IE. They are generally annotated as either trypsin or elastase inhibitors (MEROPS peptidase family S1, IPR001254 ). The proteins, found exclusively in the seeds of the cucurbitaceae, e.g. Citrullus lanatus (watermelon), Cucumis sativus (cucumber), Momordica charantia (balsam pear), are approximately 30 residues in length and contain 6 Cys residues, which form 3 disulphide bonds [ 1 ]. The inhibitors function by being taken up by a serine protease (such as trypsin),which cleaves the peptide bond between Arg/Lys and Ile residues in the N-terminal portion of the protein [ 2 , 3 ]. Structural studies have shown that the inhibitor has an ellipsoidal shape, and is largely composed of beta-turns [ 3 ]. The fold and Cys connectivityof the proteins resembles that of potato carboxypeptidase A inhibitor [ 3 ].

1. The refined 2.0 A X-ray crystal structure of the complex formed between bovine beta-trypsin and CMTI-I, a trypsin inhibitor from squash seeds (Cucurbita maxima). Topological similarity of the squash seed inhibitors with the carboxypeptidase A inhibitor from potatoes. FEBS Lett. 242, 285-92
2. Two-dimensional NMR studies of squash family inhibitors. Sequence-specific proton assignments and secondary structure of reactive-site hydrolyzed Cucurbita maxima trypsin inhibitor III. Biochemistry 31, 898-904