Functional and structural characterization of apidaecin and its N-terminal and C-terminal fragments.

Two aspects were studied to elucidate the functional and structural characterization of apidaecin and its N-terminal and C-terminal fragments: (i) Functions of the N-terminal and C-terminal fragments of apidaecin were first studied by measuring their antibacterial activity, their ability to enter Escherichia coli cells and their effects on the activities of beta-galactosidase and alkaline phosphatase. The results indicate that neither the N-terminal nor the C-terminal of apidaecin contains intracellular delivery unit or active segment. (ii) The effect of apidaecin on the ATPase activity of DnaK, and the interactions of apidaecin with E.coli lidless DnaK and DnaK D-E helix were studied. Results showed that apidaecin could interact with the E.coli lidless DnaK protein and stimulate its ATPase activity, but not with E.coli DnaK D-E helix. This indicated that the antimicrobial activity of apidaecin may be shown by stimulating the ATPase activity of DnaK by binding to its conventional substrate-binding site, to decrease its cellular concentration of DnaK by competing with natural substrates and inhibit the enzymes' activities of E. coli cells. It is the first study to suggest that the apidaecin-binding site of DnaK is the conventional substrate binging site.