CHARACTERIZATION OF A BETA-GALACTOSIDASE FUSION PROTEIN CONTAINING THE STARCH-BINDING DOMAIN OF ASPERGILLUS GLUCOAMYLASE

Granular starch,vas used as a biospecific adsorbent to investigate the possible application of the starch-binding domain (SBD) as an affinity tail for a one-step purification of target proteins from crude cell extracts. A beta-galactosidase (beta-gal) fusion protein containing the C-terminal 119 amino-acids from GA-I (BSB119) was used as a model system to study the starch binding and elution. Because of proteolysis, approximately 40% of initial beta-gal activity lacked the SBD, and the remaining fusion protein contained from to one to four SBDs per molecule of beta-gal tetramer. The fusion protein forms containing at least one intact SBD adsorbed to starch. The bound fusion protein was eluted by using 10 mM solutions of various maltooligosaccharides and cyclodextrins. The best elutants were 10 mM maltodextrin with an average degree of polymerization (<(DP)over bar>) of 10 and 10 mM beta-cyclodextrin. The elution of BSB119 with maltooligosaccharides of increasing DP suggested that the starch-binding site of the SBD consists of at least five glucosyl binding sites. SDS-PAGE gels and Western blots showed that the purity of the fusion protein eluted from starch,vas as good as or better than that obtained by conventional affinity chromatography.