Pectin methyl esterase from orange fruit: characterization and localization by in-situ hybridization and immunohistochemistry

• Published in: Planta 1998-11, Vol.206 (4), p.493-503
• Main Authors: Christensen, T.M.I.E, Nielsen, J.E, Kreiberg, J.D, Rasmussen, P, Mikkelsen, J.D
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Amino Acid Sequence; amino acid sequences; Amino acids; Base Sequence; Biological and medical sciences; Carboxylic Ester Hydrolases - metabolism; Cell walls; Citrus - enzymology; Citrus sinensis; Complementary DNA; DNA, Complementary; Economic plant physiology; enzyme activity; Enzymes; Fructification and ripening; Fructification, ripening. Postharvest physiology; fruits; Fundamental and applied biological sciences. Psychology; gene expression; Growth and development; Immunohistochemistry; In Situ Hybridization; isoelectric point; Juices; Messenger RNA; Molecular Sequence Data; molecular weight; nucleic acid hybridization; nucleotide sequences; pectinesterase; pectins; peel; Peels; Plant physiology and development; Plants; Polymerase chain reaction; Protein isoforms; Sequence Homology, Amino Acid; substrates; temperature; Vegetative and sexual reproduction, floral biology, fructification
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s004250050426

Pectin methyl esterase (PME) from orange (Citrus sinensis L.) fruit peels has been purified by ammonium sulphate precipitation, and ion-exchange and gel-filtration chromatography. Characterization of the enzyme revealed a 36-kDa protein with an isoelectric point > 9, a pH optimum at 7 and temperature optimum at 50 degrees C. The substrate specificity and kinetic experiments showed that the affinity of PME for pectin was highly dependent on the degree of esterification (DE) of the pectin, with Km values of 0.7 mg ml(-1) for pectin with a DE of 70% and 17 mg ml(-1) for pectin with a DE of 25%. The sequences of the NH2-terminal end of digested peptides from the mature protein were obtained. A DNA fragment of 501 bp was cloned by polymerase chain reaction amplification using degenerate primers and was further used for screening of a cDNA library. Two cDNA clones were isolated encoding PMEs of 584 amino acids and 362 amino acids, respectively, including a putative signal peptide. The deduced amino acid sequence showed full identity to the sequenced peptides. Polyclonal antibodies raised against orange peel PME were used for immunohistochemistry. The main localization of PMEs was in the outer cell layers of the juice vesicles, in the outer cell layers of the lamellae between the segments and in the inner cell layers of the albedo in the peel. In-situ hybridization showed that the mRNA is very abundant in the fruit and was found in the same cell layers as the native enzyme. A very intensive staining for PME mRNA was also seen in the core and in the flavedo close to the oil glands.