Poster: Cell Walls

Abs # 1244: Endoxylanase Expressed during Papaya Fruit Ripening: Purification, Cloning and Characterization

Presenter:	Paull, Robert E, paull@hawaii.edu
Authors	Paull, Robert E (A)   Chen, Nancy J (A)
Affiliations:	(A): Tropical Plant & Soil Sciences, University of Hawaii - Manoa

Papaya (Carica papaya L.) softening during fruit ripening is correlated to the activity of a 32.5 kD endoxylanase (CpaEXY1) that was purified 45,871 fold on enzymatic activity and to homogeneity by SDS electrophoresis. Degenerate primers were used to isolate a full length clone (Gene register AY138968) from papaya mesocarp cDNA library. The isolated cDNA clone (CpaEXY1) from ripening mesocarp coded for a 64.96 kD protein that had up to 78% conserved similarity with one of the twelve predicted Family 10 endoxylanases-like sequences reported in Arabidopsis. The first 27 amino acids of CpaEXY1 N-terminal were hydrophobic and a predicted secretory signal peptide. There was a predicted carbohydrate binding module, distinct from the C-terminal endoxylanase catalytic center. The carbohydrate binding module may localize the xylanase in the cell wall allowing limited xylanase activity or a developmentally controlled protease cleavage was necessary for activity. The expression data from varieties having different softening patterns supported the possibility that endoxylanase had a role in papaya mesocarp softening during ripening. The findings are consistent with the hypothesis that the endoxylanase from papaya mesocarp tissue was expressed during fruit ripening, the expression was correlated with softening and was modified by post-translational proteolysis.