Effects of dolichol monophosphate on galactose incorporation into glycoconjugates of cell cultures

Involvement of a lipid-linked intermediate in the transfer of galactose from UDP [(14)C]galactose to exogenous protein in castor bean endosperm homogenates

A crude organelle preparation from germinating castor bean endosperm catalysed the incorporation of galactose from UDP[(14)C]galactose into chloroform/methanol (2:1)-soluble glactolipids. At least two galactolipids were formed. Most of the [(14)C]galactose was present in a galactolipid synthesized by the microsomal membranes, the remainder was present in a second galactolipid synthesized by other cellular membranes, possibly Golgi-derived. The addition of asialo-agalacto-fetuin reduced incorporation of [(14)C]galactose into the microsomal galactolipid with a concomitant increase in microsomal [(14)C]galactoprotein. Asialo-agalacto-fetuin did not affect galactolipid or galactoprotein synthesis by nonmicrosomal fractions. The results suggest that the endoplasmic reticulum is a major site of protein galactosylation in castor bean endosperm cells, and that galactose transfer from UDP-galactose to protein occurs via a lipid-linked intermediate.

Incorporation of galactose from UDP-galactose into microsomal and Golgi membranes of rat liver

Rough and smooth microsomes and Golgi membranes were incubated with UDP[14C]galactose and the incorporation of radioactivity into the lipid extract and into endogenous protein acceptors were measured. Antagonistic pyrophosphatases were inhibited with ATP and interference from beta-galactosidase activity was greatly decreased by carrying out the incubation at pH 7.8. After incubation the particles were centrifuged to remove free oligosaccharide residues. Radioactivity was found in the lipid extract from Golgi membranes but not from rough and smooth microsomes. This radioactivity, however, was not associated with dolichol or retinyl phosphates. The incorporation of radioactivity into proteins of the Golgi fraction was more than double than that of the microsomal fractions. In addition, the transferases in these two types of particles exhibited different properties. Trypsin treatment of intact rough microsomal vesicles, smooth vesicles and Golgi membranes removed about 5, 15 and 50%, respectively, of newly incorporated protein-bound galactose, indicating that the proportion of the newly galactosylated proteins, which are localized at the cytoplasmic surface of the membrane, is lowest in rough microsomes, intermediate in smooth, and highest in Golgi membranes.