Biosynthesis of sphingomyelin and its delivery to the surface of baby hamster kidney (BHK) cells.

Abstract

Introduction Although there is considerable evidence that sphingomyelin is not only concentrated in the plasma membranes of eukaryotic cells but also oriented almost exclusively on the external surface [l-31, there has been some controversy about the actual site of synthesis of plasma membrane sphingomyelin. Originally it was thought to be made in situ in the plasma membrane, which represents the largest pool of sphingomyelin within most cells [4-61. Subsequently, evidence has been produced for synthesis of sphingomyelin in an internal pool (probably the early Golgi) and for its translocation to the cell surface by a vesicular route similar to that taken by membrane proteins [7-111. This evidence is based partly on the observation that inhibitors of vesicular transport block the appearance of sphingomyelin at the cell surface and partly on the results of liver cell fractionation, which suggest that sphingomyelin synthetic activity is concentrated in the early Golgi [8, 12, 131. The work described here demonstrates that although there is a site of sphingomyelin synthesis which is probably in the early Golgi, this is not the site which is responsible for the synthesis of plasma membrane sphingomyelin. The major site of plasma membrane sphingomyelin synthesis appears to be in the endosomal pathway involved in the recycling of plasma membrane.