Identification of an intracellular β-glucosidase in Aspergillus niger with transglycosylation activity

Aspergillus niger is featured with its copious amount of extracellular β-glucosidase which is generally used to balance the cellulolytic enzyme cocktails for lignocellulose saccharification. However, whether or not A. niger produces any intracellular β-glucosidase remains obscure. In this study, we analyzed a total of fifteen putative β-glucosidase genes ( bgl s) in A. niger CBS 513.88 genome and the five of them were predicted as intracellular bgl s due to the lack of signal peptide of extracellular proteins. After further characterization of these five genes through a Saccharomyces cerevisiae in vivo system, only An03g03740 (designated bgl1B ) was confirmed to be a β-glucosidase gene. Western blot and mass spectrometry analysis confirmed BGL1B protein localization inside the cell. BGL1B exhibited the maximal activity at 40 °C and pH 5.6. The K m for p -nitrophenyl-β- d -glucopyranoside and K i for glucose were 0.233 ± 0.058 mM and 119.8 ± 4.35 mM, respectively. BGL1B showed a strong transglycosylation activity while hydrolyzing cellodextrins with sophorose, laminaribiose, and cellotriose formed from cellobiose, and sophorose and laminaribiose formed from cellotriose. The confirmation of the intracellular β-glucosidase BGL1B in A. niger further extends our understanding of how A. niger utilizes lignocellulose. Key points • Identification of putative genes revealed a novel β-glucosidase in Aspergillus niger. • Newly identified β-glucosidase BGL1B was an intracellular enzyme of A. niger. • BGL1B exhibited a strong transglycosylation activity.