The smallest carbamoyl-phosphate synthetase. A single catalytic subdomain catalyzes all three partial reactions

Escherichia coli carbamoyl-phosphate synthetase (CPSase) is comprised of a 40-kDa glutaminase (GLN) and a 120-kDa synthetase (CPS) subunit. The CPS subunit consists of two homologous domains, CPS.A and CPS.B, which catalyze the two different ATP-dependent partial reactions involved in carbamoyl phosphate synthesis. Sequence similarities and controlled proteolysis experiments suggest that the CPS subdomains consist, in turn, of three subdomains, designated A1, A2, A3 and B1, B2, B3 for CPS.A and CPS.B, respectively. Previous studies of individually cloned CPS.A and CPS. B from E. coli and mammalian CPSase have shown that homologous dimers of either of these "half-molecules" could catalyze all three reactions involved in ammonia-dependent carbamoyl phosphate synthesis. Four smaller recombinant proteins were made for this study as follows: 1) A1-A2 in which the A3 subdomain was deleted from CPS.A, 2) B1-B2 lacking subdomain B3 of CPS.B, 3) the A2 subdomain of CPS.A, and 4) the B2 subdomain of CPS.B. When associated with the GLN subunit, A1-A2 and B1-B2 had both glutamine- and ammonia-dependent CPSase activities comparable to the wild-type protein. In contrast, the 27-kDa A2 and B2 recombinant proteins, which represent only 17% of the mass of the parent protein, were unable to use glutamine as a nitrogen donor, but the ammonia-dependent activity was enhanced 14-16-fold. The hyperactivity suggests that A2 and B2 are the catalytic subdomains and that A1 and B1 are attenuation domains which suppress the intrinsically high activity and are required for the physical association with the GLN subunit.