Unit 3 (post-test 1) Flashcards
what are more than half of animal membranes associated with
Over half are associated with the ER
where in the cell is the ER?
Extends enough such that all regions of the cytosol are close to at least one part of its membrane.
what do we call the lumen of the ER?
ER cisternal space
does the ER make lipids?
yes
where is Ca+ stored in the cell?
in the ER
What kind of proteins are initially sent to the ER?
Proteins that are
-on the outside of the cell
-resident to ER lumen
-resident to golgi
-resident to lysosomes
-resident to other endomembrane system compartments
Explain the difference between the rER and the sER
rER has ribsomes. It is where protein comes in and is made.
Smooth ER is where vesicles bud off (this part is called “transitional ER” It also makes lipids and stores calcium.
What kind of cell would have more rER? What kind of cell would have more sER? Which of the two ERs is more common to have more of?
More rER: cells that secrete a lot of protein
More sER: cells that do lots of lipid metabolism ex:hypatocytes which need lipoproteins OR that use Ca+2 ex: muscle cells that use it for contraction/relaxation
how do we isolate the ER and its two parts?
Homogenize cells to break the ER into small pieces
Due to their membrane, these pieces will autoreseal into closed vesicles called microsomes
Use gradient centrifugation to isolate (uses sugar gradient that causes things to move to their level of equal density after centrifugation)
The rough ER will isolate itself bc its heavy
the smooth ER will remain with the other organelles (golgi, endosome and mitochondria)
how did we learn about the ER signal sequence?
Microsomes act like mini ERs for cell-free experiments
In an experiment done with no microsomes an ER protein was found to be bigger than when it is normally secreted by a cell
This is bc the N-terminal leader peptide gets cut off when its in a cell
We thus hypothesis that this peptide is a signal sequence that directs to the ER membrane and is then cleaved via signal peptidase after translation.
what does SRP stand for?
Signal-Recognition Particle
What is the SRP
complex of 6 diff proteins + a small RNA that acts as a receptor for signal peptide.
Where can we find SRP?
It cycles between the cytosol and the ER membrane where it binds the SRP receptor
How can SRP bind multiple different signal shapes/sizes?
It has a flexible and large hydrophobic pocket of Met (flexible/unbranched a.a.) residues
What is a signal peptide?
A central stretch of 8-9 hydrophobic a.a.
what role does the SRP play in protein import?
It has a pause domain to ensure ER-destined proteins are never fully transcribed in the cytosol (could be bad if its a hydrolase)
It prevents misfolding
It means we don’t need chaperones to keep the protein unfolded
in what order does SRP binding occur?
SRP binds protein signal sequence
SRP binds SRP receptor
Everything (including the ribosome) is brought to the ER membrane protein translocator
SRP and SRP receptor released
Translocator does its job
*works with a polyribosome too
*once translation is done, the ribosome leaves to be free but the mRNA stays to be used by an ever-changing ribosome population
explain how the ER signal sequence is used twice
Firstly it is used to bind the SRP then, it is used to bind the translocator to open its pore.
which form of transport requires energy in the ER? Co-translational or post-translational
Post-translational
Explain what the ER protein translocator is
A protein made of 4 sec61s (the Sec61 complex) and its associated complexs. It has a small pore created via alpha helices and is gated by a short alpha helix such that it is closed when it rests. The pore allows unfolded soluble proteins through and a side opening to allow proteins into the membrane.
how does post-translational transport happen in the ER
Sec 62 and SEc 63 are transmembrane proteins on the lumenal side. They bring BiP chaperone proteins (which have a high affinity for unfolded proteins) onto the polypeptide chain. Bip then uses ATP to bind and unbind the polypetide as it comes out. This prevents it from slipping back in and promotes FWD motion so that more Bips can bind.
Is post- or co-translational transport more common in the ER?
Co-transport is.