Isolation and characterization of the protein body membrane of castor beans

Intact protein bodies were isolated from dry castor bean seeds (Ricinus communis L.) after homogenization in nonaqueous medium. After repeated washing with glycerol to remove trapped lipid globules, the soluble matrix proteins were removed by the addition of aqueous buffer. The membrane remained attached to the insoluble protein crystalloids and was subsequently released by sonication. Purification of the membrane vesicles in a sucrose gradient produced a single band at a density of 1.21 grams per cubic centimeter. Treatment with 6 molar urea, 1 molar KCl, or 0.25 molar galactose had no effect on the equilibrium density of the membrane. Electron microscopy revealed a highly pure and uniform collection of membrane vesicles. No enzyme activity was specifically associated with the membrane. Sodium dodecyl sulfate gel electrophoresis of the protein body fractions showed that the membrane contained unique proteins, two of which were glycosylated. The membrane contained 153 nanomoles of phospholipid per milligram of protein. The composition of the phosphoglycerides was 51% ethanolamine, 41% choline, 8% inositol, and a trace of serine.