Accumulation of the ?-subunit of polygalacturonase 1 in normal and mutant tomato fruit

Abstract

Polyclonal antiserum raised against the native PG1 isoform of tomato fruit (Lycopersicon esculentum Mill.) polygalacturonase [poly(1,4-α-d-galacturonide) glycanohydrolase, EC 3.2.1.15] bound to each of the subunits of the protein and also to a range of other fruit proteins. Affinity purification was used to remove antibody molecules that bound to the native form of the PG2 isoform. The resulting serum bound to native PG1, denatured PG2 and β-subunits of PG1 but not to native PG2 or other fruit proteins. This anti-PG1 serum was used to monitor the occurrence of the PG1 β-subunit and PG2 in detergent extracts of tomato tissues. The β-subunit polypeptide was detected in pericarp but not locule tissue of fruit, including fruit of the rin and nor mutants. It increased in amount in the pericarp tissues from an early stage to the mature green stage, clearly prior to any appreciable accumulation of the PG2 subunit. The β-subunit polypeptide was not detected in stem or leaf tissues. A PG2-specific antiserum was used to study the interaction of PG2 with the isolated β-subunit. The PG2 isoform was bound to the β-subunit over a wide range of salt concentrations and pH; the interaction was independent of the presence of reducing agents. It is concluded that strong non-covalent forces are involved in the interaction. The results are consistent with a model in which the β-subunit is positioned in the cell wall structure and provides a specific binding site for the active PG2 subunit when this is synthesised during ripening.