A NOVEL INTERSUBUNIT REPAIR MECHANISM IN THE TRYPTOPHAN SYNTHASE ALPHA(2)BETA(2)-COMPLEX - CRITICAL ROLE OF THE BETA-SUBUNIT LYSINE-167 IN INTERSUBUNIT COMMUNICATION

This study explores intersubunit communication in the tryptophan synthase alpha2beta2 complex from Salmonella typhimurium. We find that an engineered mutation in the contact region between the alpha and beta subunits remarkably alters the catalytic and spectroscopic properties of the beta subunit in the alpha2beta2 complex. Ligands that bind to the alpha subunit largely repair the deleterious effects of the beta subunit mutation in the alpha2beta2 complex. The conserved residue chosen for mutation, beta subunit lysine 167, appears to form an ion pair with alpha subunit aspartate 56 in the crystal structure of the wild type alpha2beta2 complex. Although replacement of beta subunit lysine 167 by threonine does not prevent formation of the alpha2beta2 complex, this mutation reduces the rate of synthesis of L-tryptophan from L-serine and indole (beta reaction) 25-fold. Ligands that bind to the alpha subunit (indole-3-glycerol phosphate, indole-3-propanol phosphate, alpha-glycerol 3-phosphate, or potassium phosphate) largely restore the activity of the mutant alpha2beta2 complex in the beta reaction. We conclude that beta subunit lysine 167 plays an important role in intersubunit communication in the alpha2beta2 complex. The striking allosteric effects of alpha subunit ligands on the mutant beta subunit in the alpha2beta2 complex may result from ligand-induced conformational changes in the alpha subunit that are transmitted to the beta subunit and repair the mutational defect in the beta subunit.