Translation translocation

Wilkinson BM, Tyson JR, Reid PJ, Stirling CJ (2000) Distinct domains within yeast Sec61p involved in post-translational translocation and protein dislocation. J Biol Chem 275 521-529... 

Protein translocation across the ER membrane can occur by two pathways. Most secretory proteins are cotranslationally translocated in contrast to post-translational translocation destined for other cellular compartments. Transport... 

The ability of the LamB synthetic signal sequences to inhibit protein translocation in vitro correlates with their activity in vivo (L. Chen and P. C. Tai, personal communication). The wild-type and mutant E. coli LamB signal sequences described above (Section III,H) were added to the cell-free translation/translocation system of Chen et al. (1985). The wild-type peptide blocks translocation of OmpA and alkaline phosphatase 50% inhibition is reached at a peptide concentration of 1-2 fiM. 

ATP Hydrolysis Powers Post-translational Translocation of Some Secretory Proteins in Yeast... 

The overall reaction carried out by BiP is an Important example of how the chemical energy released by the hydrolysis of ATP can power the mechanical movement of a protein across a membrane. Some bacterial cells also use an ATP-driven process for translocating completed proteins across the plasma membrane. However, the mechanism of post-translational translocation in bacteria differs somewhat from that in yeast, as we describe in Section 16.4. 

In post-translational translocation, a completed secretory protein is targeted to the ER membrane by interaction of the signal sequence with the translocon. The polypeptide chain is then pulled into the ER by a ratcheting mechanism that requires ATP hydrolysis by the chaperone BiP, which stabilizes the entering polypeptide (see Figure 16-9). 

In both cotranslational and post-translational translocation, a signal peptidase in the ER membrane cleaves the ER signal sequence from a secretory protein soon after the N-terminus enters the lumen. 

The internal signal-anchor sequence is not cleaved and remains in the translocon while the C-termlnal region of the growing chain is extruded into the ER lumen by co-translational translocation. During synthesis, the signal-anchor sequence moves laterally between the protein sub-... 

The driving force for post-translational translocation across the Inner membrane of bacteria comes from SecA protein, which uses energy derived from hydrolysis of cytosolic ATP to push polypeptides through the translocon channel (see Figure 16-23). 

In the post-translational translocation, the completed polypeptide chain is presented to the ER membrane in a complex (e.g. Sec complex) with cytosolic proteins, particularly with chaperones, and these must be released before translocation can occur. SRP is not involved in the post-translational translocation. 

The eukaryotic co-translational and post-translational translocations are diagrammatically compared in Figure 13.20. 

Figure 13.20 Diagrams for eukaryotic translocations across the ER membrane. The mammalian co-tanslational translocation (a) and yeast post-translational translocation (b) of polypeptide chain are diagrammatically represented. Abbreviations used are SRP, signal recognition particle SR, SRP receptor and TRAM, translocating chain-associated membrane protein. Sec61p spans the ER membrane multiple times and likely forms the translocation channel. The cytosolic components, SsalP and Ydjlp which maintain the nascent polypeptide chain in the unfolded state in the post-translational translocation. The nascent polypeptide-associated complex (NAC) which maintains the fidelity of co-translational precursor and the role of GTP are not shown...

The SRP exists either free in the cytosol, bound to ribosomes, or associated with the membrane. Its function is to facilitate the co-translational translocation of proteins... 

Matthews, G. M., and Colman, A. (1991) A highly efficient, cell-free translation/translocation system prepared from Xenopus eggs. Nucleic Acids Res. 19, 6405-6412. 

---