Purification of an osmotin-like protein from the seeds of Benincasa hispida and cloning of the gene encoding this protein.

A pathogenesis-related (PR) protein was purified from the seeds of Benincasa hispida, which is a medicinal plant and a member of the Cucurbitaceae family. Purification was achieved by using a procedure consisting of an acid treatment step followed by two chromatography steps. The protein is a basic protein with molecular mass of approximately 28 kDa. The sequences of the N-terminal 30 amino acids and four peptides generated from protease digestion were determined. These sequences indicated that the protein is an osmotin-like protein (OLP). Osmotin and OLPs are members of the thaumatin-like, PR-5 family of the PR proteins. A genomic clone of the gene encoding the protein was isolated and sequenced. The predicted protein has a signal peptide of 18 amino acids, and the mature protein has a molecular mass of 24.8 kDa with an isoelectric point of 7.67. The protein has 17 cysteine residues, of which 16 appear in the same positions as those appear in the sweet-tasting protein thaumatin and several other thaumatin-like proteins. Southern hybridization analysis indicated that the gene encoding the protein is a single copy gene. A computer-generated, three-dimensional model of the protein is presented.