Urea Carboxylase and Allophanate Hydrolase Are Components of a Multifunctional Protein in Yeast

Saccharomyces cerevisiae can use urea as sole nitrogen source by degrading it in two steps (urea carboxylase and allophanate hydrolase) to ammonia and carbon dioxide. We previously demonstrated that: 1) the enzymatic functions required for degradation are encoded in two tightly linked genetic loci and 2) pleiotropic mutations each resulting in the loss of both activities are found in both loci. These and other observations led to the hypothesis that urea degradation might be catalyzed by a multifunctional polypeptide. Waheed and Castric (1977) J. Biol. Chem. 252, 1628-1632), on the other hand, purified urea amidolyase from Candida utilis and reported it to be a tetramer composed of nonidentical 70- and 170-kilodalton subunits. To resolve the differing views of urea amidolyase structure, we purified the protein using rapid methods designed to avoid proteolytic cleavage. Application of these methods resulted in the isolation of a single, inducible and repressible, 204-kilodalton species. We observed no evidence for the existence of nonidentical subunits. A similar inducible, high molecular weight species was also detected in C. utilis. These biochemical results support our earlier hypothesis that urea degradation is carried out in yeast by an inducible and repressible protein composed of identical, multifunctional subunits.