ER - PRO & Lipids (13) Flashcards
how does the cell deal with misfolded PROs?
A chaperone associates with a growing polypeptide as it emerges from the cytoplasm or ER, chaperones facilitate the proper folding by supressing or unfolding incorrect structures
Chaperones aren’t always necessary; AA sequence is prone to folding a desired way A misfolded PRO that cannot be corrected is exported & destroyed by the proteasome
how do chaperones facilitate proper folding?
suppressing or unfolding incorrect structures
what triggers the unfolded PRO response?
too many unfolded PROs are produced at once
ER has sensors that monitor the levels of unfolded PROS
The unfolded PRO response mechanism
When too many unfolded PROs accumulate, BiP releases from sensors to refold PROs & activating sensors. Sensors are released alerting the cell in various ways:
o Transcription factor sensor goes to nucleus & increases the production of chaperones
o Phosphorylated translation factor inhibit the small subunit enabling the folding process to catch up
o Can also, inactivate an initiation factor to prevent more translation
how do chaperones recognize misfolded PROs?
o Recognizes unfolded PROs by looking for hydrophobic regions on the surface
what induces the cell to make chaperones?
Heat shock
Cold shock
Anoxia
what can cause PRO-PRO aggregation?
hydrophobic regions on the surface on the PRO
How can bacteria survive heat shock?
bacteria are heated allowing the cell to accumulate enough HSPs, when the temperature increases the bacteria have enough of a dose that they are protected
what protects against cardiac failure? & where was this seen?
elevated Hsp70 (chaperone)
transgenic mice
kuru
neurological disorder first seen in Fore people of New Guinea
o Practiced cannibalism, once they stopped the disease stopped
o Uncontrolled motor control
caused by prions
scrapie
a disease in sheep & goats, staggering gait & strange behavior such as scraping off the coat due to neurological damage
caused by prions
mad cow disease
neurological damage causes loss of motor control, bovine spongiform encephalopathy (BSE)
caused by prions
variant creutzfeld-jacob disease (vCJD)
brain develops holes, results in loss of neurological function
o Due to consumption of contaminated mad cow beef
caused by prions
prion
abnormal PRO, change the shape of PROs into the wrong configuration (anti-chaperone PROs)
what is the mechanism behind the disease of kuru, scrapie, mad cow & vCJD?
presence of abnormal PROs known as prions
PrPC
normal prion, enriched in neurons, mostly alpha helices
PrPSC
mutant/disease form of the normal prion, same AA as the normal prion but is mostly beta sheets
what is the diff b/w the mutant & normal prion PRO?
same AA sequence
normal –> alpha helices
mutant/disease –> beta sheets
how do the normal prion cells become harmful? & why isn’t there a defense? & what is the outcome?
PrPSc acts as an anti-chaperone and transforms PrPC into PrPSc
Mutant PRO is too similar to its own
PrPSc will aggregate into fibers, no longer functional & will obstruct the CNS
why don’t PrpSc PROs become removed?
their shape somehow protects them from being processed by the cell’s machinery that is responsible for the removal of misfolded PROs
how does a proteasome recognize a misfolded PRO targetted for destruction?
Proteasome recognizes misfolded PROs that cannot be resorted. BiP (chaperone) transports the misfolded PRO into the cytoplasm through reverse translocation through a translocon. The PRO is tagged with ubiquitin
describe the structure of a proteasome
Globular PRO structure
not membrane-bound
4 rings, 7 subunits
where are proteasomes located?
nucleoplasm or cytoplasm
Do prokaryotes or eukaryotes have proteasomes?
both
what are the 3 functions of a proteasome?
- Housekeeping: clearance of cytoplasm & nucleoplasm PROs (old & unnecessary)
Metabolic enzymes, hemoglobin & structural PROs –> last longer
Regulatory molecules (DNA rep) that are only required at certain times (S-phase) –> don’t live as long
Determine by the N-end rule - Removal of improperly folded PROs: misfolded PROs are ejected from the RER back to the cytoplasm via the translocon pores & degraded by proteasomes
- Removal of ubiquitin-tagged PROs: regulated step, allows cells to remove certain PROs as needed. Requires a polyubiquitin signal for destruction
N-end rule
PRO half-life correlates to the (single) N-terminal AA, defining half-life of PRO based on the N-terminus
half-life
the time it takes for 50% of the PRO present at time = 0 to be destroyed
ubiquitin ligase
family of related PROs recognize PROs destined for destruction, adds ubiquitin units to target PROs
what signal is required for a PRO to be destroyed?
polyubiquitin signal
what is a single ubiquitin tag responsible for?
sends the PRO to a diff place in the cell
What is the role of single ubiquitin on a trans-membrane PRO?
directs PROs to endosomes
what are the steps of the proteasome destroying PROs?
- PRO tagged by a chain of ubiquitin molecules
- PRO-ubiquitin complex binds to the cap PRO
- Ubiquitin removed & PRO unfolded by the cap & then fed through the alpha subunit gap (threaded through with the signal AAs)
- Beta subunits degrade PRO to short peptides, return to cytoplasm
which subunit in the proteasome is responsible for the proteolytic action?
beta subunit
how is membrane orientation maintained?
inner/outer orientation is maintained as membranes undergo budding & fusing or interconnecting
what membranes have different compositions?
inner & outer surfaces of the membrane
diff organelles have diff compositions
how are membranes synthesized?
Made by inserting new lipids into existing membrane
new lipid molecules are inserted on the cytosolic side (towards the cytoplasm, not the lumen)
half of the new lipids are turned over to the lumen side by flippases
where are lipids synthesized?
SER
what are membranes made of?
primarily major phosphoglycerides: Phosphatidyl serine (PS) Phosphatidyl choline (PC) Phosphatidyl ethanolamine (PE)
what is unique about specialized lipids synthesis? & what are exs of these lipids?
synthesis begins in the ER but is completed in the Golgi
sphingomyelin & glycolipids
what is unique about mitochondrial & chloroplast lipid synthesis?
some components are made locally
have lipids required for only their structure
what makes membrane lipids?
integral membrane PROs
flippases
flip half of the new lipids are over to the lumen side of the membrane
what determines the specificity of flippases?
internal/external asymmetry of the membrane
what are 3 diff ways membranes are modified?
- Enzymatic modification of the polar head groups: diff lipids have diff structural ability & can hold diff PROs in place
- Preferential incorporation: as new membranes bud off forming vesicles, there are enzymes & PROs that regulate the composition of the lipid membrane
- Phospholipid transfer PROs can carry lipids through the cytosol from one membrane to another (grabs a lipid & inserts it into a diff membrane)
phosphatidyl serine –> phosphatidyl choline is an ex of what?
membrane modification through enzymatic modification of the polar head groups
where are phospholipid transfer PROS important?
mitochondria & chloroplasts
how are lipids moved among the cell?
Move laterally on the surface of the lipid bilayer
Can flip from one membrane surface to the other
Transfer to new locations via vesicles
Single lipids can be carried from one organelle to another via carrier PROs
