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.