The Endoplasmic Reticulum and Translocation

Question 1

Name the structure of membrane-enclosed organelle ?

Answer 1

• Mitochondrion
• Endosome
• Golgi Apparatus
• Lysosome
• ER reticulum with membrane-bound ribosomes
• Cytosol
• Peroxisome
• Nucleus
• Plasma Membrane
• Free ribosomes

Question 2

Membrane-enclosed organelles import proteins by one of three mechanisms

Answer 2

1. Transport through nuclear pores
2. Transport across membranes
3. Transport by vesicles

Question 3

Briefly explain the Endoplasmic Reticulum ?

Answer 3

• Reticulum: a network of membrane tubes
• Plus outer nuclear envelope
• Closed membrane: contents separate from cytoplasm

Question 4

Explain the ER structure ?

Answer 4

• The ER have ‘cisternae’ which are long, flattened, sac-like, unbranched tubules with a diameter of 40- 50 um.
• The fluid filled interior is generally referred to as the ER lumen but also as the cisternal space.
• The lumen is where the biochemical reactions occur and the modification processes required for proteins.
• On the rough ER (RER) ribosomes are dotted on the outside showing how closely linked this organelle is to protein synthesis

Question 5

The RER is important for the ?

Answer 5

Secretory pathway and protein biosynthesis – folding and regulating misfolded proteins and responding to cellular stresses

Question 6

Tissue types that require a lot of proteins to be manufactured and secreted contain cells that are ?

Answer 6

Necessarily high in RER

Question 7

The main functions of SER are?

Answer 7

The biosynthesis of phospholipids and cholesterol and the synthesis and repair of membranes

Question 8

In hepatocytes the SER contains large amounts of ?

Answer 8

Cytochrome P450 and participates in the detoxification of metabolic waste products, drugs, and alcohol

Question 9

In myocytes the SER is known as ?

Answer 9

The sarcoplasmic reticulum and stores and releases calcium ions needed for muscle contraction

Question 10

What is the Secretory Pathway also known as ?

Answer 10

The endomembrane system

Question 11

Proteins destined for the Golgi apparatus, endosomes, lysosomes, as well as proteins destined for the cell surface all first enter the ?

Answer 11

Endoplasmic Reticulum (ER)

Question 12

Explain the Rough Endoplasmic Reticulum ?

Answer 12

• Black dots are ribosomes
• They are making membrane and secreted proteins
• These proteins are translated and translocated at the same time

Question 13

ribosomes targeted to ER membrane by ?

Answer 13

A signal sequence

Question 14

N terminal signal sequences are required and sufficient for?

Answer 14

Cellular destination determination

Question 15

What forms the translocon ?

Answer 15

The Sec61 complex (eukaryotes) Sec YEG (prokaryotes)

Question 16

What forms the channel ?

Answer 16

The central Sec61α subunit (SecY); two smaller peripheral subunits are Sec61β and Sec61γ (SecE)

Question 17

What does the translocon structure have ?

Answer 17

Transmembrane domains that span the ER membrane

Question 18

Describe the inside of the channel ?

Answer 18

It is hourglass-shaped, with a ring at the center consisting of six bulky hydrophobic amino acid residues (the pore ring) which position their side chains to the center of the pore

Question 19

What does the ring prevent ?

Answer 19

Leakage of ions through the inactive channel and during translocation of a protein substrate

Question 20

What is the plug domain ?

Answer 20

The lumenal side of the closed channel is occupied by a short helix

Question 21

ER signal sequence contains?

Answer 21

A short region of hydrophobic amino acids, usually at N-terminus

Question 22

What does Signal Recognition Particle (SRP) do?

Answer 22

• Binds to the signal and the ribosome
• stops translation
• Then it binds the SRP Receptor in the ER membrane

Question 23

SRP receptor recruits the ?

Answer 23

Translocation channel and translation continues

Question 24

Soluble proteins are translocated across ?

Answer 24

The membrane and released into the ER lumen

Question 25

Signal sequence is cleaved from ?

Answer 25

The protein by signal peptidase

Question 26

Explain the start-transfer and stop-transfer sequences in single-pass transmembrane protein ?

Answer 26

• Single-pass transmembrane protein: an N-terminal signal sequence starts transfer into the ER
• A second, longer hydrophobic sequence (~20 a.a.s) stops the transfer
• The rest of the protein is synthesised in the cytoplasm

Question 27

Explain the start-transfer and stop-transfer sequences in multi-pass transmembrane protein ?

Answer 27

• Multi-pass membrane protein: internal hydrophobic sequence bound by SRP: starts translocation into the ER
• ..but not a substrate for signal peptidase
• Next hydrophobic sequence stops translocation
• The rest of the protein is synthesised in the cytoplasm

Question 28

N terminal signal sequences determine ?

Answer 28

Targeting of proteins to RER and subsequent translocation into RER

Question 29

Co-translational translocation requires ?

Answer 29

SRP and Sec61 translocon

Question 30

Post-translational translocation requires ?

Answer 30

Hsp70 (Ydj1) and additional proteins to Sec61 (Sec62, Sec63, Sec71 and Sec72) as well as luminal chaperones to ratchet in soluble proteins)