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Terms in this set (43)

a. The energy source for cotranslational translocation comes from the translation process itself—in other words, the nascent chain is pushed through the translocon channel. Please note, however, that as translation is completed a portion of the newly synthesized protein still resides within the translocon. This portion is drawn into the ER lumen rather than being pushed.

b. In post-translational translocation, the newly synthesized polypeptide chain is drawn through the translocon by an energy input from ATP hydrolysis by BiP. BiP is luminal protein of the ER and is a member of the Hsc70 family of molecular chaperones. BiP-ATP activates by binding to the Sec63 complex that in turn binds to the Sec61 translocon complex. Activated BiP is enzymatically active and cleaves ATP to ADP plus Pi. It is BiP-ADP that binds to the entering, unfolded nascent chain. Sequential binding of BiP-ADP to the nascent chain serves to block any sliding of the chain back and forth in the translocon and to ratchet the nascent chain
through the translocon.

c. Translocation into the mitochondrial matrix occurs through a bipartite Tom/Tim complex in which Tom is the outer membrane translocon and Tim is the inner membrane translocon. Three energy inputs are required. First, ATP hydrolysis by a cytosolic Hsc70 chaperone keeps the newly synthesized mitochondrial precursor protein unfolded in the cytosol. Second, ATP hydrolysis by multiple ATP-driven matrix Hsc70 chaperones may serve to pull the translocating protein into the matrix. Matrix Hsc70s interact with Tim44 and hence may be analogous to the BiP/Sec63 interaction at the ER membrane. Third, energy input from the H+ electrochemical gradient or proton-motive force is required. The inside-negative membrane electric potential may serve to electrophorese the amphipathic matrix-targeting sequence toward the matrix.
Glycosylation: covalent addition and processing of carbohydrates. N-linked or O-linked oligosaccharides are carbohydrates that are added to proteins to form glycoproteins, which are commonly used for cell-surface signaling.
Formation of disulfide bonds: Disulfide bonds help stabilize the tertiary and quaternary structure of proteins, and these bonds are formed in the ER by the oxidation reaction of sulfhydryl (thiol) groups between two cysteine residues.
Proper folding of polypeptide chains and assembly of multi-subunit proteins: Sequential actions of ER lumen proteins, protein folding enzymes, enables efficient folding of polypeptide chains into mature, folded proteins.
Specific proteolytic cleavage: Unassembled or misfolded proteins are transported to the cytosol for degradation. ER membrane proteins recognizes the incorrectly folded protein and target it for transport into the cytosol via the process of dislocation.

Bacteria are often a poor choice for production of therapeutic proteins because prokaryotic cells do not have an ER. Although the bacteria may be able to produce the unfolded polypeptide chain as precursor for a therapeutic protein, the bacteria would have no way to efficiently and quickly make sure the protein was correctly folded or bound to carbohydrates or to other protein subunits. It is unlikely that very much of the mature folded protein could be produced efficiently by the bacteria. Eukaryotic cells with an ER are much more effective at ensuring correct and efficient protein folding and other post-translational modifications.