Exam III. 3 (Signal Hypothesis)

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  1. 1: Synthesis of secretory, cell membrane, and lysosomal proteins begins on __.
    detached or free ribosome that attach to a molecule of mRNA
  2. 2: The ribosomes start __ the info in the __. The initial portion of the polypeptide chain contains a sequence of amino acids called the __, which is __ in non-secretory proteins.
    • translating
    • mRNA
    • signal sequence (signal peptide)
    • absent
  3. Explain the signal sequence
    • 15-20 amino acids on either side
    • located at the amino end of the polypeptide
  4. 3: As the signal sequence of amino acids is made, a __ binds to both it and the __. What does it cause?
    • SRP (signal recognition particule)
    • ribosome
    • halts translation
  5. Describe the structure of the SRP.
    composed of 6 protein subunits adn one small (7S molecule of RNA)
  6. How the __ binds to the ribosome is unknown, but it may involve what?
    • SRP
    • an interaction between the RNA of the SRP and either the rRNA or mRNA
    • it may also involve an interaction between proteins of the SRP and ribosome
  7. Affinity of the __ for ribosomes translating __ is low, so it does not __.
    • SRP
    • non-secretory proteins
    • bind
  8. 4: In addition to the signal sequence, the SRP recognizes and binsd to a __ in the __, binding the __ to the __. 
    Binding to the __ releases __, which resumes when when what? The __ detaches from the membrane, the signal sequence, and the ribosome; it is __.
    • SRP receptor (docking protein)
    • RER membrane
    • ribosome
    • ER
    • SRP receptor
    • the inhibition of translation
    • the ribosome makes contact with the membrane
    • SRP
    • recycled
  9. One __ and both chains of the __ contain __. Possibly, __ during __ and __ may ensure that __ release occurs only when?
    • SRP protein
    • SRP receptor 
    • GTP-binding domains (regions)
    • shape changes
    • GTP binding
    • hydrolysis
    • SRP 
    • after the ribosome binds to the ER
  10. 5: Once the __ bidns to the SRP receptor, the ribosome also binds to an additional __. 
    What does this do?
    What happens to the SRP receptor?
    • SRP-ribosome complex
    • receptor protein (translocon, protein translocator)
    • stabilizes the attachment of the ribosome to the membrane.
    • The SRP receptor detaches and is recycled
  11. 6: When the ribosome with its growing polypeptide chain attaches to the membrane, what happens? 

    The trigger to __ may be what?

    Once open, the growing polypeptide chain is __ across the ER membrane.
    • a pore opens in the translocon (protein translocator)
    • open the pore
    • may be the binding of hte signal sequence
    • translocated
  12. 7: The __ remains bound to the __ while the rest of the polypeptide chain is translated and moves throguh the pore. Once the __ of the polypeptide chain has passed through the membrane, the __ is released from the __ and cleaved from the polypeptide chain. Removal is catalyzed by __, an enzyme located on teh inner side of the ER membrane. 
    What then happens to the signal sequence?
    • signal sequence
    • translocon (protein translocator)
    • carboxyl end 
    • signal sequence
    • translocon
    • signal peptidase

    the signal sequence is rapidly hydrolyzed by other proteases in the eR and the polypeptide chain is released into the ER lumen
  13. True or False:
    The signal sequence must always be removed.
    • False:
    • While the signal sequence from MOST polypeptide chains or proteins, it is not removed from all
  14. 8: Once released into the ER lumen, the polypeptide chain does what with the help of proteins called __. If part of a __, it binds to the other polypeptide chains in the protein. This makes the protein what?
    • folds into its normal tertiary structure 
    • chaperone (eg: BiP)
    • quaternary protein
    • too large to pass back across the ER membrane
  15. 9: What makes proteins destined to become cell membrane proteins different in their translocation?
    they travel only part of the way across the ER because they have a hydrophobic transmembrane segment (halt-transfer signal, stop-transfer sequence) some place in the protein
  16. What is the halt transfer signal (stop-transfer sequence)?
    a sequence of 20-30 hydrophobic amino acids followed by charged amino acids
  17. After the __ has entered the membrane, the charged amino acids behind it cannot follow because of their __. Translation is completed at the __.
    • hydrophobic segment
    • charge
    • cytosolic surface
  18. The location of the __ determines what?
    • halt transfer signal (stop-transfer sequence)
    • how much of the protein goes through the membrane and how much does not
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Exam III. 3 (Signal Hypothesis)
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