Intracellular proteine Trafficano lecture 2 (ER Translocation)

Question 1

Translocation

Answer 1

referes to a protein crossing a membrane

Question 2

what is glycosylation

Answer 2

refers to the addition of sugars to a protein or a lipid

Question 3

receptors are channels made of

Answer 3

proteins except the SRP

Question 4

A typical ER signal sequence

Answer 4

has one or more positively charged amino acids followed by a stretch of 6-12 hydrophobic amino acids

Question 5

ER signal sequences are always on the end terminus

Answer 5

true, its always on the n terminus

Question 6

steps in ER translocation

Answer 6

1. signal sequence on the n terminus binds to SRP (soluble), signal recognition particle, as soon it is exposed.
   This pauses translation.
2. SRP-bound ribosome attaches to SRP receptor in ER membrane
   3.Transaltion continues and translocation begins
3. SRP and and SRP receptor displaced and recycled
   5.The protein is directed(transferred) through the sec61 complex protein (channel) into the ER lumen. Sec61 complex is the protein translocator.

Question 7

what is SRP made up of

Answer 7

6 protein and 1 small RNA molecule

Question 8

GTP hydrolysis powers..

Answer 8

assembly of the nascent chain/translocon complex and release of the SRP and the SRP receptor

Question 9

ER import can be..

Answer 9

co-translational or post-translational

Question 10

post-translational ER import involves.. make sure you know it exists, rest of flashcards are abound co-transaltional er import

Answer 10

Sec 62,63,71,72 complex (receptor)
Sec 61 complex (protein channel)
hydrolysis of ATP
Bip (HSP 70 protein) which binds to the peptide and pulls into the ER lumen
similar to mitochondrial import (the protein is kept unfolded after its translated)

Question 11

the signal sequence is cleaved off by

Answer 11

signal peptidase.. the signal sequence shifts along and remains bound to the membrane as its hydrophobic

Question 12

Translocation of membrane protein the ER membrane

Answer 12

stop and start transfer sequences are hydrophobic regions

Question 13

rhodopsin has..

Answer 13

7 transmembrane spanning domains

Question 14

N-linked glycosylation occurs..

Answer 14

in the ER by oligosaccharyl transferase. The sugar is attached to N= asparagine residue. N-X-S or N-X-T

Question 15

what does gylycosylation promote

Answer 15

protein folding, can confer stability in proteins and plays a role in cell-cell adhesion

Question 16

Inside the ER,

Answer 16

polypeptides are glycosylated and and helped to fold by chaperone proteins

Question 17

what do chaperons do ?

Answer 17

help proteins to fold correctly, and mark incorrectly folded proteins for degradation

Question 18

where do disulphide bonds form

Answer 18

in the ER due to its oxidative environment. Disulphide bonds (form between cysteine residues) help stabilise the tertiary and quaternary structure of proteins.

Question 19

proteins need to taken out of the ER for degradation

Answer 19

true. it is deglycosalated. ubiquitinated. then it is degraded in the cytosolic proteasome in a process known as ER associated degradation (ERAD)

Question 20

methods to study protein translocation across membranes

Answer 20

1. transfection approach. tag gene of interest with GFP and see where the protein goes.
2. bio chemical approach. does a protein co-purify with a particular organelle.
3. genetic approach. Yeast as a model organism

Question 21

signals direct proteins to the correct place..

Answer 21

these signals can be amino acid sequences: signal sequences or signal patches
or protein modifications: glycosylation, ubuiqutination, liidi modifications

Question 22

How is a protein transported from one membrane bound compartment to another

Answer 22

1. Vesicular transport (ER to golgi transport)

2. direct fusion of compartments to form a hybrid organelle. (late endosome fuses with lysosome)

Question 23

how does vesicle form from an organelle membrane ? how does it select its cargo proteins ?

Answer 23

many transport vesicles appear to have electron dense coats. they have a structural and selective role. they select and thus concentrate the proteins that need to be transported in the vesicle.

Question 24

what controls coat recruitment ?

Answer 24

The GTPases Sar1 and ARF. they also control disassembly of COP1 and COP2 coated vesicles.
uncoating of clathrin-coated vesicles requires an Hsp70 family atpase. the vesicle has to uncoated before it can fuse with its target membrane.