Flashcards in protein sorting 3 Deck (59):
What are the major class of proteins synthesized in association with the rER?
Hydrophilic soluble proteins
integral membrane proteins
What organelles are the hydrophilic soluble proteins associated with?
they are destined for
they are completely translocated across the ER membrane into the ER lumen
What is the destination of the integral membrane proteins?
they contain large hydrophobic regions and remain trapped within the ER membrane
they function in the membrane of the ER, Golgi,
Is the mechanism similar for integral membrane spanning proteins and hydrophilic soluble proteins?
What tells the proteins to go to the ER?
By its signal sequence
Which ribosomes does the co translational , post translational and non translational proteins use?
they all use the same free floating ribosomes in the cytoplasm for translation
Where is the signal sequence for proteins bound for the ER?
On the N terminus
What is the signal sequence also called on the proteins bound for the ER?
Where is the signal sequence for integral proteins bound for the ER?
usually further back and not on the N terminus
What does ER localization signals contain?
long stretches of hydrophobic amino acids that are trapped in the ER membrane during trans location
What recognizes the hydrophobic signal peptide as it emerges from the ribosome?
the signal recognition particle (SRP)
What type of protein is the SRP?
a riboprotein (protein + RNA complex)
What is the SRP compose of?
six protein subunits and a strand of RNA
it contain two major domains
What are the two major domains of the SRP?
signal pocket - (recognizes signal sequence)
translational pause domain - (temporarily halts protein synthesis)
Where are the SRP and its receptor protein found?
In all cells, including prokaryotes
this suggest it arose early in evolution because it is highly conserved
Where does the SRP binds to on the ribosome and the proteins?
at the junction of the small and large submits and the signal sequence
it temporarily stops translation using the translational pause domain
Why does the SRP halts translation?
prevents incorrect folding of new polypeptide before it gets to the ER
gives the SRP-ribisome complex to associate with right SRP receptor on the ER
After the SRP has bound to the signal sequence and ribosome, where doe it go?
It forms a complex with:
SRP bound to signal sequence and ribosome and the mid translated mRNA and the SRP receptor on the ER and the translocator located on the ER
What type of proteins is the translocator?
a protein complex
What does the Sec61 protein have that blocks the pore?
it has a short helical segment that blocks the pore of an inactive translocator
Why is the translocator blocked?
So it wouldn't allow the diffusion of ions/molecules between the cytoplasm and the ER
When is the SRP is released from the ribosome -SRP complex?
after it is bound to the ER SRP receptor
and inhibition of protein synthesis stops
What is used to release the SRP from the SRP-ribosome complex?
GTP is hydrolyzed via a GTP-binding protein
After the SRP has disassociated from the complex what happens to the protein?
The N terminus of the signal peptide entrs the lumen of the translocator and binds to the translocator
What are GTP-binding proteins always bound?
GTP or GDP
the activity or affinity of the GTP-binding protein depending what type of guanine it is binded to
When does protein synthesis resumed?
after the SRP releases from the complex
the protein is further synthesized through the membrane
When and what cleaves the signal sequence?
a signal peptidase that is associated with the lumenal ER membrane cleaves off the signal sequence sometime during translation
the signal sequence is then released into the ER membrane
What is the signal peptidase of the ER lumen closely associated with?
the Sec61 translocator channel
How doe the Sec61 translocator releases the signal sequence?
it opens sideways and releases the signal sequence into the membrane
the signal sequence is then degraded and the AAs are reused
What happens to the newly synthesized protein?
it ten folds with the help of chaperone proteins inside the ER lumen
What is the difference in the mechanism for integral membrane proteins?
same sequence but the proteins bound for the membrane uses stop and start transfer signal sequences
How are proteins that span the membrane one time entered into the ER membrane?
1)has a signal sequence that binds to the translocator channel
2)when the protein is long enough the signal sequence it is moved to the membrane and cleaved off
3)protein synthesis continues
4)translocation is stopped by a stop transfer sequences
What is the stop transfer sequence?
hydrophobic set of amino acids separate from ER localization signal
it forms a helix that anchors the protein into the ER membrane
When is the stop transfer sequence is released fro the translocator?
it is released by the translocator into the membrane and prevents the rest of the AA from going into the ER
Which end of the protein remains in the cytoplasm?
the C terminus
Which part of the new ER protein is the membrane spanning portion of the protein?
the stop transfer sequence
Name the sequence of the signal sequence on the N terminus of a single pass protein?
1)Signal sequence on N terminus
2)signal sequence cleaved off
3)new N terminus in ER lumen
4)C terminus in cytoplasm
Name the sequence of the signal sequence internal of the protein?
1) sorting signal is the start transfer sequence
2)more of the protein is synthesized before signal sequence is produce ( still recognized y the SRP)
3) sorting signal is not cleaved off (stays part of the protein)
4)internal start transfer sequence binds to Sec61 in the N or C terminus exposed to te cytoplasm
Where is the signal peptide if the protein crosses the membrane several times?
embedded further within the AA sequence
it is not cleaved off
Proteins that span the membrane several times have?
pairs of stop and start sequences throughout the protein that dictates where the protein is going to span the membrane
What does Sec61 does to a start or stop sequence every time it encounters it?
it releases it into the membrane
What happens if a start sequence is not at the N terminus?
some of the protein stays in the cytoplasm
What is a common modification of proteins moving in the endomembrane system ?
this is adding a sugar to the protein
Where does glycosylation takes place?
in the ER/Golgi complex
Which proteins become glycosylated?
nearly all cells synthesized in association of the rER
cytoplasmic proteins have only a simple sugar attached
On what amino acids carbohydrates chains are added to?
Why when adding sugar to asparagine is it called N-linked oligosacchariddes?
because asparagine has a terminal N group that the sugar covalently linked to
What is the cosensus sequence that asparagine is added to that is recognized for glycosylation?
X represents any AA except proline
What are the most common sugars for glycosylation?
Where doe initial glycosylation takes place?
in the rER
How are the sugars added?
as a single oligosaccharide block of 14 sugars
Where is the oligosaccharide synthesized?
on the lipid carrier dolichol di-phosphate embedded in the ER membrane
What is dolichol?
a special type of very long, very hydrophobic lipid
How long is dolichol?
so long it spans the ER membrane three times firmly holding it in place
on which side of the ER is dolichol is phosphorylated?
it is phoshorylated on the cytoplasmic side of the ER and then flipped
14 sugars are then built on this lipid
Where is the oligosaccharide to from dolichol diphosphate?
it is transferred to a NH2 on the side chain of asparagine
What catalyzes the transfer of the 14 carb to the asparagine?
it is active on the luminal surface of the ER
What other modifications happen in the ER?
modifying the oligisaccharide
adding a GPI anchor on the c terminus
improperly folded proteins are recognized and brokendown in the cytoplasm