Lecture 10: Protein Sorting to the Endoplasmic Reticulum Flashcards
(23 cards)
How are proteins sorted to the ER
transmembrane transport
What are the two major functions of the ER
- synthesis and modifications of proteins
- synthesis of lipids
What proteins are sorted to the ER and what do they have in common
All have an ER signal sequence (KDEL)
Include:
- soluble proteins
- transmembrane proteins
- proteins destined for the golgi, for secretion, and for lysosomes
What is SRP
signal recognition particle that has a GTPase domain that binds GTP
What is SRP receptor
Signal recognition particle receptor; has Gtpase binding domeian
At what state is SRP low affinity
attached to a ribosome
At what state is SRP high affinity
attached to ribosome AND ER signal sequence; binds SRP receptor
What occurs when SRP binds to signal sequence
translation pauses until it has been transported to the ER (uses GTP)
What happens when ribosome with signal sequence and SRP reaches the ER surface
tight seal formed with translocator to prevent diffusion of ions and small molecules
What happens after ribosome and SRP receptor bind to translocator on surface of ER
GTP hydrolysis occurs, causing complex to dissociate and SRP to release
Describe the ER signal sequence
An N-terminal start-transfer sequence bound to the translocator that can be cleaved by a signal peptidase
What happens after ER signal sequence is cleaved
ER signal sequence laterally diffuses into the lipid bilayer and the translocater is gated in both directions
What are the three types of single-pass TM proteins
1) N-terminal start-transfer sequence & stop-transfer sequence in TM domain
2) internal start transfer sequence with N-terminus facing cytosol
3) internal start transfer sequence with N-terminus facing ER lumen
Describe single-pass 1
TM domain is the stop-transfer signal sequence
- protein synthesis continues in cytosol; so COOH must be in cytosol
Describe single-pass 2 and 3
more positive AA side faces the cytosol
TM domain is an internal start-transfer sequence and is not cleved
How does the protein translocator guide orientation of proteins
it itself is charged. Negative facing cytosol and positive towards lumen. Means that more negative side of protein goes first and more positive side faces the cytosol
What is case 1 of a multipass TM protein
1st TM domain= internal start-transfer sequence
2nd TM domain = stop-transfer sequence
starts like single-pass 3, with more positive side trying to enter first resulting in folding. only must stop before it fully unfolds.
What is case 2 of a multipass TM protein
1st TM domain = srtart-transfer sequence (where positive AAs are cytosolic)
2nd TM domain = stop-transfer sequence
**starts like single-pass 2, with negative side entering first, “properly/straight”, means that in order to fold you must instruct it to insert into the membrane a second time **
What is an example of case 2 multipass TM proteins
Rhodopsin
What are ER N-terminal, internal, and stop transfer sequences made of?
specific hydrophobic sequences predicted by stretches of hydrophobic amino acids
What are three types of membrane proteins
- integral (including transmembrane =, glycosylated on extracellular face)
- lipid anchored (lipid inserts into bilayer but AAs do not, this includes GPI anchored proteins)
- peripheral
Describe GPI-anchored protein formation
- target protein has a C-terminal hydrophobic domain that is a signal for the GPI anchor, which is pre-formed in the membrane
- ER enzyme transfers protein to GPI anchor
Where on the ER does a GPI anchored protein end up? where can it go?
ends up on ER luminal side and can travel to cell exterior surface with vesicle formation
