Figure 12–45 The insertion of the multipass membrane protein rhodopsin into the ER membrane. rhodopsin is the light-sensitive protein in rod photoreceptor cells in the mammalian retina (discussed in Chapter 15). (a) a hydropathy plot (see figure 10–20) identifies seven short hydrophobic regions in rhodopsin. (B) The hydrophobic region nearest the n-terminus serves as a start-transfer sequence that causes the preceding n-terminal portion of the protein to pass across the er membrane. subsequent hydrophobic sequences function in alternation as start-transfer and stop-transfer sequences. The green arrows indicate the paired start and stop signals inserted into the translocator.
(C) The final integrated rhodopsin has its n-terminus located in the er lumen and its C-terminus located in the cytosol. The blue hexagons represent covalently attached oligosaccharides.
er Tail-anchored proteins are integrated into the er membrane by a special mechanism
Many important membrane proteins are anchored in the membrane by a C-terminal transmembrane, hydrophobic α helix. These ER tail-anchored proteins include a large number of SNARE protein subunits that guide vesicular traffic (discussed in Chapter 13). When such a tail-anchored protein inserts into the ER membrane from the cytosol, only a few amino acids that follow the transmembrane αhelix on its C-terminal side are translocated into the ER lumen, while most of the protein remains in the cytosol. Because of the unique position of the transmembrane α helix in the protein sequence, translation terminates while the C-terminal amino acids that will form the transmembrane α helix have not yet emerged from the ribosome exit tunnel. Recognition by SRP is therefore not possible. It was long thought that these proteins are released from the ribosome and the hydrophobic C-terminal tail spontaneously partitions into the ER membrane. Such a mechanism could not explain, however, why ER tail-anchored proteins insert into the ER membrane selectively and not also into all other membranes in the cell. It is now clear that a specialized targeting machinery is involved that is fueled by ATP hydrolysis (Figure 12–46). Although the components and details differ, this post-translational targeting mechanism is conceptually similar to SRP-dependent protein targeting (see Figure 12–37).
Not all tail-anchored proteins are inserted into the ER, however. Some proteins contain a C-terminal membrane anchor that contains additional sorting information that directs the protein to mitochondria or peroxisomes. How these proteins are sorted there remains unknown.
Translocated polypeptide Chains fold and assemble in the lumen of the rough er
Many of the proteins in the lumen of the ER are in transit, en route to other destinations; others, however, normally reside there and are present at high concentrations. These ER resident proteins contain an ER retention signal of four amino acids at their C-terminus that is responsible for retaining the protein in the ER (see Table 12–3. p. 648; discussed in Chapter 13). Some of these proteins function as catalysts that help the many proteins that are translocated into the ER lumen to fold and assemble correctly.
One important ER resident protein is protein disulfide isomerase (PDI), which catalyzes the oxidation of free sulfhydryl (SH) groups on cysteines to form disulfide (S–S) bonds. Almost all cysteines in protein domains exposed to either the extracellular space or the lumen of organelles in the secretory and endocytic pathways are disulfide-bonded. By contrast, disulfide bonds form only very rarely in domains exposed to the cytosol, because of the reducing environment there.