Lecture 18 - The Endomembrane System Flashcards
how can we think of the endoplasmic reticulum?
reticulum: network of membrane tubes plus nuclear envelope
closed membrane: contents separate from the cytoplasm
ER Structure:
- 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
what is the RER important for?
the secretory pathway and protein biosynthesis - folding and regulating misfolded proteins and responding to cellular stress
what do cell types that require lots of protein manufacturing contain?
necessarily high in RER
main functions of SER:
biosynthesis of phospholipids and cholesterol and the synthesis and repair of membranes
most membrane functions are carried out by…
… proteins!
function of the lumen of the endoplasmic reticulum:
the lumen is where the biochemical reactions occur and the modification processes required for proteins
in hepatocytes, the soft endoplasmic reticulum contains:
large amounts of cytochrome P450 and participates in the detoxification of metabolic waste products, drugs & alcohol
in myocytes the soft endoplasmic reticulum is known as:
the sarcoplasmic reticulum and stores plus releases calcium ions needed for muscle contraction
what are the stages in the ‘secretory pathway’:
nuclear envelope —> endoplasmic reticulum —> golgi apparatus —> transport vesicles / early then late endoscopes which turn into lysosomes
proteins destined for the golgi apparatus, endosomes, lysosomes and proteins for the cell surface all first enter:
the endoplasmic reticulum
how are ribosomes targeted to the endoplasmic reticulum?
ribosomes are targeted to the endoplasmic reticulum by a ‘signal sequence’
what does the destination depend on when targeting ribosomes to the endoplasmic reticulum?
the destination depends on which protein is being translated
what is a signal sequence?
a signal sequence is like a post-code - sending polypeptides to a specific cellular destination
what do signal sequences specifically consist of?
signal sequences consist of short polypeptide sequences
N-terminal signal sequences are ___ and consist of:
required and sufficient for cellular destination determination
consist of a short region of hydrophobic amino acids at the N-terminus
what forms the ‘translocon’?
the Sec61 complex (eukaryotes) or the Sec YEG complex in prokaryotes
translocon structure:
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
what binds to the signal sequence and the ribosomes?
the Signal Recogniition Particle (SRP) binds to the signal sequence and the ribosome
events that occur as the Signal Recognition Particle (SRP) bind to the ribosome and signal sequence:
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
how are endoplasmic reticulum lumenal proteins translocated?
ER lumenal proteins are co-translationally translocated: soluble proteins are translocated across the membrane and released into the ER lumen
how are signal sequences cleaved from he protein?
signal sequences are cleaved from the protein by signal peptidase
what form of network is present in the ER?
a reticular network where the nuclear envelope is continuous with the ER
rough ER membranes (________) have ________ along them to enable __-_________ ________.
(1) cisternae
(2) ribosomes
(3) co-translational translocation
N-terminal sequences determine:
the targeting of proteins to the RER and subsequent translocation into the RER
co-translational translocation requires: (2)
SRP (signal recognition particle) and the Sec61 translocon
post-translational translocation requires: (3)
(1) Hsp70 (Ydj1)
(2) Sec61
(3) luminal chaperones to ratchet soluble proteins
what sort of sequences help the transfer process across the membrane?
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
When ER destined proteins have a signal sequence and two internal hydrophobic regions the protein will:
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.
