Endoxylanase expressed during papaya fruit ripening: purification, cloning and characterization

Papaya (Carica papaya L.) softening during fruit ripening is correlated with the activities of an endoxylanase (EC 3.2.1.8). A 32.5-kDa xylanase (CpaEXY1) from ripening fruit mesocarp was purified 45 871-fold on enzymatic activity and to homogeneity by SDS electrophoresis. The enzyme had endo- and not exo-xylanase activity, a pH optimum of 5-7 and was inhibited by Ca²⁺, Co²⁺, and Zn²⁺. Degenerate primers were constructed from five peptides obtained from the purified enzyme, and a full-length cDNA clone (AY138968) was isolated from a library constructed from ripening mesocarp. CpaEXY1 coded for a 64.96-kDa protein that had up to 61% identity with the 12 predicted Arabidopsis Family 10 endoxylanase-like sequences and 40% to the barley aleurone xylanase. The peptide sequences, obtained from the trypsin-digested purified protein, were all found between amino acid 267and 426 out of the predicted 584 amino acids. The N-terminal 27 amino acids were hydrophobic and formed a predicted secretory signal peptide. A predicted carbohydrate-binding module was located between amino acids 60and 182, distinct from the C-terminal endoxylanase catalytic center. CpaEXY1 was developmentally expressed during fruit ripening and the expression correlated with the variation in softening patterns of different varieties. The findings are consistent with the hypothesis that CpaEXY1 was expressed during fruit ripening; the expression was correlated with softening and was modified by post-translational proteolysis. This modification may take place in the cell wall, and regulate enzyme activity and cell-wall-microdomain-specific hydrolysis.