Allosteric regulation and substrate channeling in multifunctional pyrimidine biosynthetic complexes: analysis of isolated domains and yeast-mammalian chimeric proteins.

The initial steps of pyrimidine biosynthesis in yeast and mammals are catalyzed by large multifunctional proteins of similar size, sequence and domain structure, but appreciable functional differences. The mammalian protein, CAD, has carbamyl phosphate synthetase (CPSase), aspartate transcarbamylase (ATCase) and dihydroorotase (DHOase) activities. The yeast protein, ura2, catalyzes the first two reactions and has a domain, called pDHO, which is homologous to mammalian DHOase, but is inactive. In CAD, only CPSase is regulated, whereas both CPSase and ATCase in the yeast protein are inhibited by UTP. These functional differences were explored by constructing a series of mammalian yeast chimeras. The isolated ATCase domain is catalytically active, but is not regulated. The inclusion of the yeast sequences homologous to the mammalian regulatory domain (B3) and the intervening pDHO domain did not confer regulation. Chimeric proteins in which the homologous regions of the mammalian protein were replaced by the corresponding domains of ura2 exhibited full catalytic activity, as well regulation of the CPSase, but not the ATCase, activities. The yeast B3 subdomain confers UTP sensitivity on the mammalian CPSase, suggesting that it is the locus of CPSase regulation in ura2. Taken together, these results indicate that there are regulatory site(s) in ura2. Channeling is impaired in all the chimeric complexes and completely abolished in the chimera in which the pDHO domain of yeast is replaced by the mammalian DHO domain.