Cellular membranes: The biosynthesis of glycoprotein and glycolipid in HeLa cell membranes

Abstract

The biosynthesis of glycoprotein and glycolipid in HeLa cells was studied by following the distribution of glucosamine- 14C, fucose- 14C, and leucine- 3H as a function of time. The HeLa cells were useful for studying the biosynthesis of membrane components; nearly all of the glycoprotein and glycolipid synthesized by these cells is membrane-bound or intracellular. The extracellular material constituted only 1–4%. Most of the glucosamine- 14C and fucose- 14C was incorporated into glycoprotein and glycolipid found in the smooth internal and plasma membranes; by contrast, the majority of the leucine- 3H was found in the microsomal fraction and soluble protein. It was concluded that glucosamine is not attached to protein at the ribosomal or microsomal level. From chase experiments it appeared that the soluble and smooth membrane glycoprotein and glycolipid was first labeled; the labeled material then migrates progressively to the plasma membrane. A hypothesis was proposed concerning the events involved in membrane glycoprotein biosynthesis and the localization of these events within the cell. The smooth internal membranes were specified as the subcellular site for assembly of membrane glycoprotein and glycolipid. The incorporation of glycoprotein and glycolipid membrane subunits, which are eventually integrated into the substructure of the plasma membrane, was discussed.