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The Endoplasmic Reticulum of Mung Bean Cotyledons

ROLE IN THE ACCUMULATION OF HYDROLASES IN PROTEIN BODIES DURING SEEDLING GROWTH ¹

Department of Biology C-016, University of California, San Diego, La Jolia, California 92093

The subcellular localization of two hydrolases (ribonuclease and vicilin peptidohydrolase) which are synthesized de novo in the cotyledons of mung bean seedlings was studied. Earlier experiments had shown that both enzymes accumulate in the protein bodies in the course of seedling growth. Two methods to fractionate subcellular organelles were used to demonstrate that a significant proportion of the enzymes is organelle-associated. This proportion is highest (up to 50% for vicilin peptidohydrolase and 15% for ribonuclease) when synthesis of the enzymes has just started. Evidence obtained with isopycnic sucrose gradients indicates that both hydrolases are associated with membranes rich in NADH-cytochrome c reductase, a marker enzyme for the endoplasmic reticulum (ER). The hydrolases band with the NADH-cytochrome c reductase under conditions where the ribosomes remain attached or are detached from the ER-derived vesicles. Treatment of the ER-derived vesicles with Triton X-100 shows that vicilin peptidohydrolase and vesicle membranes can be physically separated without dissolving the membranes, indicating that the proteinase is soluble within the vesicles. These data support the conclusion that the ER is involved in the transport of ribonuclease and proteinase to the protein bodies.

³ To whom reprint requests should be addressed.

¹ This work was supported by grants from the National Science Foundation (Metabolic Biology). This is the third paper in a series on the endoplasmic reticulum of mung bean cotyledons. The first two papers are listed as references 12 and 17.

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