The p Subunit of Tomato Fruit Polygalacturonase lsoenzyme 1: Isolation, Characterization, and ldentification of Unique Structural Features

We have purified and isolated cDNAs encoding the p subunit of tomato fruit polygalacturonase isoenzyme 1 (PGl), a cell wall protein that associates with, and apparently regulates, the catalytic PG2 polypeptides. Expression of the p subunit is fruit specific and temporally separated from the expression of PG2 during fruit development. The 37to 39-kD p subunit is encoded as a 69-kD precursor protein containing a signal sequence and two propeptide domains. The mature protein is composed almost entirely of the nove1 14-amino acid motif FTNYGxxGNGGxxx in which many of the phenylalanine residues are post-translationally modified. The unique structural features of the motif suggest an important role in the function of the protein and hence in the activity of PG1. The p subunit may represent a class of bifunctional plant proteins that interact both with structural components of the cell wall and catalytic proteins to localize andlor regulate metabolic activities within the cell wall.