SUNY at Albany
June 19-23, 2001
DNA Hydrolysis Catalyzed by a HNH Endonuclease- Colicin E7 Protein
Colicin E7 (ColE7) protein is a bacteria toxin secreted by Escherichia coli to kill other Escherichia coli. Immediately after production, ColE7 forms a one-to-one noncovalent complex with the immunity protein Im7 resulting in neutralization of its toxicity toward the host cell. ColE7 contains a DNase domain by which it cleaves DNA in target cells and leads to the cell death. We have determined the crystal structures of the Im7 and the one-to-one complex between the DNase domain of ColE7 and Im7 at 1.8 Å and 2.3 Å resolution, respectively.
The DNase domain of ColE7 contains a HNH motif, which has been identified in more than fifty proteins and some of these HNH proteins are capable of cleaving DNA site-specifically or nonspecifically. However, how HNH family proteins bind and cleave DNA are still unknown. The crystal structure of the DNase domain of ColE7/Im7 shows that a HNH motif contains two b-strands and one a-helix folded in a topology similar to that of zinc-finger motif. A zinc ion, bound to three histidine residues and one water molecule in a distorted tetrahedron geometry, is identified in the center of the HNH motif. A divalent metal ion is required for the DNA hydrolysis reaction catalyzed by ColE7 which is most active with the presence of magnesium or nickel. The crystal structure of the phosphate-bound complex determined at 2.0 Å resolution further shows that a phosphate ion is bound to the zinc site. This result suggests that the metal ion in ColE7 is involved in DNA-binding and it is likely that the endonuclease active site is located around the zinc-binding site. However, this putative DNase active site is not directly blocked by the immunity protein. We suggest that the immunity protein blocks the DNA-binding site or induces conformational change in the colicin so that a colicin cannot bind to DNA for hydrolysis.
Meng-Jiun Sui (1), Wen-Yen Ku (1), Yu-Wen Liu (1, 2), Kin-Fu Chak (2) and Hanna S. Yuan (1)*
Institute of Molecular Biology (1), Academia Sinica, and