CYTOSOL ER LUMEN CYTOSOL EXTRACELLULAR SPACE SecA ATPase SecY complex Sec62,63,71,72 complex BiP SRP SRP receptor Sec61 complex ATPADPER signal sequence Pi+ bacterial inner (plasma) membrane
Figure 12–41 Three 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 then engages 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) post-translational translocation in eukaryotic cells requires an additional complex composed of sec62, sec63, sec71, and sec72 proteins, which is attached to the sec61 translocator and deposits Bip molecules onto the translocating chain as it emerges from the translocator in the lumen of the er. aTp-driven cycles of Bip binding and release pull the protein into the lumen, a mechanism that closely resembles the mechanism of mitochondrial import in figure 12–23. (C) post-translational translocation in bacteria. The completed polypeptide chain is fed from the cytosolic side into the bacterial homolog of the sec61 complex (called the secY complex in bacteria) 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 12–26B).
Whereas the sec61 translocator, srp, and srp receptor are found in all organisms, seca is found exclusively in bacteria, and the sec62, 63, 71, 72 complex is found exclusively in eukaryotic cells. (adapted from p. Walter and a.e. Johnson, Annu. Rev. Cell Biol. 10:87–119, 1994. With permission from annual reviews.) signal sequence binds to a specific site inside the pore itself, thereby opening the pore. An ER signal sequence is therefore recognized twice: first by an SRP in the cytosol and then by a binding site in the pore of the protein translocator, where it serves as a start-transfer signal (or start-transfer peptide) that opens the pore (for example, see Figure 12–35 for how this works for a soluble protein). Dual recognition may help ensure that only appropriate proteins enter the lumen of the ER.
While bound in the translocation pore, a signal sequence is in contact not only with the Sec61 complex, which forms the walls of the pore, but also, along the lateral seam, with the hydrophobic core of the lipid bilayer. This was shown in chemical cross-linking experiments in which the signal sequence and the hydrocarbon chains of lipids were covalently linked together. When the nascent polypeptide chain grew long enough, the ER signal peptidase cleaved off the signal sequence and released it from the pore into the membrane, where it was rapidly degraded to amino acids by other proteases in the ER membrane. To release the signal sequence into the membrane, the translocator opens laterally along the seam (see Figures 12–35 and 12–39). The translocator is therefore gated in two directions: it opens to form a pore across the membrane to let the hydrophilic portions of proteins cross the lipid bilayer, and it opens laterally within the membrane to let hydrophobic portions of proteins partition into the lipid bilayer. Lateral gating of the pore is an essential step during the integration of transmembrane proteins.