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

Abstract

Excised casto bean (Ricinus communis L.) endosperm tissue supplied with [(14)C]galactose incorporates radioactivity into particulate cell components. Fractionation of homogenates established that (14)C-labeled trichloroacetic acid-insoluble material was located primarily in the microsomal and glyoxysomal fractions. The capacity of the tissue to incorporate [(14)C]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 [(14)C]-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.