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) f ruit. Chemical deglycosylation of this protein decreased the molecular mass to 34 kD. Gas chromatographic analysis suggests that N-linked gl ycosylation accounts for the majority of sugar moieties. Partial amino acid sequence analysis of the purified polygalacturonase inhibitor pr otein provided information used to amplify a corresponding cDNA by pol ymerase chain reactions. Multiple cloned products of these reactions w ere sequenced and the same open reading frame was identified in all of the products. It encodes a 36.5-kD polypeptide containing the amino a cid 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.