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.
translating
mRNA
signal sequence (signal peptide)
absent
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)
ribosome
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?
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
Affinity of the __ for ribosomes translating __ is low, so it does not __.
SRP
non-secretory proteins
bind
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
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
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
translocated
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
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
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
charge
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
