Expression and secretion of the Candida wickerhamii extracellular β-glucosidase gene, bglB, in Saccharomyces cerevisiae
The yeast Candida wickerhamii exports a cell-associated β-glucosidase that is active against cellobiose and all soluble cellodextrins. Because of its unique ability to tolerate end-product inhibition by glucose, the bglB gene that encodes this enzyme was previously cloned and sequenced in this laboratory. Using several different promoters and constructs, bglB was expressed in the hosts Escherichia coli, Pichia pastoris, and Saccharomyces cerevisiae. Expression was initially performed in E. coli using either the lacZ or tac promoter. This resulted in intracellular expression of the BglB protein with the protein being rapidly fragmented. Secretion and glycosylation of active β-glucosidase was achieved with several different S. cerevisiae constructs utilizing either the adh1 or the gal1 promoter on 2-µ replicating plasmids. When either the invertase (Suc2) or the BglB secretion signal was used, BglB protein remained associated with the cell wall and appeared to be hyperglycosylated. Expression in P. pastoris was also examined to determine if higher activity and expression could be achieved in a yeast host that usually does not hyperglycosylate. Using the alcohol oxidase promoter in conjunction with either the pho1 or the α-factor secretion signal, the recombinant enzyme was successfully secreted and glycosylated in P. pastoris. However, levels of protein expression from the chromosomally integrated vector were insufficient to detect activity.
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