Glycoprotein synthesis in Drosophila Kc cells. Biosynthesis of dolichol-linked saccharides.

The biosynthesis of dolichol and dolichol-linked saccharide intermediates in glycoprotein synthesis was studied in an embryonic Drosophila cell line (Kc) that lacks the squalene-cholesterol branch of the polyisoprenoid biosynthetic pathway. Kc cells were labeled with [5-3H]mevalonic acid and the radioactive lipids formed were analyzed. Although the major labeled product was coenzyme Q, dolichol and a variety of dolichol derivatives could be readily detected. On the basis of their chromatographic and chemical properties, these derivatives were identified as dolichyl phosphate, glucosylphosphoryldolichol, mannosylphosphoryldolichol, and oligosaccharylpyrophosphoryldolichol. Both short term (4-h) and steady state (4-day) labeling experiments with mevalonate, rather than sugars as previously used, were performed to assess the level of these intermediates. The results of these studies, using a precursor common to all the intermediates, reveal that the early intermediates, N-acetylglucosaminylpyrophosphoryldolichol and N,N'-diacetylchitobiosylpyrophosphoryldolichol, are present at very low levels (less than 5%) relative to the other intermediates on the pathway to oligosaccharylpyrophosphoryldolichol. The total amount of dolichol intermediates remained essentially constant during the chase phase of pulse-chase experiments, indicating the absence of a major catabolic pathway for the polyisoprenoid backbone. As expected, however, the sugar moiety, studied with mannosylphosphoryldolichol, underwent rapid turnover. These results are discussed in the context of our current understanding of the pathway whereby dolichol derivatives participate in glycoprotein synthesis.