1.10: Protein Sorting Flashcards
proteins are sorted to the er by what transport
transmembrane transport
after proteins make it to the er, how can they be further sorted
by vesicular transport to other compartments or to the cell surface
t/f protein orientation during transport will stay consistent (no flip flop of termini)
true (eg if it starts in lumen then it’ll always stay in lumen)
what are major functions of the er
synthesis and modifications of proteins, synthesis of lipids
what kinds of protein are sorted to the er
soluble proteins, transmembrane proteins
proteins destined for: golgi, secretion, lysosomes
er signal sequences are encoded for by the ________
genome
describe the (broad) steps of protein sorting to the er
- mRNA + ribosomes
- translation starts - er signal seq emerges first
- ribosomes directed to er membrane
- co-translational translocation
what is the er signal seq made up of
hydrophobic aa at n-terminus
state the substance that directs the partially translated protein to the er
srp (signal recognition particle)
srp and srp receptors have which domain (ATP or GTP)
gtpase domain
srp + ribosome = ____ affinity
srp + ribosome + er signal seq = ____ affinity
srp + ribosome = low affinity
srp + ribosome + er signal seq = high affinity
which components are necessary to bind to the srp receptor
srp + ribosome + er
binding of srp causes translation to _______
pause
from the ribosome it goes to the ___________ channel
protein translocator channel
is the protein translocator channel/translocon a gated or non gated channel
gated
in order, describe how from the cytosol, a protein is directed by srp into the er
- ribosome forms a tight seal w the translocator which prevents diffusion of ions and small molecules
- srp + srp receptor causes gtp hydrolysis and the complex dissociates
- srp released and recycled while the translation of the protein continues and translocation begins
describe protein sorting to the er (soluble proteins)
- er signal seq is an n terminal start transfer seq that is bound to the translocator
- signal peptidase cleaves the er signal seq
- the er signal seq laterally diffuses into the lipid bilayer (the translocator is gated in a 2nd direction)
- translocated protein is released into the er
how many different types of insertions are there for single pass transmembrane proteins
3 types
describe protein sorting into the er of transmembrane proteins (single pass (stop transfer seq))
er signal seq: (nh2) start-transfer
- tm domain is a stop transfer signal which laterally diffuses into lipid bilayer
- feed the protein into the translocator for cotranslational translocation to occur (translocator binds to signal seq)
- the stop transfer seq makes the protein stop moving through
- signal peptidase cuts off signal
- protein synthesis continues in the cytosol == COOH in cytosol
describe protein sorting into the er of transmembrane proteins (single pass - without stop transfer seq)
- tm domain is an internal start-transfer seq and is NOT cleaved
- it laterally diffuses into lipid bilayer
- orientation is determined by aa flanking the internal start transfer seq (cytosolic side is more +)
- the protein translocator has charged aa on either side (it is more - on cytosolic side bc opps attract). if aa at n side more + then it faces cytosol and c side on er lumen. if aa at c side more + then that faces cytosol and n side faces er lumen
what is the diff between the single pass tm proteins regarding the start transfer seq and the tm domain and orientation
single pass 1: tm domain is stop transfer seq and start transfer seq gets cleaved, orientation stays the same
single pass 2 and 3: tm domain is internal start transfer seq and is not cleaved, orientation is determined by aa flanking the internal start transfer seq
describe both example of multipass tm proteins (ex 1 doesn’t have a name but ex 2 is rhodopsin)
1: 1st tm domain has internal start transfer seq ( + side is nh2 and faces cytosol, - side faces er lumen) and 2nd tm domain has stop == cooh and nh in cytosol
2: 1st tm is start (+aa, cytosolic), 2nd tm is start (we don’t need to know 3rd (stop) and 4th (start)) == nh2 in lumen and depending on how many tm then it’s same or diff side)
er targeting seq (n-terminal, internal) and stopt transfer seq are specific ___________ __________
hydrophobic seq - these are prediced by stretches of hydrophobic aa
describe the formation of glycosylphosphadtidylinositol (gpi) anchored proteins
- target protein has c terminal hydrophobic domain
- gpi anchor is preformed in the membrane
- er enzyme transfer protein to gpi anchor
- gpi anchored protein ends up on er luminal side and can go to cell exterior surface
which of the following bind to the er signal seq?
a. ribosome
b. srp
c. translocator protein
d. b and c
d
