Cloning, sequence analysis, and characterization of a novel β-glucosidase-like activity from Pichia etchellsii

Genomic DNA fragment encoding a novel β-glucosidase-like activity of the yeast Pichia etchellsii was cloned and expressed in Escherichia coli. An open-reading frame of 1515 bp, termed mugA, coding for a protein of predicted molecular mass of approximately 54 kDa was confirmed for this activity. The sequence of the deduced protein did not show homology with the generic β-glucosidases but a high degree of identity was seen with several Ser-Asp (SD)-rich cell-surface-associated proteins. The secondary structure prediction program 3D-PSSM indicated the protein to be composed of largely helical and coiled structures, which was confirmed by circular dichroism spectroscopy. The encoded protein, MUGA, was purified by about 53-fold and characterized as a monomer of 52.1 kDa by SDS-PAGE and MALDI-TOF. The protein displayed high hydrolytic activity on methylumbelliferyl β-d-glucoside but relatively very little hydrolysis of p-nitrophenyl β-d-glucoside and gentiobiose, characteristic substrates for β-glucosidases. The binding experiments performed between P. etchellsii cells and the purified E. coli expressed MUGA indicated binding with the cell surface, which was monitored by fluorescence microscopy. In competition experiments with the SD dipeptide, less protein was shown to bind to the cell surface, in a concentration-dependent manner.