A possible model for the structure of the Neurospora carbamoyl phosphate synthase-aspartate carbamoyl transferase complex enzyme

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Abstract
Summary 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 suggestions of two dissimilar ACT subunits. Proteolysis of an extract from a heterokaryon formed from two of the above CPS+ACT- alleles (α and β) 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 construct a model for the structure of the native enzyme complex.