Regulation of CTP:phosphocholine cytidylyltransferase in HeLa cells. Effect of oleate on phosphorylation and intracellular localization.

The mechanism by which oleate induced the translocation of CTP:phosphocholine cytidylyltransferase from the soluble to particulate fraction was studied in HeLa cells. Addition of 0.5 mM oleate caused a 10-fold stimulation of [methyl-3H]choline incorporation into phosphatidylcholine. The stimulation of phosphatidylcholine synthesis was correlated with a decrease in phosphocholine and an increase in CDP-choline, indicating that cytidylyltransferase is regulatory under these conditions. The stimulation of cytidylyltransferase activity upon oleate treatment was concomitant with the translocation of this enzyme from the soluble to particulate fraction. Immunoblot analysis revealed the soluble form as multiple slowly migrating bands, and the particulate form as a faster migrating, single band. The slowly migrating form could be converted to the fast migrating form by phosphatase treatment, suggesting that the mobility difference was due to phosphorylation. In vivo 32P labeling showed that the soluble cytidylyltransferase was highly phosphorylated and the particulate form was much less phosphorylated. The kinetics of the translocation showed that the conversion of the majority of cytidylyltransferase from the soluble to the particulate form occurred within about 15 min following addition of oleate and that the translocation showed a good correlation with dephosphorylation. In contrast, the relocation of cytidylyltransferase from the particulate to the soluble fraction after the removal of oleate occurred within 1 min, and extensive phosphorylation was not required for cytidylyltransferase to leave the membrane. The relocated, soluble cytidylyltransferase was phosphorylated at a much slower rate than that at which the enzyme left the membrane. Immunolocalization showed that the cytidylyltransferase translocated to the nuclear envelope instead of the endoplasmic reticulum. These results indicate that activation of cytidylyltransferase by dephosphorylation and translocation to the nuclear envelope may be a general mechanism occurring in many cell types. Moreover, in HeLa cells, multiple mechanism exist for regulating cytidylyltransferase activity.