Cooperative interactions in hybrids of aspartate transcarbamylase containing succinylated regulatory polypeptide chains

Abstract

Succinylated derivatives of the regulatory subunit of aspartate transcarbamylase of Escherichia coli were prepared by treating the intact enzyme with succinic anhydride followed by dissociation of the modified protein into catalytic and regulatory subunits which were separated by ion exchange chromatography. The succinylated regulatory subunits were used in hybridization experiments with native subunits to study the organization of the six regulatory polypeptide chains in the intact enzyme. Rapid mixing of succinylated and native regulatory subunits with native catalytic subunits yielded a four-membered hybrid set of reconstituted enzyme-like molecules; hence, the assembly process involves three regulatory combining units and the six regulatory polypeptide chains in the intact enzyme must be arranged as three dimeric subunits. When the modified and native regulatory subunits were incubated together for only brief periods (less than 1 min) followed by the addition of catalytic subunits, the resulting hybrid set was complex with no resolution of discrete species. Apparently the isolated regulatory dimers dissociate readily and reversibly into single polypeptide chains due to relatively weak intra-subunit bonding domains. In contrast, after reconstitution of enzyme-like molecules, the incorporated succinylated regulatory subunits did not exchange with free subunits. Enzyme-like molecules containing three extensively succinylated regulatory subunits show reduced binding of the inhibitor, CTP, and lack both the homotropic and heterotropic effects characteristic of native aspartate transcarbamylase. Preparations containing only slightly succinylated regulatory subunits showed only little inhibition by CTP and considerable cooperativity. The decrease in homotropic effects in these reconstituted molecules correlated with the reduction in the succinate-promoted change in the sedimentation coefficient. Reconstituted enzyme-like molecules containing regulatory subunits which had been extensively succinylated in the presence of CTP retained their binding capacity even though they were only slightly inhibited by CTP and exhibited reduced cooperativity. Hybrid molecules containing both native and succinylated regulatory subunits also possessed reduced allosteric behavior.