7.4-7.14

Question 1

SRP

Answer 1

ribonucleoprotein particle

Question 2

composition of SRP

Answer 2

• ribonucleoprotein particle - made of 6 polypeptides and small 7S RNA molecule

Question 3

part of SRP has ____________ activity

Answer 3

GTPase

Question 4

true or false: SRP is a G protein

Answer 4

true

Question 5

the part of SRP that binds to the nascent polypeptide’s signal sequence has a large number of what?

Answer 5

Met residues (hydrophobic)

Question 6

part of SRP binds to where?

Answer 6

the ribosome

Question 7

when does SRP slow tsl until

Answer 7

docking at the ER

Question 8

why does the SRP slow tsl

Answer 8

to prevent outpacing available machinery

Question 9

SRP receptor (SR) composition

Answer 9

dimer of two subunits

Question 10

2 subunits of SR

Answer 10

• SRa (c-face, interacts with SRP)
• SRB (transmembrane)

Question 11

SRa and SRB are _____-binding proteins

Answer 11

GTP (GTPases)

Question 12

what is required by SRP and SR for proper targeting of nascent chains to the ER and for transfer to the translocation channel and recycling of SRP to the cytosol?

Answer 12

coordinated GTP binding and hydrolysis

Question 13

when SRP releases the ribosome, what happens?

Answer 13

ribosome engages the translocon and nascent polypeptide begins translocation

Question 14

transport of the polypeptide into the ER lumen is through what?

Answer 14

an aqueous channel

Question 15

translocon

Answer 15

comprised of the channel + other proteins closely associated with it

Question 16

true or false: ions can cross the membrane while a polypeptide is being translocated through the channel

Answer 16

false

Question 17

after the polypeptide is released and the ribosome is still attached, can ions permeate through the channel?

Answer 17

yes

Question 18

Sec61

Answer 18

part of the translocon that actually forms the channel through which the translocating protein passes

Question 19

composition of Sec61

Answer 19

heterotrimeric complex shaped like an hourglass in a cross section
- central pore occluded by small plug

Question 20

what commits the chain to translocation?

Answer 20

recognition and insertion of signal sequence

Question 21

what is probably displaced as translocation begins?

Answer 21

channel plug

Question 22

how are some loops formed during translocation

Answer 22

some proteins able to transiently slip out of the gap between the ribosome and the translocon to create a loop

Question 23

post-translational translocation

Answer 23

tsl proteins completely in the cytosol, keep them unfolded, and then translocate them into the ER

Question 24

what is post-translational translocation independent of?

Answer 24

SRP or SR - use other Sec proteins instead

Question 25

true or false: the protein is still associated with the ribosome during post-translational translocation

Answer 25

false

Question 26

what prevent folding of protein during post-translational translocation

Answer 26

chaperoning of the hsp70 family

Question 27

what do all chaperonins require

Answer 27

ATP

Question 28

what is the channel for post-translational translocation

Answer 28

Sec61

Question 29

what is most important for recognition of a signal sequence by a channel

Answer 29

hydrophobicity of the signal

Question 30

what seems to be the main energy source driving post-translational translocation?

Answer 30

ATP hydrolysis by the ER-lumenal hsp70 BiP or SecA

Question 31

role of BiP in post-translational translocation

Answer 31

ATP hydrolysis causes conformational change in BiP, causing polypeptide to be actively pulled through the channel (euks)

Question 32

role of SecA in post-translational translocation

Answer 32

ATP hydrolysis also causes conformational change, and pushes protein through (prokaryotes)

Question 33

how do some proteins become integrated in the membrane

Answer 33

hydrophobic anchor sequences recognized as they emerge from the ribosome, then are transferred out of the translocon into the bilyaer

Question 34

true or false: a signal/anchor sequence can only be used for recognition

Answer 34

false, serve as dual-purpose signal and anchor sequences

Question 35

when is the N-terminal signal likely cleaved

Answer 35

shortly after the beginning of translocation

Question 36

how do signal anchor proteins target?

Answer 36

by using an internal transmembrane domain (the signal anchor)

Question 37

2 orientations of proteins being translocated

Answer 37

1. N-terminal translocated into ER lumen 2. C-terminus translocated into ER lumen

Question 38

what dictates whether the N or C terminus will enter the ER lumen first

Answer 38

distribution of charged residues on either side of the transmembrane domain

Question 39

polytopic proteins

Answer 39

proteins that span the membrane multiple times

Question 40

how are polytonic proteins integrated

Answer 40

one at-a-time or in pairs

Question 41

if the ER-signal is _____________, it is almost always cleaved after it has served its purpose

Answer 41

N-terminal

Question 42

signal peptidase complex composition

Answer 42

5 subunits: 2 proteolytic, 3 regulatory

Question 43

what is the N-terminal cleavage site influenced by

Answer 43

aa residues in immediate vicinity

Question 44

what is the signal peptide processed by after its removal?

Answer 44

signal peptide peptidase - cuts it into pieces