Semliki Forest virus does not inhibit phosphatidylcholine biosynthesis in BHK-21 cells

The mechanism by which Semliki Forest virus inhibits the incorporation of [methyl-3H]choline into phosphatidylcholine has been investigated. Decreased labeling of the lipid was not due to altered uptake of [methyl-3H]choline. The specific activities of choline kinase and CTP:phosphocholine cytidylyltransferase were unchanged. The previously observed inhibition (Vance, D. E. & Burke, D. C. (1974) Eur. J. Biochem. 43, 327-336) of CDP-choline:1,2-diacylglycerol phosphocholinetransferase was confirmed. Since the decreased activity of the phosphocholinetransferase may not have caused the reduced labeling of phosphatidylcholine, the amounts of this lipid and its precursors were measured. We observed changes in the concentration of phosphocholine (34 +/- 12 and 120 +/- 40 nmol x g cells-1 in mock- and virus-infected cells, respectively) and CTP (116 +/- 35 and 36 +/- 13 nmol x g cells-1 in mock- and virus-infected cells, respectively). Pulse-chase studies with [methyl-3H]choline demonstrated that, initially, most of the radioactivity was in phosphocholine. As it disappeared from this compound, it appeared in phosphatidylcholine. From these results, we calculated the rate of phosphatidylcholine biosynthesis to be 0.56 and 1.23 nmol x min-1 x g cells-1 in mock- and virus-infected BHK-21 cells, respectively. We conclude that phosphatidylcholine biosynthesis is not inhibited in Semliki Forest virus infected BHK cells, but rather is stimulated 6.75 h after infection. The decreased labeling observed during pulse studies with [methyl-3H]choline is due to dilution of the labeled choline into a pool of phosphocholine which is 3.5 times larger in the infected cells.