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<p></p> <p><b>新页面</b></p><div> The export and <br /> degradation of misfolded ER proteins. <br /> Misfolded soluble proteins in the ER <br /> lumen are recognized and targeted <br /> to a translocator complex in the ER <br /> membrane. They first interact in the <br /> ER lumen with chaperones, disulfide <br /> isomerases, and lectins. The chaperones <br /> maintain the misfolded protein in an <br /> unfolded conformation and prevent their <br /> aggregation. The disulfide isomerases <br /> reduce disulfide bonds to fully unfold the <br /> protein. The lectins selectively recognize <br /> trimmed N-linked oligosaccharides that <br /> are generated when a protein spends <br /> too long in the ER. The lectins have <br /> binding sites on a membrane-embedded <br /> protein translocator built around an E3 <br /> ubiquitin ligase. The unfolded protein is <br /> then exported into the cytosol through <br /> the translocator. The E3 ubiquitin ligase <br /> ubiquitylates the unfolded protein as it <br /> emerges on the cytosolic side of the <br /> translocator. The ubiquitin prevents <br /> backsliding of the protein into the ER <br /> and provides a molecular handle for <br /> an AAA-ATPase that completes the <br /> extraction reaction. The unfolded protein <br /> is then de-glycosylated and degraded <br /> in proteasomes. Misfolded membrane <br /> proteins follow a similar pathway but <br /> are thought to engage the translocator <br /> sideways within the lipid bilayer. Multiple <br /> translocator complexes containing different <br /> E3 ubiquitin ligases reside in the ER. <br /> They are thought to handle different <br /> subsets of proteins that are misfolded in <br /> different ways.</div>

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