Functional role and folding properties of the glucan-binding domain of oral bacterial glucansucrase

A Streptococcus sobrinus glucansucrase (i.e., glucosyltransferase-I) elongates α-1, 3-linked glucose chains at pre-existing branch points of dextran, and its activity is enhanced by primer dextran. The glucansucrase contains a catalytic domain and a glucan-binding domain (GBd) with six tandem repeats. We here examined the role of GBd in the glucansucrase, focusing on its impact on enzymatic activity, dextran binding, and structural stability. By generating seven deletion mutants and a circularly permuted protein, our research demonstrates that the first four tandem repeats in the GBd and proper domain orientation are required for efficient glucosyl transfer. Moreover, characterization of circular dichroism and thermal unfolding indicated that glucosyl transfer efficiency is linked to cooperative interdomain folding. These findings highlight the importance of interdomain allostery in optimizing glucansucrase function.