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The endoplasmic reticulum (ER) is an organelle of cells in eukaryotic organisms that forms an interconnected network of membrane vesicles. According to the structure, the endoplasmic reticulum is classified into two types, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). These are the sites of protein synthesis. The rough endoplasmic reticulum is predominantly found in hepatocytes where protein synthesis occurs actively.
The surface of the rough endoplasmic reticulum (often abbreviated RER) is studded with protein-manufacturing ribosomes giving it a "rough" appearance (hence its name). The binding site of the ribosome on the rough endoplasmic reticulum is the translocon. However, the ribosomes bound to it at any one time are not a stable part of this organelle's structure as they are constantly being bound and released from the membrane. A ribosome binds to the endoplasmic reticulum only when it begins to synthesize a protein destined for the secretory pathway. There, a ribosome in the cytosol begins synthesizing a protein until a signal recognition particle recognizes the signal peptide of 5–30 hydrophobic amino acids, sometimes preceded by a positively charged amino acid. This signal sequence allows the recognition particle to bind to the ribosome, causing the ribosome to bind to the rough endoplasmic reticulum and pass the new protein through the rough endoplasmic reticulum membrane. The signal peptide is then cleaved off within the lumen of the ER by a signal peptidase. Ribosomes at this point may be released back into the cytosol; however, non-translating ribosomes are also known to stay associated with translocons.
The membrane of the rough endoplasmic reticulum forms large double membrane sheets that are located near, and continuous with, the outer layer of the nuclear envelope. Although there is no continuous membrane between the endoplasmic reticulum and the Golgi apparatus, membrane-bound vesicles shuttle proteins between these two compartments. Vesicles are surrounded by coating proteins called COPI and COPII. COPII targets vesicles to the golgi apparatus and COPI marks them to be brought back to the rough endoplasmic reticulum. The rough endoplasmic reticulum works in concert with the golgi complex to target new proteins to their proper destinations. A second method of transport out of the endoplasmic reticulum involves areas called membrane contact sites, where the membranes of the endoplasmic reticulum and other organelles are held closely together, allowing the transfer of lipids and other small molecules.
The rough endoplasmic reticulum is key in multiple functions:
- Manufacture of lysosomal enzymes with a mannose-6-phosphate marker added in the cis-Golgi network
- Manufacture of secreted proteins, either secreted constitutively with no tag or secreted in a regulatory manner involving clathrin and paired basic amino acids in the signal peptide.
- Integral membrane proteins that stay embedded in the membrane as vesicles exit and bind to new membranes. Rab proteins are key in targeting the membrane; SNAP and SNARE proteins are key in the fusion event.
- Initial glycosylation as assembly continues. This is N-linked (O-linking occurs in the golgi).
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- Endoplasmic reticulum. (n.d.). McGraw-Hill Encyclopedia of Science and Technology. Retrieved September 13, 2006, from Answers.com Web site: http://www.answers.com/topic/endoplasmic-reticulum
- Levine T (September 2004). Short-range intracellular trafficking of small molecules across endoplasmic reticulum junctions. Trends Cell Biol. 14 (9): 483–90.
- Levine T, Loewen C (August 2006). Inter-organelle membrane contact sites: through a glass, darkly. Curr. Opin. Cell Biol. 18 (4): 371–8.
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