A, mammalian protein kinase C (0.5 unit/ml) was incubated with human CTP synthetase 1 (50 µg/ml) and the indicated concentrations of [γ-32P]ATP (11,000 cpm/pmol) for 10 min. B, mammalian protein kinase C (0.5 unit/ml) was incubated with 50 µM [γ-32P]ATP (11,000 cpm/pmol) and the indicated concentrations of human CTP synthetase 1 for 10 min. After the phosphorylation reactions, the samples were subjected to SDS-PAGE. The SDS-polyacrylamide gels were dried and the phosphorylated proteins were subjected to phosphorimaging analysis. The relative amounts of phosphate incorporated into CTP synthetase 1 were quantified using ImageQuant software. Each data point represents the average of duplicate determinations.