Intracellular transport and posttranslational cleavage of oat globulin precursors.

The synthesis, transport, and posttranslational processing of reserve globulin in Avena sativa L. seeds were studied by pulse-chase labeling. Developing oat seeds were labeled with radioactive sulfate and tissue homogenates were used for globulin extraction.Two globulin precursors (58-62 kilodaltons) were labeled after 1 hour pulse. The alpha and beta globulin subunits appeared between 2 and 10 hours later, while simultaneously the 58 to 62 kilodaltons polypeptides gradually disappeared. This confirmed a precursor-product relationship. In a second pulse-chase experiment, the tissue extracts were fractionated on a sucrose gradient. The major portion of radioactivity was initially (1 hour pulse) associated with the endoplasmic reticulum. However, a significant amount of radioactivity shifted from the endoplasmic reticulum to protein bodies after 20 hours chase, suggesting the transport of the newly synthesized proteins. Protein bodies isolated from pulse-chased seeds were analyzed for the arrival of the newly synthesized globulin. Labeled precursors were detected after 2 hours chase and gradually disappeared. The alpha and beta subunits appeared during the same chase period and assembled into a 12S oligomer.The data indicated that oat globulin was synthesized as two large precursors which were transported from endoplasmic reticulum into protein bodies where they were processed to the alpha and beta subunits forming a 12S oligomer.