Molecular characterization of a polygalacturonase inhibitor from Pyrus communis L. cv Bartlett.

A polygalacturonase inhibitor glycoprotein with an apparent molecular mass of 43 kD was purified from pear (Pyrus communis L. cv Bartlett) fruit. Chemical deglycosylation of this protein decreased the molecular mass to 34 kD. Gas chromatographic analysis suggests that N-linked glycosylation accounts for the majority of sugar moieties. Partial amino acid sequence analysis of the purified polygalacturonase inhibitor protein provided information used to amplify a corresponding cDNA by polymerase chain reactions. Multiple cloned products of these reactions were sequenced and the same open reading frame was identified in all of the products. It encodes a 36.5-kD polypeptide containing the amino acid sequences determined by protein sequencing and predicts a putative signal sequence of 24 amino acids and seven potential N-glycosylation sites. The expression of polygalacturonase inhibitor is regulated in a tissue-specific manner. Activity and mRNA level were much higher in fruit than in flowers or leaves.