Lecture 18 - The Endomembrane System

Question 1

how can we think of the endoplasmic reticulum?

Answer 1

reticulum: network of membrane tubes plus nuclear envelope

closed membrane: contents separate from the cytoplasm

Question 2

ER Structure:

Answer 2

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

Question 3

what is the RER important for?

Answer 3

the secretory pathway and protein biosynthesis - folding and regulating misfolded proteins and responding to cellular stress

Question 4

what do cell types that require lots of protein manufacturing contain?

Answer 4

necessarily high in RER

Question 5

main functions of SER:

Answer 5

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

Question 6

most membrane functions are carried out by…

Answer 6

… proteins!

Question 7

function of the lumen of the endoplasmic reticulum:

Answer 7

the lumen is where the biochemical reactions occur and the modification processes required for proteins

Question 8

in hepatocytes, the soft endoplasmic reticulum contains:

Answer 8

large amounts of cytochrome P450 and participates in the detoxification of metabolic waste products, drugs & alcohol

Question 9

in myocytes the soft endoplasmic reticulum is known as:

Answer 9

the sarcoplasmic reticulum and stores plus releases calcium ions needed for muscle contraction

Question 10

what are the stages in the ‘secretory pathway’:

Answer 10

nuclear envelope —> endoplasmic reticulum —> golgi apparatus —> transport vesicles / early then late endoscopes which turn into lysosomes

Question 11

proteins destined for the golgi apparatus, endosomes, lysosomes and proteins for the cell surface all first enter:

Answer 11

the endoplasmic reticulum

Question 12

how are ribosomes targeted to the endoplasmic reticulum?

Answer 12

ribosomes are targeted to the endoplasmic reticulum by a ‘signal sequence’

Question 13

what does the destination depend on when targeting ribosomes to the endoplasmic reticulum?

Answer 13

the destination depends on which protein is being translated

Question 14

what is a signal sequence?

Answer 14

a signal sequence is like a post-code - sending polypeptides to a specific cellular destination

Question 15

what do signal sequences specifically consist of?

Answer 15

signal sequences consist of short polypeptide sequences

Question 16

N-terminal signal sequences are ___ and consist of:

Answer 16

required and sufficient for cellular destination determination

consist of a short region of hydrophobic amino acids at the N-terminus

Question 17

what forms the ‘translocon’?

Answer 17

the Sec61 complex (eukaryotes) or the Sec YEG complex in prokaryotes

Question 18

translocon structure:

Answer 18

the translocon strucutre has transmembrane domains that span the ER membrane

hourglass channel with the ring consisting of 6 bulky amino acid residues which prevent ion leakage

lumenal side of the closed channel is occupied by a short helix called the plug domain

Question 19

what binds to the signal sequence and the ribosomes?

Answer 19

the Signal Recogniition Particle (SRP) binds to the signal sequence and the ribosome

Question 20

events that occur as the Signal Recognition Particle (SRP) bind to the ribosome and signal sequence:

Answer 20

the SRP particle binds to the signal sequence and the ribosome - this stops translation

the SRP then binds to the SRP receptor in the ER membrane which recruits the translocation channel - allowing translation to continue

Question 21

how are endoplasmic reticulum lumenal proteins translocated?

Answer 21

ER lumenal proteins are co-translationally translocated: soluble proteins are translocated across the membrane and released into the ER lumen

Question 22

how are signal sequences cleaved from he protein?

Answer 22

signal sequences are cleaved from the protein by signal peptidase

Question 23

what form of network is present in the ER?

Answer 23

a reticular network where the nuclear envelope is continuous with the ER

Question 24

rough ER membranes (________) have ________ along them to enable __-_________ ________.

Answer 24

(1) cisternae

(2) ribosomes

(3) co-translational translocation

Question 25

N-terminal sequences determine:

Answer 25

the targeting of proteins to the RER and subsequent translocation into the RER

Question 26

co-translational translocation requires: (2)

Answer 26

SRP (signal recognition particle) and the Sec61 translocon

Question 27

post-translational translocation requires: (3)

Answer 27

(1) Hsp70 (Ydj1)

(2) Sec61

(3) luminal chaperones to ratchet soluble proteins

Question 28

what sort of sequences help the transfer process across the membrane?

Answer 28

start-transfer N-terminal signal sequence starts transfer into the ER, then a second and longer hydrophobic sequence stops the transfer - the rest of the protein is then synthesised in the cytoplasm

Question 29

When ER destined proteins have a signal sequence and two internal hydrophobic regions the protein will:

Answer 29

start translocation at N terminus (and will be cleaved so that N is in the ER lumen). This will leave a loop of protein in the cytoplasm and both N and C terminus will be in the ER lumen.