The control of CTP:choline-phosphate cytidylyltransferase activity in pea (Pisum sativum L.)

Several possible control mechanisms for CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15) activity in pea (Pisum sativum L.) stems were investigated. Indol-3-ylacetic acid (IAA) treatment of the pea stems decreased total cytidylyltransferase activity but did not affect its subcellular distribution. Oleate (2 mM) caused some stimulation of enzyme activity by release of activity from the microsomal fraction into the cytosol, but neither phosphatidylglycerol nor monoacyl phosphatidylethanolamine had an effect on activity or subcellular distribution. A decrease in soluble cytidylyltransferase protein concentrations was found in IAA-treated pea stems, but this was not sufficient to account for all of the decrease in cytidylyltransferase activity. A 50% inhibition of enzyme activity could be obtained with 0.2 mM-CMP, which indicated possible allosteric regulation. Similar inhibition was obtained with 1.5 mM-ATP, but other nucleotides had no effect. The cytidylyltransferase enzyme protein was not directly phosphorylated, and the inhibition with 1.5 mM-ATP occurred with the purified enzyme, thus excluding an obligatory mediation via a modulator protein. The results indicate that the cytosolic form of cytidylyltransferase is the most important in pea stem tissue and that the decrease in cytidylyltransferase activity in IAA-treated material appears to be brought about by several methods.