Briefly describe the mechanism by which the presence

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Briefly describe the mechanism by which the presence of an internal stop-transfer sequence in a protein causes the protein to become embedded in the lipid bilayer as a transmembrane protein with a single-membrane-spanning region? Assume that the protein has an amino ter- minal signal sequence and just one internal hydrophobic stop-transfer sequence. 192 Essential Cell Biology Test Bank
14–19 Intermediate/difficult, art labeling A plasma membrane protein X traverses the membrane three times in the orientation shown in Figure Q14–19. (N = amino terminus; C = carboxyl terminus.) The hydrophobic membrane- spanning regions are shown as open boxes. (A) Sketch the arrangement of the newly synthesized protein chain after it has completed its entry into the ER membrane but before any action of signal pep- tidase. Label the cytosol and ER lumen in your diagram. (B) Label the location of stop-transfer (S) and start-transfer (T) sequences on your diagram. Q14–19 14–20 Intermediate/difficult, multiple choice (Note to instructors: this is an alter- native question to 14–19 that does not require the student to sketch any- thing; giving them both in the same test will give away the answer to 14–19) Figure Q14–20B shows the orientation of a multipass membrane protein in the plasma mem- brane. This protein also had an amino-terminal signal sequence, which was cleaved off in the endoplasmic reticulum (Figure Q14–20A). All of the membrane-spanning regions (depicted as open boxes) in this protein have the same amino acid sequence, leading to the hypothesis that any hydrophobic signal sequence has the potential to act as either a start or a stop signal; the actual behavior of the signal is determined by the position of the signal relative to other signals in the protein. If this hypothesis were true, which of the following modifications would cause the protein depicted in Figure Q14–20B to be inserted in the membrane in exactly the opposite orientation relative to the cytoplasm?
Q14–20

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