Dodecins: a family of lumichrome binding proteins.

Dodecin is a small dodecameric flavoprotein from Halobacterium salinarum that contains two flavins stacked between two tryptophan residues to form an aromatic tetrade. The functional properties of heterologously expressed dodecin were investigated by fluorescence spectroscopy, which allowed the determination of dissociation constants for a number of protein-ligand complexes. The values obtained were in the nanomolar to micromolar range and correlate positively with the ligand size. These data were supplemented by X-ray crystal structures of the apododecin and holocomplexes with lumichrome, lumiflavin, riboflavin and FMN at resolutions between 1.55 to 1.95 A to unravel a gating mechanism as the structural basis for the preferential binding of the small ligands lumichrome and lumiflavin. The detailed analysis of the dodecin manifold for preferential binding of lumichrome and lumiflavin provides insight on a subatom level into a protein's strategy to gain selectivity for low molecular mass compounds by steric restrictions rather than specific interactions. Investigations on the ligand composition of a wild-type dodecin crystal (1.32 A resolution) support conclusions of functional and structural investigations on heterologously expressed dodecin, and strongly suggest that lumichrome, a molecule associated with the flavin metabolism, is a ligand of dodecin in vivo. Studies on mutant protein and a Halorhodospira halophila homologue spread the idea of a lumichrome binding system as a possible "waste"-trapping device, widely distributed in prokaryotes.