IDENTIFICATION AND CHARACTERIZATION OF A FRUIT-SPECIFIC, THAUMATIN-LIKE PROTEIN THAT ACCUMULATES AT VERY HIGH-LEVELS IN CONJUNCTION WITH THE ONSET OF SUGAR ACCUMULATION AND BERRY SOFTENING IN GRAPES

The protein composition of the grape (Vitis vinifera cv Muscat of Alex andria) berry was examined from flowering to ripeness by gel electroph oresis. A protein with an apparent molecular mass of 24 kD, which was one of the most abundant proteins in extracts of mature berries, was p urified and identified by amino acid sequence to be a thaumatin-like p rotein. Combined cDNA sequence analysis and electrospray mass spectrom etry revealed that this protein, VVTL1 (for V. vinifera thaumatin-like protein 1), is synthesized with a transient signal peptide as seen fo r apoplastic preproteins. Apart from the removal of the targeting sign al and the formation of eight disulfide bonds, VVTL1 undergoes no othe r posttranslational modification. Southern, northern, and western anal yses revealed that VVTL1 is found in the berry only and is encoded by a single gene that is expressed in conjunction with the onset of sugar accumulation and softening. The exact role of VVTL1 is unknown, but t he timing of its accumulation correlates with the inability of the fun gal pathogen powdery mildew (Uncinula necator) to initiate new infecti ons of the berry. Western analysis revealed that the presence of thaum atin-like proteins in ripening fruit might be a widespread phenomenon.