endoplasmic reticulum (ER)
Membrane system within the cytoplasm of a eukaryotic cell (see eukaryote), important in the synthesis of proteins and lipids. The ER usually makes up more than half the membrane of the cell and is continuous with the outer membrane of the nuclear envelope (see nucleus). There are two distinct regions of ER: the rough ER, or RER (so called because of the protein-synthesizing ribosomes attached to it), and the smooth ER (SER), which is not associated with ribosomes and is involved in the synthesis of lipids and the detoxification of some toxic chemicals.
endoplasmic reticulum [¦en·də¦plaz·mik rə′tik·yə·ləm]
Endoplasmic reticulum
An intracellular membrane system that is present in all eukaryotic cells. In most cells the endoplasmic reticulum is thought to consist of only one continuous membrane enclosing only a single space. However, in protozoa, some unicellular algae, and possibly some fungi, the endoplasmic reticulum occurs as separate, multiple vesicles. See Cell membranes
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Several morphologically and functionally distinct domains of this continuous membrane system can be distinguished. At the level of the nuclear pores, the inner nuclear membrane is continuous with the outer nuclear membrane; both membranes together are referred to as the nuclear envelope. The outer nuclear membrane in turn is continuous with the rough endoplasmic reticulum, which contains specialized regions, termed transitional elements, and is continuous with the smooth endoplasmic reticulum. The two membranes of the nuclear envelope enclose the perinuclear space. The rough and smooth endoplasmic reticula and the transitional element enclose a space called the intracisternal space, or lumen. Both intracisternal and perinuclear spaces form a single compartment. All nucleated cells contain at least a nuclear envelope, but the amount of smooth and rough endoplasmic reticula varies greatly among different cell types. See Cell nucleus
The term rough endoplasmic reticulum is based on the morphologic appearance of attached ribosomes, which are absent from smooth endoplasmic reticulum. (Ribosomes are also associated with the outer nuclear membrane; in fact, the outer nuclear membrane and the rough endoplasmic reticulum appear to be functionally equivalent.) Another morphologic distinction is the organization of the rough endoplasmic reticulun in interconnected flattened sacs (called cisternae), whereas the smooth endoplasmic reticulum forms a tubular network (see illustration). See Ribosomes
The rough endoplasmic reticulum is the site of translocation of secretory and lysosomal proteins from the cytosol to the intracisternal space, and of integration into the membrane of integral membrane proteins. Except for integral membrane proteins of chloroplast, mitochondria, and peroxisomes, essentially all other integral membrane proteins are integrated into the endoplasmic reticulum and either remain there (resident endoplasmic reticulum membrane proteins) or are subsequently distributed to other cellular membranes.
The signal hypothesis was formulated to explain how these proteins are targeted to and then translocated across or integrated into the endoplasmic reticulum membrane. Its tenets are that all polypeptides targeted to this membrane contain a discrete sequence (termed the signal sequence), that a complex machinery recognizes this sequence, and that recognition triggers the opening of a proteinaceous channel through which the polypeptide passes across the membrane. In the case of membrane proteins, the existence of an additional topogenic sequence, the so-called stop-transfer sequence, was postulated. This sequence is thought to trigger opening of the channel to the lipid bilayer to abort translocation and thus integrate the protein into the lipid bilayer.
The rough endoplasmic reticulum also contains numerous enzymes, most of which are involved in the modification of the nascent protein chain on the cisternal side. Thus the main function of the rough endoplasmic reticulum and the outer nuclear membrane is to serve as a port of entry of secretory, lysosomal, and integral membrane proteins and as the site of their initial modification.
Secretory and lysosomal proteins as well as those integral membrane proteins that are not residents of the endoplasmic reticulum are next transported to the cis Golgi cisternae. The transitional elements represent sites of transport from the rough endoplasmic reticulum. Coated vesicles carrying proteins to be transported form at these sites and, after uncoating, eventually fuse with the cis Golgi cisternae. See Golgi apparatus
Smooth endoplasmic reticulum contains enzymes for phospholipid biosynthesis, steroid biosynthesis, and drug detoxification. See Cell (biology), Cell organization, Enzyme
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