3.2/3 - Endomembrane System & Secretory Pathway Flashcards
cisternae
long, flattened, sac-like, unbranched tubules present in endoplasmic reticulum
ER lumen (or cisternal space)
fluid filled interior of endoplasmic reticulum
role of ER lumen
where the biochemical reactions occur and the modification processes required for proteins
importance of RER (2)
- secretory pathway
- protein biosynthesis (folding and regulating misfolded proteins and responding to cellular stresses)
type of tissue types that contain cells with lots of RER
tissue types that require lots of proteins to be manufactured and secreted
main function of SER (2)
- biosynthesis of phospholipids and cholesterol
- synthesis and repair of membranes
what do SER contain large amounts of in hepatocytes and why? (2)
- cytochrome p450
- participates in detoxification of metabolic waste products, drugs and alcohol
what is SER known as in myocytes and what is its role? (2)
- sarcoplasmic reticulum
- stores and releases calcium ions needed for muscle contraction
how are ribosomes targeted to ER membrane?
signal sequence
signal sequence
like a post code - sending polypeptides to specific cellular destinations
what forms the translocon in eukaryotes?
Sec61 complex
what forms the translocon in prokaryotes?
Sec YEG
peripheral subunits of eukaryotic translocon (2)
- Sec61B
- Sec61y (SecE)
translocon channel structure
- inside channel is hourglass-shaped
- ring at centre consisting of six bulky hydrophobic amino acid residues (pore ring)
translocon structure
transmembrane domains that span the ER membrane
role of translocon pore ring
prevents leakage of ions through inactive channel and during translocation of a protein substrate
plug domain
short helix that occupies lumenal side of closed channel (pore ring)
binding site for signal recognition particle (SRP)
ER signal sequence contains short region of hydrophobic amino acids (usually at N-terminus)
role of signal recognition particle (SRP) (2)
- binds to ER signal and ribosome, stopping translation
- then binds to SRP receptor in ER membrane recruiting translocation channel and translation continues
role of Hsp70 chaperones
maintain translocation competence of post-translational precursors prior to them encountering ER membrane
ER protein N-terminal signal peptide topology
internal stop-transfer sequence: N-terminus of protein inside ER and C-terminus in cytoplasm)
ER protein internal start-transfer sequence topology
C-terminus inside ER and N-terminus in cytoplasm
single-pass transmembrane protein (3)
- signal sequence starts transfer into the E
- second, longer hydrophobic sequence stops the transfer
- rest of protein synthesised in cytoplasm
multi-pass membrane protein (4)
- internal hydrophobic sequence bound by SRP
- starts translocation into ER
- next hydrophobic sequence stops translocation
- rest of protein synthesised in cytoplasm
what happens when ER destined proteins have signal sequence and 2 internal hydrophobic regions (2)
- protein will start translocation at N-terminus
- will be cleaved so N is in ER lumen
result of signal sequence with 2 internal hydrophobic transfer sequences (2)
- translocation will stop at first transfer sequence and start again at next transfer sequence
- leaves loop of protein in cytoplasm and both N and C terminus will be in ER lumen
what do disulfide bonds form between?
cysteine amino acids
how are disulfide bonds prevented in the cytosol
reducing environment prevents cysteine residues interacting and forming bonds
where can disulfide bonds form and why?
ER lumen because it has an oxidative environment
role of protein disulfide isomerase (PDI)
ER lumenal oxidoreductase, catalyses redox reactions that alter positions of disulfide bonds on proteins
why is the altering of disulfide bonds on proteins by protein disulfide isomerase essential?
essential for correct folding of proteins and/or functionality of some lysosomal proteins and external domains of proteins
what has occurred to many proteins in the ER lumen?
glycosylation
glycosylation
preformed branched oligosaccharide added en bloc to asparagine (N) residue
why are only extracellular parts of proteins glycosylated?
occurs in the ER lumen
what does ER perform on protein folding
quality control
ER: fate of properly folded proteins
exported from ER to golgi apparatus
ER: fate of improperly folded proteins (2)
- held in ER by chaperone proteins until properly folded
- if this doesnt happen, degraded
chaperone proteins in ER (2)
- hsp40/hsp70
- BiP/Kar2p
role of hsp40/hsp70 in ER
combine with a nucleotide exchange factor to ratchet in and enable translocating protein to fold
role of BiP/Kar2p in ER
involved in stress response signalling and ER associated degradation
what happens to proteins when trimming and folding is wrong?
proteins targeted to proteasome for degradation
what do unfolded proteins in the rER trigger?
production of chaperones and expansion of the ER
mechanism allowing ER to respond to unfolded proteins
transmembrane receptors activate transcription factor in cytoplasm when bound to unfolded proteins
feedback response to unfolded proteins (2)
- transcription factor enters nucleus
- activates transcription of chaperone genes and other ER components
role of smooth ER (2)
- synthesises phospholipids and cholesterol
- metabolises hormones and drugs
what is smooth ER. major site of? (sarcoplasmic reticulum)
intracellular Ca2+
why do myofibrils have a huge network of sarcoplasmic reticulum?
to store calcium ions
role of inositol 1,2,5-trisphosphate receptors (IP3R), ryanodine receptors and calcium channels in the ER
responsible for releasing Ca2+ from ER into cytosol when intracellular levels are low
how does depolarisation of t-tubule membranes lead to Ca2+ release
depolarisation leads to conformational changes in voltage-dependant Ca2+ channels (such as dihydropyridine receptors (DHPRs)) which interact leading to Ca2+ release
role of sarcoplasmic reticulum in muscles
t-tubules conduct impulses from sarcolemma into cell and sarcoplasmic reticulum
sarcoplasmic reticulum role in muscle contraction mechanism (4)
- action potential spreads along sarcolemma and t-tubules into muscle
- DHP receptor senses membrane depolarisation, changes conformation and activates RyR receptor on sarcoplasmic reticulum
- causes SR to release Ca2+
- Ca2+ binds to troponin activating contraction process
one of the most well-studied Ca2+ release events
fertilisation following sperm entry
what does an autophagy induction signal lead to
formation of a phagophore (sequestering membrane)
autophagy (4)
- ubiquination-like reactions, LC3 conjugates to membrane and elongates membrane of the phagophore
- cytoplasmic components enwrapped by phagophore
- at end of elongation double membrane vesicle formed (autophagosome)
- autophagosome fuses with lysosome to form autolysosome and will degrade components
how does the ER-localised transmembrane protein: vacuole membrane protein 1 (VMP1) mediate ER-phagophore dissociation?
via activating ER Ca2+ channel sarcoplasmic Ca2+ ATPase (SERCA) and perturbing the local Ca2+ concentration
