MOLECULAR-STRUCTURE OF THE LIPOAMIDE DEHYDROGENASE DOMAIN OF A SURFACE-ANTIGEN FROM NEISSERIA-MENINGITIDIS

The protein p64k from the surface of the Neisseria meningitidis bacter ia has been characterized as a two-domain protein. It contains a dihyd rolipoamide dehydrogenase domain of 482 residues, involving a FAD pros thetic group as a cofactor, and a smaller lipoic acid binding domain o f 86 residues. The two domains are joined by a flexible segment rich i n alanine and proline residues. The structure of the dihydrolipoamide dehydrogenase domain was determined by X-ray diffraction. It was solve d by a combination of molecular replacement and multiple isomorphous r eplacement techniques and refined to 2.7 Angstrom resolution. In the c rystal, the recombinant p64k mimics the functional homo-dimer by using one of the crystallographic 2-fold axes. The reactive disulphide brid ge Cys161-Cys166 is in the oxidised state and the FAD is bound in an e xtended conformation. This main domain contains the major antigenic de terminant of the protein, an extended loop of 32 residues at the surfa ce of the protein. A mis-attribution at residue 553 in the sequence ha s been detected by inspection of electron density maps and the geometr y. However, when compared to the other dihydrolipoamide dehydrogenases , there are some significant differences: (1) an unusual number of cis -proline residues and (2) a new motif built around a 2-fold axis by th e sulphur atoms of residues Met558, Cys560 and their symmetry related equivalents. (C) 1997 Academic Press Limited.