Soluble proteins—destined for the ER lumen, for secretion, or for transfer to the lumen of other organelles—pass completely into the ER lumen. Transmembrane proteins destined for the ER or for other cell membranes are translocated part way across the ER membrane and remain anchored there by one or more membrane-spanning α-helical segments in their polypeptide chains. These hydrophobic portions of the protein can act either as start-transfer or stop-transfer signals during the translocation process. When a polypeptide contains multiple, alternating start-transfer and stop-transfer signals, it will pass back and forth across the bilayer multiple times as a multipass transmembrane protein.
The asymmetry of protein insertion and glycosylation in the ER establishes the sidedness of the membranes of all the other organelles that the ER supplies with membrane proteins.
how do nuclear import receptors negotiate the tangled gel-like interior of a nuclear pore complex so efficiently?
is the nuclear pore complex a rigid structure or can it expand and contract, depending on the cargo transported?
sequence comparisons show that signal sequences for an individual protein such as insulin are quite conserved across species, much more so than would be expected from our current understanding that all that matters for their function are general structural features such as hydrophobicity. What other functions might signal sequences have that could account for their evolutionary sequence conservation?
how are polyribosomes on the endoplasmic reticulum membrane arranged so that the next initiating ribosome will find an unoccupied translocator?
Why does the signal-recognition particle have an indispensable rna subunit?
Which statements are true? Explain why or why not.
12–1 Like the lumen of the ER, the interior of the nucleus is topologically equivalent to the outside of the cell.
12–2 ER-bound and free ribosomes, which are structurally and functionally identical, differ only in the proteins they happen to be making at a particular time.
12–3 To avoid the inevitable collisions that would occur if two-way traffic through a single pore were allowed, nuclear pore complexes are specialized so that some mediate import while others mediate export.
12–4 Peroxisomes are found in only a few specialized types of eukaryotic cell.
Discuss the following problems.
12–5 What is the fate of a protein with no sorting signal?
12–6 The rough ER is the site of synthesis of many classes of membrane proteins. Some of these proteins remain in the ER, whereas others are sorted to compartments such as the Golgi apparatus, lysosomes, and the plasma membrane. One measure of the difficulty of the sorting problem is the degree of “purification” that must be achieved during transport from the ER. Are proteins bound for the plasma membrane common or rare among all ER membrane proteins?
A few simple considerations allow one to answer this question. In a typical growing cell that is dividing once every 24 hours, the equivalent of one new plasma membrane must transit the ER every day. If the ER membrane is 20 times the area of a plasma membrane, what is the ratio of plasma membrane proteins to other membrane proteins in the ER? (Assume that all proteins on their way to the plasma membrane remain in the ER for 30 minutes on average before exiting, and that the ratio of proteins to lipids in the ER and plasma membranes is the same.) 12–7 Before nuclear pore complexes were well understood, it was unclear whether nuclear proteins diffused passively into the nucleus and accumulated there by binding to residents of the nucleus such as chromosomes, or whether they were actively imported and accumulated regardless of their affinity for nuclear components.