~Polygalacturonase isozyme 1 (PC1) is a heterodimer comprising a catalytic and noncatalytic or B subunit, whereas polygalacturonase isozyme 2 (PC2) comprises only the catalytic subunit. To assess the state of assembly of PCI in vivo, both subunits were purified to homogeneity and used to study assembly of the heterodimer.PC1 could be reconstituted in vitro from purified B subunit and purified PC2 under a wide range of salt and pH conditions, and PC1 reconstituted in vitro was indistinguishable from PC1 isolated from tomato (Lycopersicon esculentum) fruit. Specific antibodies indicated that the B subunit was present in fruit of all developmental stages, but absent in vegetative tissue. l h e state of assembly of PC1 in vivo was tested based on the differential thermal stability of PC1 and PC2 by heating segments of ripe fruit pericarp tissue. Temperatures well below those required to inactivate PC1 in vitro caused the loss of activity of both PC1 and PC2, suggesting that only heat-labile PC2 is present in vivo. In addition, when extracts of ripe fruit were rigorously maintained and analyzed at 4"C, PC1 was absent or barely detectable. These results are consistent with the hypothesis that PG1 can assemble spontaneously and i s essentially absent in intact tomato fruit but forms artifactually from PG2 and the B subunit during the extraction of tomato fruit tissue when low temperatures are not rigorously maintained.