Cellulose-binding domains: potential for purification of complex proteins.

The endoglucanase CenA and the exoglucanase Cex from Cellulomonas fimi each contain a discrete cellulose-binding domain (CBD), at the amino-terminus or carboxyl-terminus respectively. The gene fragment encoding the CBD can be fused to the gene of a protein of interest. Using this approach hybrid proteins can be engineered which bind reversibly to cellulose and exhibit the biological activity of the protein partner. Alkaline phosphatase (PhoA) from Escherichia coli, and a beta-glucosidase (Abg) from an Agrobacterium sp. are dimeric proteins. The fusion polypeptides CenA-PhoA and Abg-CBC(Cex) are sensitive to proteolysis at the junctions between the fusion partners. Proteolysis results in a mixture of homo- and heterodimers; these bind to cellulose if one or both of the monomers carry a CBD, e.g. CenA-PhoA/CenA-PhoA and CenA-PhoA/PhoA. CBD fusion polypeptides could be used in this way to purify polypeptides which associate with the fusion partner.