CRYSTAL-STRUCTURE OF A FLAVOPROTEIN RELATED TO THE SUBUNITS OF BACTERIAL LUCIFERASE

The molecular structure of the luxF protein from the bioluminescent ba cterium Photobacterium leiognathi has been determined by X-ray diffrac tion techniques and refined to a conventional R-factor of 17.8% at 2.3 angstrom resolution. The 228 amino acid polypeptide exists as a symme trical homodimer and 33% of the monomer's solvent-accessible surface a rea is buried upon dimerization. The monomer displays a novel fold tha t contains a central seven-stranded beta-barrel. The solvent-exposed s urface of the monomer is covered by seven alpha-helices, whereas the d imer interface is primarily a flat surface composed of beta-strands. T he protein monomer binds two molecules of flavin mononucleotide, each of which has C6 of the flavin isoalloxazine moiety covalently attached to the C3' carbon atom of myristic acid. Both myristyl groups of thes e adducts are buried within the hydrophobic core of the protein. One o f the cofactors contributes to interactions at the dimer interface. Th e luxF protein displays considerable amino acid sequence homology with both alpha- and beta-subunits of bacterial luciferase, especially the beta-subunit. Conserved amino acid residues shared between luxF and t he luciferase subunits cluster predominantly in two distinct regions o f the luxF protein molecule. These homologous regions in the luciferas e subunits probably share a three-dimensional fold similar to that of the luxF protein.