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Microsomes were prepared from 20 day old Bomi barley endosperm by sucrose density gradient centrifugation. Electron microscopy revealed the presence of ribosome-studded vesicles in the material banding at the 1.75-2.26 M sucrose interface. The isolated microsomes were active in the wheat-germ cell-free protein synthesizing system. Hordeins were present among the in vitro synthesized products and were identified by their solubility in 55% isopropanol and by their co-migration with native hordeins on SDS-polyacrylamide gels. The microsomeand polysome-directed, in vitro synthesized hordeins were analysed before and after chymotrypsin treatment by SDS-polyacrylamide gel electrophoresis. The hordeins were degraded when polysomes were used as a template. However, hordeins synthesized on microsomes showed significant protection from proteolysis. This protection could be abolished by treatment with membrane-solubilizing detergents. The reported experiments show that hordeins synthesized on microsomes were discharged vectorially into the lumen of the microsomes.

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Membrane bound and free polyribosomes were isolated from 20 day old barley endosperms. Sucrose gradient analysis revealed distinct polysomal peaks up to heptamers. The isolated polysomes were active in a cell-free protein synthesizing system employing wheat germ extract. SDS-polyacrylamide gel electrophoresis showed that proteins with molecular weights ranging from 200,000 to 10,000 daltons were synthesized. A substantial part of the polypeptides coded for by the template associated with the membrane bound polysomes was identified as hordeins by their solubility in 55% isopropanol and by their co-migration with native hordein on SDS-polyacrylamide gels. Membrane bound endosperm polysomes from a barley mutant defective in hordein synthesis produced in the cell-free protein synthesizing system only a small amount of hordein. Conversely membrane bound polysomes from the endosperm of a mutant giving rise to an increased content of some hordein polypeptides catalyzed a preferential synthesis of these polypeptides in vitro. SDS-polyacrylamide gel electrophoresis revealed that the in vitro template activities of the free polyribosomes from the wild type and mutant endosperms were very similar. The resulting polypeptides had not the solubility characteristics of hordeins.