Incorporation of D-[14C]galactose into organelle glycoprotein in castor bean endosperm

Excised casto bean (Ricinus communis L.) endosperm tissue supplied with ¹⁴C]galactose incorporates radioactivity into particulate cell components. Fractionation of homogenates established that ¹⁴C-labeled trichloroacetic acid-insoluble material was located primarily in the microsomal and glyoxysomal fractions. The capacity of the tissue to incorporate ¹⁴C]galactose into organelle glycoprotein varied during seedling development, increasing during the first 3 days of germination and subsequently declining. The kinetics of incorporation into the major organelle fractions of 2-day old endosperm tissue showed that the endoplasmic reticulum was immediately labeled whereas a lag period preceded the labeling of glyoxysomes. Sub-fractionation of the isolated organelles established that the greatest proportion of the ¹⁴C]-galactose labeled glycoprotein was located in the membrane, although a significant incorporation into the matrix protein was also observed.The results indicate that the addition of the carbohydrate moiety to the polypeptide cores occurs in the endoplasmic reticulum during or immediately after their synthesis on membrane-bound ribosomes.Abbreviations ER endoplasmic reticulum - SDS sodium dodecyl sulphate - TCA trichloroacetic acid