Co-translational translocation

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 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... 

Vectorial translation [31,32]. Polypeptides are made on membrane-bound polysomes. Many of these proteins are synthesized with a 16-30 amino acid extension at the NH2-terminus. This signal sequence is hydrophobic in nature. Protein synthesis and translocation, into or across the membrane, are obligatorily linked. Therefore, the transmembrane movement is co-translational and it is coupled to the elongation of the polypeptide chain. Consequently, the completed polypeptide chain is never present in the compartment where it is synthesized. The polypeptides that do not yet cross the membrane are shorter than the mature protein. Addition of inhibitors of protein synthesis immediately arrest movement of the polypeptide across the membrane. 

Two translocation pathways are operative in eukaryotes, i.e. co-translational and post-translational. In the co-translational pathway, transport occurs while the polypeptide chain is being synthesized on a membrane-bound ribosome. In the post-translational pathway, the polypeptide chain is completed in the cytoplasm before being transported. In prokaryotic, ribosomes do not seem to be tightly bound to the membrane and most proteins may be transported post-translationally or after much of the chain has been synthesized. 

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 addition of lipids as defined artificial liposomes, bicelles, or nanodiscs into the RM can provide alternative hydrophobic environments for the production of soluble MPs. However, the co-translational insertion of MPs into lipid bilayers may require additional compounds (such as chaperones) to support translocation and membrane targeting. The L-CF expression mode is an emerging technique and efficient membrane insertion could require intensive protocol optimizations for particular targets. Lipids tolerated at high concentrations by CF systems include DMPC, DOPC, POPC, and E. coli polar lipids. 

The first isolation of an active OST complex, composed of three subunits (ribo-phorin I and II, and OST48) succeeded from dog pancreas cells [17]. The ribo-phorins had been characterized earlier as possible receptors for ribosomes and thought to be involved in protein translocation [119-121]. Since OST activity could be depleted from detergent solubilized extract by anti-ribophorin I antibodies, it became clear that ribophorins are indeed constituents of OST [17], They are no longer believed to be involved in ribosome binding, but in accord with the co-translational nature of A-glycosylation, the identification of ribophorins as part of the OST may indicate a close location of the enzyme to the protein translocation channel. 

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