Shuttling of CTP : phosphocholine cytidylyltransferase between the nucleusand endoplasmic reticulum accompanies the wave of phosphatidylcholine synthesis during the G(0) -> G(1) transition

The transition from quiescence (G(0)) into the cell division cycle is marke d by accelerated phospholipid turnover. We examined the rates of phosphatid ylcholine (PC) synthesis and the activity, membrane affinity, and intracell ular localization of the rate-limiting enzyme in the synthesis of PC, CTP:p hosphocholine cytidylyltransferase (CT) during this transition, The additio n of serum to quiescent IIC9 fibroblasts resulted in a wave of PC synthesis beginning at similar to 10 min, peaking at similar to 3 h with a >10-fold increase in rate, and declining to near basal rates by 10 h, CT activity, m onitored in situ, was elevated similar to 3-fold between I and 2 h postseru m, Neither CT mass nor its phosphorylation state changed during the surge i n PC synthesis and CT activity, On the other hand, the ratio of particulate /soluble CT surged and then receded in concert with the wave of PC synthesi s. During quiescence, CT was confined to the nucleus, as assessed by indire ct immunofluorescence. Within 10 min after serum stimulation, a portion of the CT fluorescence appeared in the cytoplasm, where it intensified until s imilar to 4 h postserum, Thereafter, the cytoplasmic CT signal waned, while the nuclear signal increased, and by 8 h CT was once again predominantly n uclear. The dynamics of CT's apparent translocation in and out of the nucle us paralleled the wave of PC synthesis and the solubility changes of CT. Cy toplasmic CT co-localized with BiP, a resident endoplasmic reticulum protei n, in a double labeling experiment. These data suggest that the wave of PC synthesis that accompanies the G(0) --> G(1) transition is regulated by the coordinated changes in CT activity, membrane affinity, and intracellular d istribution. We describe for the first time a redistribution of GT from the nucleus to the ER that correlates with an activation of the enzyme. We pro pose that this movement is required for the stimulation of PC synthesis dur ing entry into the cell cycle.