Glucan-binding domain of a glucosyltransferase from Streptococcus sobrinus: isolation of a 55-kilodalton peptide from a trypsin digest of glucosyltransferase prebound to insoluble glucan.

We isolated a glucan-binding domain of water-insoluble glucan synthase (GTF-I) of Streptococcus sobrinus B13. Mild trypsin digestion of GTF-I bound to a water-insoluble glucan (IG) produced one predominant large fragment (55 kilodaltons). The fragment was easily recovered in IG precipitate. The isolated fragment had the same degree of affinity to IG as did the native GTF-I but no glucan synthesis activity. By the same method, a similar 55-kilodalton fragment was protected for GTF-Sd but not for GTF-Si. Immunological comparisons using specific antisera against the purified glucan-binding fragment of GTF-I from strain B13 indicated that GTF-I and GTF-S have a distinct glucan-binding domain.