CEL1: a novel cellulose binding protein secreted by Agaricus bisporus during growth on crystalline cellulose

The cell gene of Agaricus bisporus encodes a protein (CEL1) that has an architecture resembling the multi‐domain fungal cellulases, although the sequence of its putative catalytic core is not matched by any other in the protein and nucleic acid data bases. The N‐terminal half of the putative catalytic domain of CEL1 was expressed in Escherichia coli as a fusion protein with glutathione‐S‐transferase. The fusion protein was used to raise a CEL1‐specific antibody. CEL1 was detected as an extracellular 49.8 kDa protein in A. bisporus cellulose‐grown cultures, where it bound strongly to cellulose. CEL1 was neither an endoglucanase, a cellobiohydrolase able to hydrolyze fluorogenic cellobiosides, a β‐glucosidase, a xylanase, nor a cellobiose: quinone oxidoreductase. CEL1 was present in some fractions of culture fluid separated by electrophoresis which released soluble sugars from crystalline cellulose.