Sequestration of pea reserve proteins by rough microsomes.

Free polysomes, polysomes released from membranes, and rough microsomal vesicles isolated from developing cotyledons of Pisum sativum L. cv. Burpeeana were used to direct cell-free protein synthesis in a wheat germ system. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that the polypeptide products had molecular weights ranging from 12,000 to 74,000. Some of the polypeptides migrated during electrophoresis with the same mobility as polypeptides present in legumin and vicilin preparations. By the use of rabbit antibodies raised against pea reserve proteins it was established that polysomes released from membranes and rough microsomes directed the synthesis of polypeptides that were related to reserve proteins whereas free polysomes did not.Centrifugation studies indicated that the majority of the radioactivity incorporated by rough microsomes was specifically associated with the microsomes. The incorporated radioactivity of sedimented microsomes was not released by treatment with KCl and was resistant to proteolysis unless detergent was present. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that the sequestered translation products were related to pea reserve proteins. It is concluded that the reserve proteins of pea cotyledons are synthesized exclusively by membrane-bound polysomes and that they are sequestered within the rough endoplasmic reticulum.