Allosteric Signal Transmission Involves Synergy between Discrete Structural Units of the Regulatory Subunit of Aspartate Transcarbamoylase

Previous studies have shown that the S5′ β-strand (r93–r97) of the regulatory polypeptides of the aspartate transcarbamoylases (ATCases) from Serratia marcescens and Escherichia coli are responsible for their diverged allosteric regulatory patterns, including conversion of CTP from an inhibitor in E. coli to an activator in S. marcescens. Similarly, mutation of residues located in the interface between the allosteric and the zinc domains resulted in conversion of the ATP responses of the E. coli enzyme from activation to inhibition, suggesting that this interface not only mediates but also discriminates the allosteric responses of ATP and CTP. To further decipher the roles and the interrelationships of these regions in allosteric communication, allosteric–zinc interface mutations (Y77F and V106A) have been introduced into both the native and the S5′ β-strand chimeric backgrounds. While the significance of this interface in the allosteric regulation has been confirmed, there is no direct evidence supporting the presence of distinct pathways for the ATP and CTP signals through this interface. The analysis of the mutational effects reported here suggested that the S5′ β-strand transmits the allosteric signal by modulating the hydrophobic allosteric–zinc interface rather than disturbing the allosteric ligand binding. Intragenic suppression by substitutions in the hydrophobic interface between the allosteric and the zinc domains of the regulatory chains resulted in the partial recovery of allosteric responses in the EC:rS5′sm chimera and reduced the activation by ATP in the Sm:rS5′ec chimera. Thus, it seems that there is a synergy between these two structural units.