SQAPI, aspartic acid inhibitor first isolated from squash, inhibits a wide range of aspartic proteinases. This particular family of PAAPIs (proteinaceous aspartic acid inhibitors) seems to have evolved quite recently from an ancestral cystatin. Structurally it consists of a four-stranded anti-parallel beta-sheet gripping an alpha-helix in much the same manner that a hand grips a tennis racket. The unstructured N-terminus and the loop connecting beta-strands 1 and 2 are important for pepsin inhibition, but the loop connecting strands 3 and 4 is not [1].

1: Solution structure of the squash aspartic acid proteinase inhibitor (SQAPI) and mutational analysis of pepsin inhibition. Headey SJ, Macaskill UK, Wright MA, Claridge JK, Edwards PJ, Farley PC, Christeller JT, Laing WA, Pascal SM; J Biol Chem. 2010;285:27019-27025. PMID:20538608