ALLOSTERIC REGULATION IN A FAMILY OF ENTEROBACTERIAL ASPARTATE TRANSCARBAMYLASES - INTRAMOLECULAR TRANSMISSION OF REGULATORY SIGNALS IN CHIMERIC ENZYMES

Several enterobacterial aspartate transcarbamylases (ATCases) exhibit a [2(c(3)):3(r(2))] quaternary structure analogous to that of the Esch erichia coli enzyme. Despite their conserved quaternary structures, th ese enzymes present substantial differences in the co-operativity of s ubstrate binding and in their allosteric regulation by nucleotide effe cters. A comparison between different enzymatic species provides an op portunity to expand our understanding of the molecular basis of allost ery in ATCase. Chimeric ATCases were constructed by exchanging subdoma in regions involved in quaternary structural features, such as the r1- c4 regulatory-catalytic subunit interface analyzed in this study, in o rder to define the involvement of this interface in the several compon ents of allosteric regulation. The r1-c4 interface was found to consti tute an essential element for the recognition and the transmission of the ATP regulatory signal in the Serratia marcescens and the Proteus v ulgaris ATCases, as it does in the E. coli ATCase. Besides, the specif ic amino acid composition of the C-terminal region of the regulatory c hain and its interactions with the amino acid residues in the 240s loo p of the catalytic chain (r1-c4 interactions) were found to modulate t he amplitude of the enzyme's response to ATP. The C-terminal region of the regulatory chain did not appear to participate directly in the re gulation of the three native ATCases by CTP. Even when CTP acts as an activator, as in the P. vulgaris and S. marcescens ATCases, its signal follows a route distinct from that of the general activator ATP. Syne rgistic inhibition by CTP and UTP was found to involve the transmissio n of a specific UTP signal. This signal appeared different in the vari ous ATCases, involving the C-terminal region of the regulatory chain i n the E. coli and S. marcescens ATCases but not in the P. vulgaris ATC ase. (C) 1996 Academic Press Limited