Do multiple forms of tomato fruit endopolygalacturonase exist in situ?

Abstract

Extracts of tomato fruit contain three isoforms of endopolygalacturonase (PG). The two of lower molecular weight (PG2 A and B) are heat labile isoforms, but the higher molecular weight, thermo-tolerant isoform, PG1, can be formed in vitro from PG2 and a second glycopeptide, the so-called 'converter' molecule. The existence of PG1 as an entity in vivo has been challenged. Observations that have been interpreted to either support or deny the existence of PG1 were repeated, and some assumptions in the interpretations tested. We determined that soluble PG2 can react with insoluble 'converter' immobilized in situ to form PG1-like proteins. This precludes the use of differential extractions of PG2 and PG1 to demonstrate their presence or absence in vivo. Heating whole fruit and pericarp portions did not affect the properties or solubility of extractable PG1 and PG2. Thermal decay curves demonstrated the presence in fruit tissues of two forms of PG with the thermo-tolerances of PG1 and PG2 and in the relative abundance found for PG1 and PG2 in extracts. This evidence supports the thesis that PG1 exists as such in vivo and may be formed by the interaction of the PG2 polypeptide with an immobile 'converter' peptide. Show more