Molecular and biochemical characterizations of a novel arthropod endo-β-1,3-glucanase from the Antarctic springtail, Cryptopygus antarcticus, horizontally acquired from bacteria

Collembolan species have been known to have β-1,3-glucanase activity and yet the genes coding such enzymes have not been demonstrated. We report here a novel arthropod endo-β-1,3-glucanase gene CaLam from the Antarctic springtail, Cryptopygus antarcticus. The open reading frame consists of 813bp encoding 270 amino acids with a putative signal peptide and a typical motif of glycosyl hydrolase family 16 (GHF16), E-I-D-I-T-E. The recombinant protein expressed in E. coli shows the hydrolytic activity toward laminarin (K_m ~9.98mg/mL) with an optimal temperature 50°C and an optimal pH 6.0. CaLam digests laminarin and laminarioligosaccharides except laminaribiose as an endo-β-1,3-glucanase, releasing glucose, laminaribiose and laminaritriose as the major products. Analyses of molecular phylogeny of CaLam and its protein structure reveal that CaLam is closely related with bacterial β-1,3-glucanases more than with the eukaryotic homologues. Even so, the genomic structure of the CaLam gene consisting of six exons interspersed with approximately 57 to 63bp introns confirms that it is endogenous in the genome of the Antarctic springtail. These results suggest that CaLam should have been transferred from bacteria to the lineage of the Collembolan species by horizontal gene transfer.