Immunoaffinity chromatographic purification of cellobiohydrolase II mutants from recombinant <i>Trichoderma reesei</i> strains devoid of major endoglucanase genes

Efficient purification of Trichoderma reesei cellobiohydrolase II (CBHII) requires the use of affinity chromatography based on a substrate analogue. Due to altered substrate binding, the purification of many active-site mutants of CBHII from the complex fungal culture media represents a considerable challenge. Here we describe a combination of two approaches to facilitate the purification: the first is based on the construction of novel engineered T. reesei strains devoid of the major contaminating endoglucanases, and the second uses immunoaffinity chromatography as the final purification step. Two different procedures for the preparation of the antibody matrix were tested. Crosslinking of the monoclonal antibody to Protein G matrix instead of the conventional immobilization via cyanogen bromide increased the binding efficiency. Three different active-site mutants of CBHII bound to the immunoaffinity column in neutral pH and were eluted in pH 2.7. The purity of the CBHII mutant preparations was tested using small chromophoric substrates and hydroxyethyl cellulose, which are hydrolyzed by many other cellulases but not by CBHII. The immunoaffinity column purified the CBHII mutants over 800-fold in a single step and resulted in homogeneous protein preparations free of proteolytically cleaved forms of CBHII. The use of the double replacement T. reesei production strains, especially the one lacking the genes coding for both the endogeneous CBHII and the endoglucanase II (EGII), helped to reduce the total endoglucanase activity in the preparations