Organization of a multifunctional protein in pyrimidine biosynthesis. Analyses of active, tryptic fragments

The multifunctional protein which catalyzes the first three steps of pyrimidine biosynthesis in hamster cells can be cleaved by trypsin into enzymatically active fragments. When the fragments are separated by nondenaturing polyacrylamide gel electrophoresis, three major polypeptide bands appear. Carbamyl phosphate synthetase (EC 2.7.2.9), aspartate transcarbamylase (EC 2.1.3.2), and dihydroorotase (EC 3.5.2.3) activities are associated with 129,000-, 660,000-, and 94,000-dalton bands, respectively. Further analysis of these fragments by denaturing polyacrylamide gel electrophoresis has shown that the aspartate transcarbamylase band seen on the nondenaturing gel is actually a large aggregate of 39,000-dalton fragments and the dihydroorotase band is a dimer of 44,000-dalton fragments. The data reported here indicate that (i) this multifunctional protein is composed of three enzymatically independent domains, (ii) the sum of the molecular weights of these three domains (129,000 + 39,000 + 44,000 = 212,000) is similar to that of the undigested monomer (220,000 daltons), and (iii) a site important to the formation of the native multimeric protein is probably near the aspartate transcarbamylase domain.