A beta-glucosidase/xylosidase from the phytopathogenic oomycete, Phytophthora infestans

An 85-kDa beta-glucosidase/xylosidase (BGX1) was purified from the axenically grown phytopathogenic oomycete, Phytophthora infestans. The bgx1 gene encodes a predicted 61-kDa protein product which, upon removal of a 21 amino acid leader peptide, accumulates in the apoplastic space. Extensive N-mannosylation accounts for part of the observed molecular mass difference. BGX1 belongs to family 30 of the glycoside hydrolases and is the first such oomycete enzyme deposited in public databases. The bgx1 gene was found in various Phytophthora species, but is apparently absent in species of the related genus, Pythium. Despite significant sequence similarity to human and murine lysosomal glucosylceramidases, BGX1 demonstrated neither glucocerebroside nor galactocerebroside-hydrolyzing activity. The native enzyme exhibited glucohydrolytic activity towards 4-methylumbelliferyl (4-MU) beta-D-glucopyranoside and, to lesser extent, towards 4-MU-D-xylopyranoside, but not towards 4-MU-beta-D-glucopyranoside. BGX1 did not hydrolyze carboxymethyl cellulose, cellotetraose, chitosan or xylan, suggesting high substrate specificity and/or specific cofactor requirements for enzymatic activity.