A CHERRY PROTEIN AND ITS GENE, ABUNDANTLY EXPRESSED IN RIPENING FRUIT, HAVE BEEN IDENTIFIED AS THAUMATIN-LIKE

A 29-kD polypeptide is the most abundant soluble protein in ripe cherr y fruit (Prunus avium L); accumulation begins at the onset of ripening as the fruit turns from yellow to red. This protein was extracted fro m ripe cherries and purified by size-exclusion and ion-exchange chroma tography. Antibodies to the purified protein were used to screen a cDN A library from ripe cherries. Numerous recombinant plaques reacted pos itively with the antibodies; the DNA sequence of representative clones encoded a polypeptide of 245 amino acid residues. A signal peptide wa s indicated, and the predicted mature protein corresponded to the puri fied protein in size (23.3 kD, by mass spectrometry) and isoelectric p oint (4.2). A search of known protein sequences revealed a strong simi larity between this polypeptide and the thaumatin family of pathogenes is-related proteins. The cherry thaumatin-like protein does not have a sweet taste, and no antifungal activity was seen in preliminary assay s. Expression of the protein appears to be regulated at the gene level , with mRNA levels at their highest in the ripe fruit.