Mapping of catalytic domains and phosphorylation sites in the multifunctional pyrimidine‐biosynthetic protein CAD

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Abstract
We have examined the domain organization, and the locations of the sites phosphorylated by the cyclic‐AMP‐dependent protein kinase, in the multifunctional polypeptide of the pyrimidine‐biosynthetic protein, CAD. Fragments produced after limited proteolysis by elastase or trypsin were separated by SDS/polyacrylamide gel electrophoresis and transferred onto nitrocellulose. The blots were probed with antibodies raised against the core aspartate carbamoyltransferase (ACTase) and dihydroorotase (DHOase) fragments to locate fragments containing these domains, and we also examined the locations of the phosphorylation sites by complete tryptic digestion of blotted, 32P‐labelled fragments, followed by analytical isoelectric focussing. Our results are consistent with the domain order glutaminase(GLNase)‐carbamoyl‐phosphate synthetase‐(CPSase)‐DHOase‐ACTase, as suggested by recently reported homologies between the predicted amino acid sequence for the Drosophila rudimentary gene product, and monofunctional CPSases/ACTases/DHOases. In particular, the finding of a 95‐kDa elastase fragment which cross‐reacted with both anti‐DHOase and anti‐ACTase antibodies rules out the previously suggested domain order: DHOase‐GLNase‐CPSase‐ACTase. Phosphorylation by cyclic‐AMP‐dependent protein kinase accelerates cleavage of native CAD by both elastase and trypsin, and abolishes the protective effect of UTP. Site 1 is located close to the C‐terminal end of the 160‐kDa GLNase/CPSase region. Comparison with the predicted amino acid sequence of the Drosophila rudimentary gene revealed a strong homology between the tryptic peptide containing site 1 from hamster CAD, and a region at the extreme C‐terminal end of the CPSase II domain of the Drosophila enzyme. Alignment of the Drosophila sequence and that of rat liver CPSase I, which is not phosphorylated by cyclic‐AMP‐dependent protein kinase, revealed that this putative site 1 region is missing in CPSase I. Site 2 could not be located with certainty, either from the limited proteolysis data, or from comparison of the sequence around this site and the sequence of the rudimentary gene. There were also one or more previously undetected minor phosphorylation site(s) located in the protease‐sensitive hinge region between the DHOase and ACTase domains.