intracellular protein trafficking

Question 1

what is protein targeting/sorting

Answer 1

mechanism by which proteins are transported to their appropriate destinations in the cell or outside it

Question 2

when does the first step of protein sorting take place

Answer 2

while translation is still taking place

Question 3

where are proteins destined for the ER, Golgi apparatus, lysosomes, plasma membrane synthesized on

Answer 3

ribosomes that are bound to the ER membrane

Question 4

where are proteins destined for the nucleus, mitochondria, chloroplast and peroxisome synthesized on

Answer 4

free (non-bound) ribosomes

Question 5

when can proteins be translated to the ER (2)

Answer 5

1. during their synthesis on membrane-bound ribosomes (co-translational translocation)
2. After their translation have been completed on free ribosomes in the cytosol (post-translational translocation)

Question 6

what is the first step of the Co-translational pathway

Answer 6

association of ribosomes with ER

Question 7

what is the specific aa (amino acid) sequence that target ribosomes for binding to the ER membrane

Answer 7

signal sequence at the amino (N) terminus of polypeptide

Question 8

what removes the signal sequences

Answer 8

proteolytic cleavage.

Question 9

what binds to the ribosome and signal sequence, prevents further translation and targets the complex to the ER

Answer 9

signal recognition particle (SRP)

Question 10

what does signal recognition particle (SRP) contain

Answer 10

six polypeptides and a small cytoplasmic RNA (srpRNA)

Question 11

after translation inhibition, what does the complex bind to 1

Answer 11

SRP receptor on the ER membrane

Question 12

after the SRP is released what does the ribosome bind to

Answer 12

translocon

Question 13

what is the translocon a complex of

Answer 13

three transmembrane proteins called Sec61

Question 14

what does cleavage of the signal sequence by signal peptidase release

Answer 14

polypeptide into the lumen of the ER

Question 15

what maintains the unfolded conformation of the proteins

Answer 15

cytosolic chaperones

Question 16

during post-translational import, what recognizes the signal sequences

Answer 16

Sec62/63 complex

Question 17

what does Sec63 protein associate with

Answer 17

chaperone protein (BiP)

Question 18

what is the function of chaperone protein (BiP)

Answer 18

acts as a molecular ratchet to drive protein translocation into the ER

Question 19

what happens to proteins destined for secretion from the cell or residence within the lumen of ER, Golgi, or lysosome

Answer 19

(i) released into the lumen of ER

(ii) translocated across the ER membrane

Question 20

what are the two ways in which insertion occurs

Answer 20

(1) with a cleavable signal sequence and a single stop-transfer sequence
(2) with an internal non-cleavable signal sequence

Question 21

what are the N and C terminus of membrane proteins

Answer 21

amino (N) or carboxy (C) terminus

Question 22

in the first type of insertion, what happens when the polypeptide chain crosses the membrane

Answer 22

signal sequence is cleaved and amino terminus of the polypeptide chain is exposed in the ER lumen

Question 23

what halts translocation in the ER membrane, for the first type of insertion

Answer 23

stop transfer sequence

Question 24

in the second type of insertion what does the signal sequence act as

Answer 24

anchors the protein in the membrane with its C terminus or N terminus in the ER lumen

Question 25

what determines whether C-terminus or the N-terminus is in the cytosol

Answer 25

orientation of the polypeptide chain

Question 26

how does Insertion of a protein that spans the membrane multiple times occur

Answer 26

after the signal sequence by repeatedly stopping the sequence

Question 27

ER is the site of

Answer 27

``` protein folding assembly of multisubunit proteins, disulfide bond formation, the initial stages of glycosylation, the addition of glycolipid anchors to some plasma membrane proteins ```

Question 28

what assists in the 3D folding of polypeptides in the ER

Answer 28

Hsp70 chaperone (BiP)

Question 29

what is the function of a) cytosolic chaperones b) Mitochondrial chaperones

Answer 29

a) stabilize the un-folded configuration | b) facilitate transport and subsequent folding of the polypeptide chain within the organelle

Question 30

what happens to proteins that travel along the secretory | pathway in transport vesicles

Answer 30

they bud from the membrane of one organelle and then fuse with the membrane of another

Question 31

what are transmembrane proteins that enter the transitional ER marked with

Answer 31

sequences that signal either their export from or retention within the ER

Question 32

what are the export signal sequences

Answer 32

* di-acidic (eg., Asp-Asp or Glu-Glu) | * Di-hydrophobic (eg., Met-Met)

Question 33

what are proteins destined to remain in the ER lumen marked by

Answer 33

sequence Lys-Asp-Glu-Leu (KDEL). they exported from ER to Golgi, and are recognized by a receptor in the ERGIC and are transported back to the ER

Question 34

where does protein modification occur in the Golgi

Answer 34

at its medial and trans Golgi stacks

Question 35

what are proteins transported through after being processed in the Golgi

Answer 35

through the secretory pathway which involves aorting of proteins into different kinds of transport vesicles which bud from the trans Golgi network and deliver their contents to appropriate locations

Question 36

what are the types of coated vesicles

Answer 36

COP-coated vesicles (COP1 and COP II) | Clathrin-coated vesicles

Question 37

what vesicles carry secretory proteins from the ERGI or Golgi, budding from the transitional ER and carrying their cargo forward along the secretory pathway

Answer 37

COPII coated vesicles

Question 38

describe the function of COP1 coated vesicles

Answer 38

bud from the ERGIS or Golgi. It is the coat protein on vesicles moving from one Golgi cisternae to another during secretion and on the retrieval vesicles that return resident ER proteins marked with KDEL signals

Question 39

what are Clathrin coated vesicles responsible for

Answer 39

uptake of extracellular molecules from the plasma membrane by endocytosis and transport of molecules from the trans Golgi network to endosomes, lysosomes or the plasma membrane