Cloning and characterization of two structurally and functionally divergent rhamnogalacturonases from Aspergillus...

Two rhamnogalacturonases from the filamentous fungus Aspergillus aculeatus have been cloned and characterized. A cDNA library from A. aculeatus was constructed, and a novel rhamnogalacturonase B was isolated by expression cloning in yeast. For this purpose a new plate screening assay was developed, specific for the detection of rhamnogalacturonase activity. The rhamnogalacturonase A, known from previous reports, was shown not to be expressed in yeast in an active form. Therefore, rhamnogalacturonase A was purified, peptide sequences were obtained, and full-length cDNAs encoding the enzyme were isolated using a polymerase chain reaction-generated product as a probe. Comparison of the deduced primary structures indicates that the two rhamnogalacturonases are structurally different. This is further supported by the finding that polyclonal antibodies raised against native rhamnogalacturonase A do not cross-react with rhamnogalacturonase B. The cloned genes were transformed into Aspergillus oryzae for high level expression. The recombinant enzymes were purified and characterized, revealing significant differences in glycosylation pattern and substrate specificity as well as in pH and temperature optima and stability. Data from the hydrolysis of apple rhamnogalacturonan with the recombinant rhamnogalacturonases suggest that the two enzymes exert their action at different sites in the backbone.