Subunit communication in the tryptophan synthase alpha 2 beta 2 complex. Effects of beta subunit ligands on proteolytic cleavage of a flexible loop in the alpha subunit.

To probe the structural basis for ligand-mediated communication between the alpha and beta subunits in the tryptophan synthase alpha 2 beta 2 complex, we have determined the effects of ligands of the alpha and beta subunits on proteolysis of a flexible loop in the alpha subunit. We find that addition of a ligand of the beta subunit (L-serine, D-tryptophan, or L-tryptophan) in combination with a ligand of the alpha subunit (alpha-glycerol 3-phosphate) almost completely prevents the tryptic cleavage of the alpha subunit loop. Thus, the binding of a ligand to the beta-site affects the conformation of the alpha subunit 25-30 A distant.