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Effects of Truncation at the Non-homologous Region of a Family 3 β-Glucosidase from Agrobacterium tumefaciens

The function of the non-homologous region of a family 3 β-glucosidase from Agrobacterium tumefaciens (Cbg1) was studied by analyzing the properties of mutant enzymes that have internal truncated amino acid sequences in the region. Five truncated mutants named Cbg1-d4, Cbg1-d31, Cbg1-d62, Cbg1-d89, and Cbg1-d119 having deletions of 4, 31, 62, 89, and 119 amino acid residues starting from Phe417, respectively, were expressed in Escherichia coli and purified. All the mutants exhibited β-glucosidase activity, indicating that the non-homologous region was not essential for the activity. The truncation caused thermal instability, decrease in pK a of the proton donor residue (Glu616), and deficient transglycosylation activity. The thermal stability and the pK a of Glu616 were partially recovered with longer truncation, suggesting that the truncation perturbed the structure and that their presence in the region was not essential. The main role of the non-homologous region could be formation of a hydrophobic atmosphere at the acceptor site to make the enzyme suitable for hydrolyzing hydrophobic glucosides.