1.10: Protein Sorting

Question 1

proteins are sorted to the er by what transport

Answer 1

transmembrane transport

Question 2

after proteins make it to the er, how can they be further sorted

Answer 2

by vesicular transport to other compartments or to the cell surface

Question 3

t/f protein orientation during transport will stay consistent (no flip flop of termini)

Answer 3

true (eg if it starts in lumen then it’ll always stay in lumen)

Question 4

what are major functions of the er

Answer 4

synthesis and modifications of proteins, synthesis of lipids

Question 5

what kinds of protein are sorted to the er

Answer 5

soluble proteins, transmembrane proteins
proteins destined for: golgi, secretion, lysosomes

Question 6

er signal sequences are encoded for by the ________

Answer 6

genome

Question 7

describe the (broad) steps of protein sorting to the er

Answer 7

1. mRNA + ribosomes
2. translation starts - er signal seq emerges first
3. ribosomes directed to er membrane
4. co-translational translocation

Question 8

what is the er signal seq made up of

Answer 8

hydrophobic aa at n-terminus

Question 9

state the substance that directs the partially translated protein to the er

Answer 9

srp (signal recognition particle)

Question 10

srp and srp receptors have which domain (ATP or GTP)

Answer 10

gtpase domain

Question 11

srp + ribosome = ____ affinity
srp + ribosome + er signal seq = ____ affinity

Answer 11

srp + ribosome = low affinity
srp + ribosome + er signal seq = high affinity

Question 12

which components are necessary to bind to the srp receptor

Answer 12

srp + ribosome + er

Question 13

binding of srp causes translation to _______

Answer 13

pause

Question 14

from the ribosome it goes to the ___________ channel

Answer 14

protein translocator channel

Question 15

is the protein translocator channel/translocon a gated or non gated channel

Answer 15

gated

Question 16

in order, describe how from the cytosol, a protein is directed by srp into the er

Answer 16

1. ribosome forms a tight seal w the translocator which prevents diffusion of ions and small molecules
2. srp + srp receptor causes gtp hydrolysis and the complex dissociates
3. srp released and recycled while the translation of the protein continues and translocation begins

Question 17

describe protein sorting to the er (soluble proteins)

Answer 17

• 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

Question 18

how many different types of insertions are there for single pass transmembrane proteins

Answer 18

3 types

Question 19

describe protein sorting into the er of transmembrane proteins (single pass (stop transfer seq))

Answer 19

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

Question 20

describe protein sorting into the er of transmembrane proteins (single pass - without stop transfer seq)

Answer 20

• 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

Question 21

what is the diff between the single pass tm proteins regarding the start transfer seq and the tm domain and orientation

Answer 21

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

Question 22

describe both example of multipass tm proteins (ex 1 doesn’t have a name but ex 2 is rhodopsin)

Answer 22

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)

Question 23

er targeting seq (n-terminal, internal) and stopt transfer seq are specific ___________ __________

Answer 23

hydrophobic seq - these are prediced by stretches of hydrophobic aa

Question 24

describe the formation of glycosylphosphadtidylinositol (gpi) anchored proteins

Answer 24

• 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

Question 25

which of the following bind to the er signal seq?
a. ribosome
b. srp
c. translocator protein
d. b and c

Answer 25

d