Figure 12–46 The insertion mechanism for tail-anchored proteins. in this post-translational pathway for the insertion of tail-anchored er membrane proteins, a soluble pre-targeting complex captures the hydrophobic C-terminal α helix after it emerges from the ribosomal exit tunnel and loads it onto the Get3 aTpase. The resulting complex is targeted to the er membrane by interaction with the Get1– Get2 receptor complex that functions as a membrane protein insertion machine. after Get3 hydrolyzes its bound aTp, the tail-anchored protein is released from the receptor and inserted into the er membrane. aDp release and renewed aTp binding recycles Get3 back to the cytosol.
Another ER resident protein is the chaperone protein BiP. We have already discussed how BiP pulls proteins post-translationally into the ER through the Sec61 ER translocator. Like other chaperones (discussed in Chapter 13), BiP recognizes incorrectly folded proteins, as well as protein subunits that have not yet assembled into their final oligomeric complexes. It does so by binding to exposed amino acid sequences that would normally be buried in the interior of correctly folded or assembled polypeptide chains. An example of a BiP-binding site is a stretch of alternating hydrophobic and hydrophilic amino acids that would normally be buried in a β sheet with its hydrophobic side oriented towards the hydrophobic core of the folded protein. The bound BiP both prevents the protein from aggregating and helps keep it in the ER (and thus out of the Golgi apparatus and later parts of the secretory pathway). Like some other members of the hsp70 family of chaperone proteins, which bind unfolded proteins and facilitate their import into mitochondria and chloroplasts, BiP hydrolyzes ATP to shuttle between highand low-affinity binding states, which allow it to hold on to and let go of its substrate proteins in a dynamic cycle.
most proteins synthesized in the rough er are Glycosylated by the addition of a Common N-linked oligosaccharide
The covalent addition of oligosaccharides to proteins is one of the major biosynthetic functions of the ER. About half of the soluble and membrane-bound proteins that are processed in the ER—including those destined for transport to the Golgi apparatus, lysosomes, plasma membrane, or extracellular space—are glycoproteins that are modified in this way. Many proteins in the cytosol and nucleus are also glycosylated, but not with oligosaccharides: they carry a much simpler sugar modification, in which a single N-acetylglucosamine group is added to a serine or threonine of the protein.
During the most common form of protein glycosylation in the ER, a preformed precursor oligosaccharide (composed of N-acetylglucosamine, mannose, and glucose, and containing a total of 14 sugars) is transferred en bloc to proteins. Because this oligosaccharide is transferred to the side-chain NH2 group of an asparagine in the protein, it is said to be N-linked or asparagine-linked (Figure 12–47A). The transfer is catalyzed by a membrane-bound enzyme complex, an protein with N-glycosylation site