Exam III. 4

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Exam III. 4
2013-10-30 18:44:49
Cell Bio

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  1. The __ and its receptor are thought to act in concert. The __ binds to both the __ and the __, thereby inducing __.
    • SRP
    • SRP 
    • exposed ER signal sequence
    • ribosome
    • a pause in translation
  2. The __ in the eR membrane, which is composed of __, binds the __ and directs it to the __.
    • SRP receptor
    • two different polypeptide chains
    • SRP-ribosome complex
    • translocator
  3. In a poorly understood reaction, the __ and __ are then released, leaving the ribosome bound to the __ in the ER membrane.
    The __ then inserts the polypeptide chain into the membrane and transfers it across the __.
    • SRP 
    • SRP receptor
    • translocator x2
    • lipid bilayer
  4. Because one of the __ proteins and both chains of the __ contain __, it is thought that conformational changes that occur during cycles of __ and __ ensure that __ release occurs only after the ribosome has become properly engaged with the __ in the ER membrane.
    • SRP
    • SRP receptor
    • GTP-binding domains
    • GTP binding and hydrolysis
    • SRP 
    • translocator
  5. The __ is closed until the ribosome has bound, so that the __ of teh ER membrane is maintained at all times.
    • translocator
    • permeability barrier
  6. What is an SRP?
    • an elongated complex containing six protein subunits and one RNA molecule.
    • One end of the SRP binds to an ER signal peptide on a growing polypeptide chain, while the other end binds to the ribosome itself and stops translation.
    • The RNA in the particle may mediate an interaction with rRNA
  7. One molecule of RNA that reacts with SRP has __. The size of the RNA is __.
    The __ gives the measure of how fast something will sediment in centrifuge. What affects this rate?
    Most RNA molecules are characterized by their __.
    • 7 polypeptide chians
    • 7S
    • Svedberg unit
    • mw and shape
    • S value
  8. Why is it important to stop translation when the SRP binds to the signal peptide?
    if it continued, the length grows and it will be difficult to both align the complex to the translocon and allow it to go in
  9. When the ribosome, signal sequence and SRP all bind to each other, what happens?
    Once bound to the translocon, what happens?
    • pause in translation
    • translation begins again
  10. What does the SRP bind to?
    both the signal sequence and ribosome
  11. Brief summary of the ER signal sequence steps?
    • 1) binding of SRP to signal peptide causes translation to pause
    • 2) SRP-bound ribosome attaches to SRP receptor in ER membrane
    • 3) Translation continues and translocation begins
    • 4) SRP and SRP receptor displaced and recycled
  12. Affinity of a non-secretory protein that does not synthesize a signal sequence with an SRP molecule?
    low; if Signal sequence is absent, SRP doesn't bind
  13. True or False:
    The complex bumping into the ER is completely directed.
    • False:
    • random
  14. Usually as the protein binds to the translocon, what happens?
    it activates it and opens it up (shape change)
  15. In addition to SRP binding to receptor proteins, ribosomes bind to the __, allowing what?
    • translocon
    • ribosome to stay attached
  16. Does SRP detach before the complex binds to the translocon?
    • it is simultaneous. 
    • the SRP binds to the SRP receptor, then binding complex to translocon
  17. What is a signal peptidase?
    an enzyme in the inner surface of hte ER membrane that cleaves off the signal sequence, releasing the mature proteins into the ER lumen
  18. Why won't the protein go back through the channel? What helps the protien fold?
    • too big
    • chaperones
  19. On binding an __ (which acts as a __), the __ opens its pore, allowing the transfer of the polypeptide chain across the lipid bilayer as a loop. After the proteinhas been completely translocated, the pore closes, but the __ now opens __ within the lipid bilayer, allowing the __ to diffuse into the bilayer, where it is rapidly degraded.
    • ER signal sequence
    • start-transfer signal
    • translocator
    • translocator
    • hydrophobic signal sequence
  20. True or False: In some cases, the signal sequence is still thre.
  21. If a protein is destined to be a single-pass transmembrane protein, it will have a __ integrated into the ER membrane. The protein contains a __, which enters teh transloactor and interacts with a __, causign the translocator to change conformation and discharge the protein laterarlly into teh bilayer.
    • cleaved ER signal sequence
    • stop-transfer sequence
    • binding site
  22. The stop transfer sequence is followed by __. The sequence is __. 
    Why does it stay in the membrane?
    • charged amino acids
    • hydrophobic
    • due to teh charge of amino acids, which prevents passage
  23. If it has no stop-halt tranfer sequence, what?
    If it has neither stop nor start, it is?
    • it is in the ER lumen
    • in the cytosol
  24. In some protein s(hypothetical), an internal ER signal sequence that functions as a __ binds to the __ in such a way that its more positively charged end remains in the cytosol.
    A) IF there are more positively charged amino acids immediately preceding the hydrophobic core of the __ than there are following it, the __ is inserted into the translocator in which way? THe part of the protein __to the start transfer sequence will therefore be passed across the membrane.
    • start-transfer signal
    • translocator
    • start-transfer sequence x2
    • with both ends out and the middle in the translocon
    • C-terminal
  25. If there are more positively charged amino acids immediately follwing the hydrophobic core of the __ than there are preceding it, the __ is inserted into the translocator in what way?

    The part of hte protein __ to the start-transfer sequence will be passed across the membrane. Because a translocation cannot start before a __ appears outside the ribosome, translocation of the N-terminal portion of hte protein can occur only after this portion has been __.
    • start-transfer sequence x2
    • with both ends out, but the C-terminal end more prominent in the cytosol

    • N-terminal
    • start-transfer sequence
    • fully synthesized
  26. Either the amino or carboxyl end can go through the membrane. WHat does it depend on?
    the positivel ycharged amino acid because those with the positive charge stay in the cytosol
  27. The __ stays outside. What goes through determines what?
    what goes through the translocon
  28. True or False:
    You can only have a signal sequence at hte end.
    False: in the middle