| Abstract: | The relationship between the synthesis of phospholipids and the intrinsic proteins of the sarcoplasmic reticulum was investigated in differentiating L6 cells in culture. The rates of lipid synthesis and turnover in L6 showed no large variations over the course of differentiation from myoblasts to myotubes while the rate of synthesis of the sarcoplasmic reticulum Ca2+-ATPase steadily increased. Removal of choline from the culture medium after the onset of fusion resulted in a 2-fold inhibition of phosphatidylcholine (PC) synthesis and a 40-50% reduction in total cellular PC content within 36 h. The synthesis and content of phosphatidylethanolamine also declined subsequent to the effect on PC. The amount of newly synthesized phospholipid in the microsomal fraction also decreased 50% in choline-deprived cells. Choline deprivation of myotubes for up to 4 days had no effect on the rates of synthesis of the Ca2+-ATPase or two intrinsic glycoproteins of 53,000 and 160,000 daltons. The newly synthesized proteins were incorporated into PC-deficient microsomal membranes. The synthesis of total cellular protein and total membrane protein was not altered, thus phospholipid:protein ratios declined 2-fold. These observations suggest that the assembly of the sarcoplasmic reticulum is not tightly coordinated with the rate of phospholipid synthesis. |
