CIRCULAR-DICHROISM ANALYSIS OF THE GLUCAN BINDING DOMAIN OF STREPTOCOCCUS-MUTANS GLUCAN BINDING PROTEIN-A

The glucan binding domain (GBD) of the glucan binding protein-A (GBP-A ) from the cariogenic bacterium Streptococcus mutans was studied using circular dichroism (CD) analysis, Chuu-Fasman-Rose secondary structur e prediction, and absorption and fluorescence spectroscopy. Our data s how that the binding domain undergoes a conformational shift upon bind ing to the ligand dextran. The CD spectrum shows two positive bands at 280 nm and 230 nm which were assigned to aromatic residues. The 230-n m band was seen at 20 degrees C and 30 degrees C, lost intensity at 40 degrees C, and was eliminated at 45 degrees C coinciding with complet e denaturation. The protein was stable at physiological pH, but precip itated at pH 5. A pH of 10 changed the secondary structure but had no effect on the 230-nm band. Analysis of the CD data in the Far UV using the SELCON computer program revealed a high content of beta-sheets an d a lack of alpha-helical structures, Secondary structure prediction b ased on the amino acid sequence of GBD agreed with the CD analysis. Th e fluorescence emission maximum at 339 nm suggested that the majority of the tryptophans were located in the interior of the protein. This m aximum shifted to higher energy upon binding to the ligand dextran. (C ) 1998 Elsevier Science B.V.