Plant trypsin inhibitors
SMART accession number:SM00286
Interpro abstract (IPR000737):

Peptide proteinase inhibitors can be found as single domain proteins or as single or multiple domains within proteins; these are referred to as either simple or compound inhibitors, respectively. In many cases they are synthesised as part of a larger precursor protein, either as a prepropeptide or as an N-terminal domain associated with an inactive peptidase or zymogen. This domain prevents access of the substrate to the active site. Removal of the N-terminal inhibitor domain either by interaction with a second peptidase or by autocatalytic cleavage activates the zymogen. Other inhibitors interact direct with proteinases using a simple noncovalent lock and key mechanism; while yet others use a conformational change-based trapping mechanism that depends on their structural and thermodynamic properties.

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 [(PUBMED:2914611)]. 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 [(PUBMED:1731946), (PUBMED:2914611)]. Structural studies have shown that the inhibitor has an ellipsoidal shape, and is largely composed of beta-turns [(PUBMED:2914611)]. The fold and Cys connectivity of the proteins resembles that of potato carboxypeptidase A inhibitor [(PUBMED:1731946)].

GO function:serine-type endopeptidase inhibitor activity (GO:0004867)
Family alignment:
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There are 483 PTI domains in 46 proteins in SMART's nrdb database.

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