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TRAPP complex : ウィキペディア英語版 | TRAPP complex TRAPP (TRAnsport Protein Particle) is a protein involved in particle transport between organelles. ==Protein folding and the Endoplasmic Reticulum (ER)== Proteins that are destined for the plasma membrane or export to the extracellular environment in eukaryotic cells are translated on ribosomes that sit on the rough endoplasmic reticulum (RER). Most proteins are co-translationally transported into the ER (i.e., while the ribosome is translating the mRNA code into a polypeptide, the polypeptide is simultaneously inserted via the translocon pore into the ER). The ER provides an environment that helps nascent polypeptides fold into and become functional or partially functional proteins. The ER provides an oxidizing environment (for formation of disulfide bonds) and the necessary chaperones (folding assisting agents that are not part of the final protein). Numerous exported proteins form disulfide bonds—covalent bonds that stabilize the protein structure in harsh extracellular environments. A classic example are the disulfide-linked heavy and light chain polypeptides of antibodies secreted by B-cells of the immune system. Another key event that takes place in the ER is N-linked glycosylation. In this process, polypeptides that have a unique stretch of 3 amino acids (asparagine - X - serine/threonine, where X represents any amino acid except proline) are modified with a complex sugar moiety on the amide group of asparagine. Other types of glycosylations include S-linked (via cysteine residues), C-linked (via tryptophan) and O-linked (via serine or threonine). By far, N-linked glycosylation is the most abundant post-translational modification found in eukaryotic cells.
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