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
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
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
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
The __ is closed until the ribosome has bound, so that the __ of teh ER membrane is maintained at all times.
translocator
permeability barrier
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
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
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
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
What does the SRP bind to?
both the signal sequence and ribosome
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
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
True or False:
The complex bumping into the ER is completely directed.
False:
random
Usually as the protein binds to the translocon, what happens?
it activates it and opens it up (shape change)
In addition to SRP binding to receptor proteins, ribosomes bind to the __, allowing what?
translocon
ribosome to stay attached
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
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
Why won't the protein go back through the channel? What helps the protien fold?
too big
chaperones
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
True or False: In some cases, the signal sequence is still thre.
True
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
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
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
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
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
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
The __ stays outside. What goes through determines what?
what goes through the translocon
True or False:
You can only have a signal sequence at hte end.
