Post-translational 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... 

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 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...

Suzuki T, Seko A, Kitajima K, Inoue Y, Inoue S (1994) Purification and enzymatic properties of peptide N-glycanase from C3H mouse-derived L-929 fibroblast cells Possible widespread occurrence of post-translational remodification of proteins by N-deglycosylation. J Biol Chem 269 17611-17618 Swaminathan S, Amerik AY, Hochstrasser M (1999) The Doa4 deubiquitinating enzyme is required for ubiquitin homeostasis in yeast. Mol Biol Cell 10 2583-2594 Tanaka K, Yoshimura T, Tamura T, Fujiwara T, Kumatori A, Ichihara A (1990) Possible mechanism of nuclear translocation of proteasomes. FEBS Lett 271 41-46... 

However, some receptors are constitutively expressed in the nucleus and this type of receptor would not be amenable to a nuclear translocation assay. The activities of nuclear receptors may be dependent upon complex interactions with a number of coregulatory proteins, commonly known as coactivators or corepressors, and modifications by post-translational means. Cell type-specific expression levels of receptors and coregulators may contribute to some, but not all, of the molecular bases for gene and functional selectivity of receptor activity. Therefore selecting a cell line that expresses both the target receptor and the necessary cofactors may be required to design an appropriate assay. 

Figure 2. Pre-system representation of the ER. Schematic representation of the endoplasmic reticulum including various molecular machines such as protein synthesis and translocation of newly synthesized proteins (from the ribosome through the translocation channel), protein folding and post-translational modification in the lumen of the ER (presence of numerous chaperones and PTM enzymes) and finally either export to later compartment or degradation.

---