Crystal structure of CcdB, a topoisomerase poison from E-coli

The crystal structure of CcdB, a protein that poisons Escherichia coli gyra se, was determined in three crystal forms. The protein;consists of a five-s tranded antiparallel beta-pleated sheet followed by a C-terminal alpha-heli x. In one of the loops of the sheet, a second small three-stranded antipara llel beta-sheet is inserted that sticks out of the molecule as a wing. This wing contains the LysC proteolytic cleavage site that is protected by CcdA and, therefore, forms a Likely CcdA recognition site. A dimer is formed by sheet extension and by extensive hydrophobic contacts involving three of t he five methionine residues and the C terminus of the a-helix. The surface of the dimer on the side of the alpha-helix is overall negatively charged, while the opposite side as well as the wing sheet is dominated by positive charges. We propose that the CcdB dimer binds into the central hole of the 59 kDa N-terminal fragment of GyrA, after disruption of the head dimer inte rface of GyrA. (C) 1999 Academic Press.