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Glucose on the sugar side chains of proteins keeps unfolded proteins in the ER. Mannoses on sugar side chains act as a timer for protein folding. Unfolded proteins in the ER are threaded through a retrotranslocon into the cytosol where they are ubiquitylated and digested. IRE1, ATF6 and PERK sense the amount of unfolded protein and generates several responses to reduce the effects and amount of unfolded protein.
Main article: Protein primary structure. The primary structure of a protein refers to the linear sequence of amino acids in the polypeptide chain. The primary structure is held together by covalent bonds such as peptide bonds, which are made during the process of protein biosynthesis or translation. structural domain is an element of the protein's overall structure that is self-stabilizing and often folds independently of the rest of the protein chain. Many domains are not unique to the protein products of one gene or one gene family but instead appear in a variety of proteins.