INTRAMOLECULAR TRANSMISSION OF THE ATP REGULATORY SIGNAL IN ESCHERICHIA-COLI ASPARTATE-TRANSCARBAMYLASE - SPECIFIC INVOLVEMENT OF A CLUSTERED SET OF AMINO-ACID INTERACTIONS AT AN INTERFACE BETWEEN REGULATORY AND CATALYTIC SUBUNITS

Aspartate transcarbamylase from Escherichia coli is stimulated by ATP and feedback-inhibited by CTP and UTP. Previous work allowed the identification of the hydrophobic interface between the two domains of the regulatory chain as a structural element specifically involved in the transmission of the Am regulatory signal toward the catalytic sites. The present work describes the identification of a cluster of amino acid interactions at an interface between the regulatory chains and the catalytic chains of the enzyme as another structural feature involved in the transmission of the ATP regulatory signal but not in those of CTP and UTP. These interactions involve residues 146 to 149 of the regulatory chain and residues 242 to 245 of the catalytic chain. Perturbations of these interactions also alter to various extents the co-operativity between the catalytic sites for aspartate binding. These findings are in agreement with the idea that the primary effect of ATP might consist, in part, of a modulation of the stability of the interfaces between regulatory and catalytic subunits, thereby facilitating the T to R transition induced by aspartate binding, as was put forward in two recently proposed models, the ''effector modulated transition'' model and the ''nucleotide perturbation'' model. This does not exclude that this cluster of interactions could also act as a relay to transmit the ATP regulatory signal to the catalytic sites according to the previously proposed ''primary-secondary effects'' model.