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1: Synthesis of secretory, cell membrane, and lysosomal proteins begins on __.
detached or free ribosome that attach to a molecule of mRNA
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.
- signal sequence (signal peptide)
Explain the signal sequence
- 15-20 amino acids on either side
- located at the amino end of the polypeptide
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)
- halts translation
Describe the structure of the SRP.
composed of 6 protein subunits adn one small (7S molecule of RNA)
How the __ binds to the ribosome is unknown, but it may involve what?
- 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
Affinity of the __ for ribosomes translating __ is low, so it does not __.
- non-secretory proteins
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
- SRP receptor
- the inhibition of translation
- the ribosome makes contact with the membrane
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
- after the ribosome binds to the ER
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
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
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
- signal peptidase
the signal sequence is rapidly hydrolyzed by other proteases in the eR and the polypeptide chain is released into the ER lumen
True or False:
The signal sequence must always be removed.
- While the signal sequence from MOST polypeptide chains or proteins, it is not removed from all
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
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
What is the halt transfer signal (stop-transfer sequence)?
a sequence of 20-30 hydrophobic amino acids followed by charged amino acids
After the __ has entered the membrane, the charged amino acids behind it cannot follow because of their __. Translation is completed at the __.
- hydrophobic segment
- cytosolic surface
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