Solution structure of the cellulose-binding domain of the endoglucanase Z secreted by Erwinia chrysanthemi

Two-dimensional proton nuclear magnetic resonance spectroscopy has been used to determine the three-dimensional structure of the 62 amino acid C-terminal cellulose-binding domain (CBD) of the endoglucanase Z (CBDEGZ), secreted by Erwinia chrysanthemi. An experimental data set comprising 958 interproton nOe-derived restraints was used to calculate 23 structures. The calculated structures have an average root mean-square deviation between Cys4 and Cys61 of 0.91 +/- 0.11 Angstrom for backbone atoms and 1.18 +/- 0.12 Angstrom for the heavy atoms. The CBDEGZ exhibits a skiboot shape based mainly on a triple antiparallel beta-sheet perpendicular to a less-ordered summital loop. Three aromatic rings (Trp18, Trp43, and Tyr44) are localized on one face of the protein and are exposed to the solvent in a conformation compatible with a cellulose-binding site. Based on its original folding, we have been able to relate the CBD sequence to those of several domains of unknown function occurring in several bacterial chitinases as well as other proteins. This study also provides a structural basis for analyzing the secretion-related information specific to the CBDEGZ.