Structural organization of ultrarapidly frozen barley aleurone cells actively involved in protein secretion.
The ultrastructural organization of actively secreting barley (Hordeum vulgare L. cv. Himalaya) aleurone cells was examined using ultrarapid-freezing (<-10 000°C s(-1)) followed by freeze-fracture and freeze-substitution. Our analysis indicates that much of the evidence supporting a direct pathway from the endoplasmic reticulum (ER) to the plasma membrane (i.e. bypassing the Golgi apparatus) for the secretion of α-amylase (EC 3.2.1.1) may not be valid. Cryofixed ER cisternae show no sign of vesiculation during active α-amylase secretion in gibberellic acid (GA3)-treated cells. At the same time, Golgi complexes are abundant and numerous small vesicles are associated with the edges of the cisternae. Vesicles appear to be involved in the delivery of secretory products to the plasma membrane since depressions containing excess membrane material appear there. Treatment with GA3 also induces changes in the composition of Golgi membranes; most notably, the density of intramembrane particles increases from 2700 μm(-2) to 3800 μm(-2) because of an increase of particles in the 3-8.5-nm size range. A slight decrease in 9-11-nm particles also occurs. These changes in membrane structure appear to occur as the Golgi complex becomes committed to the processing and packaging of secretory proteins. We suggest that secretory proteins in this tissue are synthesized in the abundant rough ER, packaged in the Golgi apparatus, and transported to the plasma membrane via Golgi-derived secretory vesicles. Mobilization of reserves is also accompanied by dynamic membrane events. Our micrographs show that the surface monolayer of the lipid bodies fuses with the outer leaflet of the bilayer of protein-body membranes during the mobilization of lipid reserves. Following the breakdown of the protein reserves, the protein bodies assume a variety of configurations.
Department of Molecular, Cellular and Developmental Biology, University of Colorado, 80309, Boulder, CO, USA.
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Fernandez
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Staehelin