The beta subunit of tomato fruit polygalacturonase isoenzyme 1 defines a new class of plant cell proteins involved in pectin metabolism: AroGPs (Aromatic Amino Acid Rich Glyco Proteins)

Understanding the biochemical and molecular mechanisms governing the synthesis, regulation, structure and function of plant-encoded cell wall modifying enzymes, particularly those involved in pectin metabolism, has been a long term goal of research in my laboratory. The ripening tomato fruit has proven to be a highly tractable system for this work and several laboratories have focused on studying a single, ripening-induced pectin degrading enzyme, tomato fruit polygalacturonase or PG. This chapter first reviews the field of tomato fruit PG and then focuses on our recent work with the beta subunit of tomato PG isoenzyme 1. Two PG isozymes can be isolated from ripe fruit, PG2 and PG1, the latter of which is formed by the association of a catalytic PG2 polypeptide with an ancillary glycoprotein, the beta subunit. Multiple lines of evidence suggesting the beta subunit plays an important role in regulating polyuronide degradation have prompted us to clone the beta subunit and study its structure, expression and function. beta subunit mRNA accumulates to high levels in developing fruit and is to a large degree temporally separated from PG2 expression. The beta subunit is encoded as a large precursor protein whose mature domain is composed almost entirely of the novel repeating motif FTNYGxxGNGGxxx in which the phenylalanine residues are post-translationally modified. Results from beta subunit antisense experiments have conclusively demonstrated that the protein plays a major role in restricting PG2 catalytic activity in vivo during fruit ripening. We have isolated the complete tomato beta subunit gene family, related genes from Arabidopsis and identified homologous sequences in a number of dicots and monocots, suggesting that beta subunit structure and function may be evolutionarily conserved in plants. We propose the tomato beta subunit as the archetypal member of a new class of plant cell wall proteins, AroGPs (for Aromatic Amino Acid Rich GlycoProteins). AroGPs are defined by: 1) their overall protein sequence homology and common precursor structure, 2) a conserved 14 amino acid repeating motif in the mature AroGP protein and 3) a high percentage of aromatic amino acids in the mature AroGP protein. Based on the available data, AroGPs appear to be specifically expressed in PG-producing tissues, where they presumably interact with cell wall components and/or catalytic enzymes to regulate cell wall enzymatic activities in vivo.