Non-catalytic subunit of the polygalacturonase isozyme 1 (PG1). Necessary and sufficient to convert the polygalacturonase from its monomeric form PG2 to its heterodimeric form PG1. Seems to limit the depolymerization and solubilization of cell wall polyuronides mediated by PG2 during ripening, probably by recruiting PG2 to form PG1. Interacts with polygalacturonase-2 (isoenzymes PG2A and PG2B) to form heterodimers called polygalacturonase-1 (PG1). Associated to the cell wall. Mostly expressed in fruit pericarp. Also detected at low levels in cell wall of roots, leaves and flowers (at protein level). Expressed in ripening fruits from the 20th day after anthesis and increase during the ripening (at protein level). Transiently repressed by ethylene.