Characterization of an exo-beta-1,3-D-galactanase from Streptomyces avermitilis NBRC14893 acting on arabinogalactan-proteins

A gene belonging to glycoside hydrolase family 43 (GH43) was isolated from Streptomyces avermitilis NBRC14893. The gene encodes a modular protein consisting of N-terminal GH43 module and a family 13 carbohydrate-binding module at the C-terminus. The gene corresponding to the GH43 module was expressed in Escherichia coli, and the gene product was characterized. The recombinant enzyme specifically hydrolyzed only beta-1,3-linkage of two D-galactosyl residues at non-reducing ends of the substrates. The analysis of the hydrolysis products indicated that the enzyme produced galactose from beta-1,3-D-galactan in an exo-acting manner. When the enzyme catalyze hydrolysis of the arabinogalactan-protein, the enzyme produced oligosaccharides together with galactose, suggesting that the enzyme is able to accommodate beta-1,6-linked D-galactosyl side chains. These properties are the same as the other previously reported exo-beta-1,3-D-galactanases. Therefore, we concluded the isolated gene certainly encodes an exo-beta-1,3-D-galactanase. This is the first report of exo-beta-1,3-D-galactanase from actinomycetes.