Figure 12-45. Three ways in which protein translocation can be driven through structurally similar translocators.

Figure 12-45Three ways in which protein translocation can be driven through structurally similar translocators

(A) Co-translational translocation. The ribosome is brought to the membrane by the SRP and SRP receptor and forms a tight seal with the Sec61 protein translocator. The growing polypeptide chain is threaded across the membrane as it is made. No additional energy is needed, as the only path available to the growing chain is to cross the membrane. (B) Posttranslational translocation in eucaryotic cells. An additional complex composed of the Sec62, Sec63, Sec71, and Sec72 proteins is attached to the Sec61 translocator and deposits BiP molecules onto the translocating chain as it emerges into the ER lumen. ATP-driven cycles of BiP binding and release pull the protein into the lumen, a mechanism that closely resembles the thermal ratchet model for mitochondrial import in Figure 12-28. (C) Posttranslational translocation in bacteria. The completed polypeptide chain is fed from the cytosolic side into a translocator in the plasma membrane by the SecA ATPase. ATP-hydrolysis-driven conformational changes drive a pistonlike motion in SecA, each cycle pushing about 20 amino acids of the protein chain through the pore of the translocator. The Sec pathway used for protein translocation across the thylakoid membrane in chloroplasts uses a similar mechanism (see Figure 12-30B).

Whereas the Sec61 translocator, SRP, and SRP receptor are found in all organisms, SecA is found exclusively in bacteria, and the Sec62, Sec63, Sec71, and Sec72 proteins are found exclusively in eucaryotic cells. (Adapted from P. Walter and A.E. Johnson, Annu. Rev. Cell Biol. 10:87–119, 1994.)

From: The Endoplasmic Reticulum

Cover of Molecular Biology of the Cell
Molecular Biology of the Cell. 4th edition.
Alberts B, Johnson A, Lewis J, et al.
New York: Garland Science; 2002.
Copyright © 2002, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter; Copyright © 1983, 1989, 1994, Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson .

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