The ER lumen differs from the cytosol in its concentration of calcium ions ( Meldolesi and Pozzan, 1998) and probably of oxidized glutathione ( Hwang et al., 1992), but whether other small molecules are concentrated or depleted in the ER is unclear. Nevertheless, the mere existence of gradients does not indicate how tight a membrane is.Īlthough the plasma and lysosomal membranes are considered to be highly impermeable, the situation with the endoplasmic reticulum (ER) membrane is less clear. However, leakiness of a membrane is usually not a problem because the pumps that generate the gradients are strong. Cellular membranes generally show some leakiness, caused by the imperfect nature of the bilayer, particularly by kinks in unsaturated hydrocarbon chains of phospholipids and by the presence of proteins that disturb its organization ( de Gier, 1992). The imbalance of these small molecules across the membrane is the result of both the barrier that the phospholipid bilayer represents and of pumps that generate the gradients. These data indicate that the ER membrane is significantly more leaky than other cellular membranes, a property that may be required for protein folding and other functions of the ER.Ĭellular membranes generate compartments in which the concentration of ions and other small molecules can differ dramatically from that of the surroundings. Permeation of the small molecules is passive because it occurs at low temperature in the absence of energy. A larger polar reagent of ∼5 kDa is unable to pass through the ER membrane. In permeabilized cells, the ER membrane allows the passage of a small, charged modification reagent that is unable to cross the plasma membrane or the lysosomal and trans-Golgi membranes. We report that isolated ER vesicles allow different chemical modification reagents to pass from the outside into the lumen with little hindrance. Here, we have tested the permeability of the ER membrane to small, nonphysiological molecules. This is the case with cells of the liver, for example.The lumen of the endoplasmic reticulum (ER) differs from the cytosol in its content of ions and other small molecules, but it is unclear whether the ER membrane is as impermeable as other membranes in the cell. Since the RER is engaged in modifying proteins (such as enzymes, for example) that will be secreted from the cell, the RER is abundant in cells that secrete proteins. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane. The RER also makes phospholipids for cellular membranes. These modified proteins will be incorporated into cellular membranes-the membrane of the ER or those of other organelles -or secreted from the cell (such as protein hormones, enzymes ). Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope. Rough Endoplasmic Reticulum: This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell.
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