Substitutions at histidine 74 and aspartate 278 alter ligand binding and allostery in lactose repressor protein

In the inducer-bound structure of the lac repressor protein, the side chains of H74 and D278 are positioned to form an ion pair between monomers that appears to be disrupted upon operator binding (Lewis, M., Chang, G., Horton, N. C., Kercher, M. A., Pace, H. C., Schumacher, M. A., Brennan, R. G., and Lu, P. (1996) Science 271, 1247-1254). A series of single substitutions at H74 and D278 and a double mutant, H74D-D278H, were generated to determine the influence of this interaction on ligand binding and allostery in lac repressor. Introduction of apolar amino acids at H74 resulted in distinct effects on ligand binding. Alanine and leucine substitutions decreased operator binding, while tryptophan and phenylalanine increased affinity for operator DNA. Introduction of a negatively charged residue at position 74 in H74D had minimal effects, and "inverting" the side chains in H74D/D278H did not significantly alter inducer or operator binding at neutral pH. In contrast, all substitutions of D278 increased affinity for operator DNA and diminished inducer binding. These observations can be interpreted in the context of the Monod-Wyman-Changeux model. If a salt bridge were essential for stabilizing or destabilizing the inducer-bound conformation, a mutation at either residue that interrupts this interaction should have a similar effect on allostery. Because the type and degree of alteration in ligand binding properties depended on the nature of the substitution at these residues, the individual roles played by H74 and D278 in lac repressor allostery appear more important than their direct contact across the monomer-monomer interface.