Role of Endoplasmic Reticulum in Biosynthesis of Oat Globulin Precursors

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Abstract
Oat (A. sativa L.) groats were labeled with radioactive leucine and salt-soluble proteins were extracted and analyzed. Polyacrylamide gel electrophoresis followed by fluorography indicated 2 radioactive polypeptides with MW 58-62 kilodaltons [kd] which were similar in size to unreduced globulin .alpha.-.beta. dimers. The role of endoplasmic reticulum in the synthesis of these globulin polypeptides was investigated by in vivo and in vitro protein synthesis studies. Labeled tissue was fractionated by centrifugation and rough endoplasmic reticulum [ER] was isolated. Two polypeptides which had MW of 58-62 kd and were immunoprecipitable with antiglobulin immunoglobulin G were found to be transiently associated with the endoplasmic reticulum. Rough endoplasmic reticulum and membrane-bound polysomes, directed the in vitro synthesis of 2 polypeptides with MW 58-62 kd corresponding in size to unreduced .alpha.-.beta. dimers, and could be immunoprecipitated with antiglobulin immunoglobulin G. The translation products of free polysomes did not show this. In pulse-labeling, globulin polypeptides with MW 58-62 kd, as well as the .alpha. + .beta. subunits, were labeled in protein bodies. Oat globulin polypeptides are synthesized as higher MW precursors on ER-associated polysomes. These precursors are probably transported into protein bodies and cleaved into smaller .alpha. and .beta. subunits.