A molecular basis for the endo-β1,3-glucanase activity of the thaumatin-like proteins from edible fruits

Fruit-specific thaumatin-like proteins were isolated from cherry, apple and banana, and their enzymatic and antifungal activities compared. Both the apple and cherry possess a moderate endo-beta1,3-glucanase activity but are devoid of antifugal activity. In contrast, the banana thaumatin-like protein inhibits the in vitro hyphal growth of Verticillium albo-atrum but is virtually devoid of endo-beta1,3-glucanase activity. Both structural and molecular modeling studies showed that all three thaumatin-like proteins possess a h extended electronegatively charged cleft at their surface, which is believed to be a prerequisite for endo-beta1,3-glucanase activity. Docking experiments showed that the positioning of linear (1,3)-beta-D-glucans in the cleft of the apple and cherry proteins allows an interaction with the glutamic acid residues that are responsible for the hydrolytic cleavage of the glucan. Due to a different positioning in the cleft of the banana thaumatin-like protein, the linear beta-glucans cannot properly interact with the catalytic glutamic acid residues and as a result the protein possesses no enzymatic activity. The possible function of the fruit-specific thaumatin-like proteins is discussed in view of the observed biological activities and structural features. (C) 2003 Editions scientifiques et medicales Elsevier SAS and Societe francaise de biochimie et biologie moleculaire. All rights reserved.