CTP-PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE IS BOTH A NUCLEAR AND CYTOPLASMIC PROTEIN IN PRIMARY HEPATOCYTES

CTP:phosphocholine cytidylyltransferase (CT) has recently been reporte d to be a predominantly intranuclear enzyme in several cell lines (Wan g et al., J. Biol. Chem. 268, 5899-5904 (1993)). This contrasts with p revious reports that CT was a cytosolic protein that translocated to t he endoplasmic reticulum upon activation. The aim of the present study was to compare the localization of CT in CHO cells and in primary rat hepatocytes. Indirect immunofluorescence of CHO cells revealed a larg ely nuclear localization of the CT. On the other hand; immunogold elec tron microscopy and biochemical studies showed a similar density of di stribution of CT between the nucleus and cytoplasm, In primary rat hep atocytes immunofluorescence studies indicated that CT was largely cyto plasmic. Studies by immunogold electron microscopy of rat hepatocytes demonstrated that the enzyme was homogeneously distributed throughout all cytoplasmic regions and the nucleoplasm. This result was confirmed by biochemical studies using digitonin and streptolysin O, which perm eabilizes the plasma membrane of cells. Enucleation studies indicated that in CHO cells 76 % of the CT activity was in the nuclear fraction, whereas in hepatocytes only 32 % was recovered in this fraction. The data indicate that CT is found both in nuclear and cytoplasmic fractio ns of primary hepatocytes and is not predominantly a nuclear enzyme.