Structure-function relationship in allosteric aspartate carbamoyltransferase from Escherichia coli. II. Involvement of the C-terminal region of the regulatory chain in homotropic and heterotropic interactions.

The modified aspartate transcarbamylase (ATCase) encoded by the transducing phage described by Cunin et al. has been purified to homogeneity. In this altered form of enzyme (pAR5-ATCase) the last eight amino acids of the C-terminal end of the regulatory chains are replaced by a sequence of six amino acids coded for by the lambda DNA. This modification has very informative consequences on the allosteric properties of ATCase. pAR5-ATCase lacks the homotropic co-operative interactions between the catalytic sites for aspartate binding and is "frozen" in the R state. In addition, this altered form of enzyme is insensitive to the physiological feedback inhibitor CTP, in spite of the fact that this nucleotide binds normally to the regulatory sites. Conversely, pAR5-ATCase is fully sensitive to the activator ATP. However, this activation is limited to the extent of the previously described "primary effect" as expected from an ATCase form "frozen" in the R state. These results emphasize the importance of the three-dimensional structure of the C-terminal region of the regulatory chains for both homotropic and heterotropic interactions. In addition, they indicate that the primary effects of CTP and ATP involve different features of the regulatory chain-catalytic chain interaction area.