Lecture 10- Quality control and the glycocode

Question 1

bulk flow model of export

Answer 1

idea that the default position for molecules is to be exported from the ER, and signals instead indicate residency

Question 2

other hypothesis that was rejected about export

Answer 2

signal-mediated export hypothesis

Question 3

evidence for bulk flow

Answer 3

bacterial proteins targeted with SPs to the ER were exported, suggesting that this is what happens with no specific signal
ER abundance was found not to impact rate of export

Question 4

evidence for signal-mediated export

Answer 4

proteins are not exported according to their abundance in the ER, suggests export is not default

Question 5

bulk flow vs (refined) lodish theory

Answer 5

bulk flow- default is the fastest possible rate, there are no export signals and other factors impact how fast or slow it is
lodish- default is the slowest possible rate, and there are export signals that help speed it up

Question 6

experimental evidence against lodish theory

Answer 6

tripeptide secretion happened quickly without the addition of any specific sequences, suggests signals arent entirely necessary

Question 7

example of residency signals

Answer 7

K/HDEL

Question 8

what does mediate different rates of export?

Answer 8

quality control processes, which lead to some proteins being retained for longer due to slower folding rates

Question 9

what is calnexin/calreticulin

Answer 9

an ER protein- calnexin is outer membrane version, calrectulin is cytoplasmic version- associates with high-mannose glycans on peptides, as part of the tagging process

Question 10

how does calnexin/calrectulin mediated transport work

Answer 10

-CLX/CRT binding to the glycose chain, specifically single glucose version
-this glucose is removed as the protein folds correctly
-this leads the protein to be released and secreted

Question 11

how is continued CLX/CRT association maintained if folding is incorrect

Answer 11

UGT enzyme adds glucose to unfolded proteins, locates hydrophobic residues and glycan core and associates with this to add glucose

Question 12

what does mannosidase do

Answer 12

trims Man9 to Man8, but very slowly- this creates a time limit for folding as Man8 has less association w CLX CRT

Question 13

pathway which degrades unfolded proteins

Answer 13

ERAD pathway- ER assisted degradation

Question 14

how does ERAD work

Answer 14

once Man7 is reached (given time to fold), association with specific lectins
unfolded proteins fed from the lectin to the Sec61 channel, ubiquitination and breakdown of the protein at the 26s proteasome

Question 15

mechanism of protein retention in the ER

Answer 15

extensive, but loose, interaction w other ER proteins via Ca2+ cross bridges, reducing their diffusion into vesicles- and COP1 transport acts as a failsafe to get proteins back to the ER if necessary

Question 16

exclusion model of protein maintenance

Answer 16

does still have the idea of more binding for resident proteins- cargo ends up where there are less of these proteins, as they dont partake in the bonding

Question 17

role of COPII in export

Answer 17

seems to recruit cargo, e.g. by signals in the coat

Question 18

sec24p receptor system

Answer 18

GDP-GTP exchange leads to recruitment of various Sec proteins, eventually the congregation of proteins forms a vesicle coat and the cytoplasmic tails of Sec24p bind cargo

Question 19

what was the closest original theory to being correct

Answer 19

lodish bulk flow idea