3-DIMENSIONAL STRUCTURES OF THE PERIPLASMIC LYSINE ARGININE ORNITHINE-BINDING PROTEIN WITH AND WITHOUT A LIGAND

Many proteins exhibit a large-scale movement of rigid globular domains . Among these, bacterial periplasmic binding proteins involved in subs trate transport, or transport and chemotaxis, can be used as prototype s for understanding the mechanism of the movement. Such movements have been found to be associated with specific functions, such as substrat e binding, catalysis, and recognition by other biomolecules. We have d etermined the three-dimensional structures of the lysine/arginine/orni thine-binding protein (LAO) from Salmonella typhimurium with and witho ut lysine by x-ray crystallographic methods at 1.8- and 1.9-angstrom r esolution, respectively. The structures are composed of two lobes held together by two short connecting strands. The two lobes are far apart in the unliganded structure, but in contact with each other in the ly sine-liganded structure. The large movement of the lobes is a conseque nce of a 52-degrees rotation of a single backbone torsion angle in the first connecting strand and of distributed smaller changes of three b ackbone torsion angles of the second connecting strand. The absence of contact between the lysine and the connecting strands suggests that t he ligand does not induce the conformational change directly. We inste ad propose that the unliganded protein undergoes a dynamic change betw een an ''open'' and a ''closed'' conformation and that the role of the ligand is to stabilize the closed conformation. We discuss the nature of a surface area which might be recognized by the membrane-bound com plex of these amino acids transport systems.