title: A possible model for the structure of the Neurospora carbamoyl phosphate synthase-aspartate carbamoyl transferase complex enzyme.
abstract: The pyrimidine-3 locus of Neurospora crassa specifies a multienzyme complex comprising pyrimidine-specific carbamoyl phosphate synthase (CPSpyr) and aspartate carbamoyl transferase (ACT). It appears to be divided into a translationally proximal CPS-specific region and a distal ACT-specific region. Levels of complementation for ACT activity between pairs of four pyr-3 CPS+ ACT- mutants showed a range from 12% to 68% of the wild-type level of the enzyme. This is interpreted as interallelic complementation, contradicting certain earlier suggestion of two dissimilar ACT subunits. Proteolysis of an extract from a heterokaryon formed from two of the above CPS+ ACT- alleles (alpha and beta) did not lead to loss of ACT activity, but led to the formation of a fragment with ACT activity with a similar molecular weight (92,000 daltons) to that produced in extracts of wild type strain. The pyr-3 polar mutant 43-174 which is enzymatically CPS+ ACT- and which fails to complement with any other CPS+ ACT- alleles, thus suggesting its location towards the proximal end of the ACT region, has CPS activity associated with a form of 180,000 daltons molecular weight. These findings are used to contruct a model for structure of the native enzyme complex.
mesh_terms: D000595:Amino Acid Sequence; D001221:Aspartate Carbamoyltransferase / Q000378:metabolism*; D002223:Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing) / Q000378:metabolism*; D002467:Cell Nucleus; D005816:Genetic Complementation Test; D006579:Heterozygote; D008956:Models, Chemical; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D009492:Neurospora crassa / Q000166:cytology / Q000201:enzymology; D010770:Phosphotransferases / Q000378:metabolism*
chemical_list: D009097:Multienzyme Complexes; D001221:Aspartate Carbamoyltransferase; D010770:Phosphotransferases; D002223:Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing)