Oligomeric structure of the multifunctional protein CAD that initiates pyrimidine biosynthesis in mammalian cells.

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Abstract
The first three steps in mammalian de novo pyrimidine biosynthesis are catalyzed by the multifunctional protein designated CAD. Regions of the single 240-kDa poly-peptide chain are folded into separate structural domains that have discrete functions. Previous studies suggested that CAD forms predominantly trimers. The trimers are found to be in slow equilibrium with hexamers and higher oligomers composed of multiples of three copies of the CAD polypeptide chain. However, quantitative chemical crosslinking studies of CAD with dimethyl suberimidate were used here to show a progressive conversion of monomer to crosslinked hexamer. High levels of the hexamer accumulate in the reaction mixture, suggesting that the major oligomeric form is hexameric, although residual amounts of smaller oligomers remain present. Larger oligomers may form by association of hexamers and are seen after longer crosslinking times. Sucrose gradient centrifugation shows a 20.8S species to be the slowest sedimenting peak, while the larger species sediments at 27.9S. Electron microscopic studies of rotary-shadowed preparations of CAD have confirmed that, while small amounts of other oligomeric forms are present, the CAD monomer is primarily associated into cyclic hexamers with an open planar appearance.