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Palmitoylation of myelin P0 protein is independent of its synthesis and parallels that of phospholipids.

To determine whether the acyl chains modifying P0, the major protein of PNS myelin, turn over independently of the protein backbone, sciatic nerve slices from 10 to 65 day-old rats were incubated with a mixture of [3H]palmitic acid and [14C]amino acids, and proteins were analyzed by electrophoresis. Incorporation of [14C]amino acids into nerve P0 decreased approximately 10-fold between 10 and 65 days of age. In contrast, palmitoylation of P0, although maximal at 10 days-of age, decreased only 3-4-fold during the same period. In the same experiments, the incorporation of [3H]palmitate into the nerve and into various lipids classes diminished by a comparable extent (2.5-fold). Thus, if corrected by the uptake of the tritiated precursor, palmitoylation of P0 remains nearly constant throughout development, and it is therefore independent of protein synthesis. Preincubation of nerve slices with cycloheximide for one hour reduced the incorporation of [3H]palmitate into both P0 and phospholipids in a concentration-dependent manner. At 10 microM cycloheximide, palmitoylation of P0 was unaffected while its synthesis was still repressed, indicating that these events are uncoupled. The effect of cycloheximide on fatty acid uptake can be attributed to inhibition of the palmitoyl-CoA : lysophosphatidylcholine acyltransferase activity. Neither the distribution of palmitate between albumin and lipid membranes nor the activities of other lipid-metabolizing enzymes were affected by the inhibitor. In conclusion, these results indicate that P0 palmitoylation occurs mainly on the preexisting molecules, and it therefore constitutes a dynamic event.