SYNTHESIS OF SURFACE SPHINGOMYELIN IN THE PLASMA-MEMBRANE RECYCLING PATHWAY OF BHK CELLS

Sphingomyelin, which has been degraded at the BHK cell surface by exog enous sphingomyelinase. is converted back into sphingomyelin with kine tics similar to those of plasma membrane recycling. Resynthesis of sph ingomyelin under these conditions proceeds at a rate about 4-fold high er than normal biosynthesis of sphingomyelin. Neither resynthesis of s phingomvelin nor its return to the surface is inhibited by brefeldin A (BFA), which is a potent blocker of vesicular transport through the G olgi but has no effect on plasma membrane recycling. However, resynthe sis of plasma membrane sphingomyelin is greatly decreased in cells und ergoing mitosis or energy depletion, where endocytosis is inhibited. W e conclude that the main site of surface sphingomyelin synthesis in BH K cells could be in recycling endosomes and not in the Golgi apparatus as proposed previously. We also suggest a model pathway by which chol esterol may reach the plasma membrane via recycling endosomes.