Intracellular Compartments and Protein Sorting Flashcards
What percentage of the cell volume is cytosol?
About half. The other half are organelles (nucleus takes up 10%)
What organelles make up the exocytic network?
Endoplasmic reticulum
Golgi apparatus
Secretory vesicles
What organelles make up the endocytic network?
Plasma membrane
Endosomes
Lysosomes
How are proteins ‘tagged’ with a destination to travel to following synthesis?
Sorting signals address the protein to enter/exit cellular organelles
This sequence can be linear at end of protein or formed during protein folding (patch)
What property is common in ER signal peptides?
A hydrophobic center
What property is common in mitochondrial signal peptides?
Amphipathic alpha helices
Have positively charged edge on side of alpha helix
A signal peptide is leucine rich. Where is the protein most likely going?
Leucine rich signal peptides signal export from the nucleus
Describe the nuclear membrane
A double membrane that is perforated with about 4000 nuclear pores
The perinuclear space (between membranes) is continuous with the endoplasmic reticulum
Describe the structure of a nuclear pore
Nuclear pores are octamers with fibrils extending into the cytosolic and nuclear sides of the pores
On the nuclear side, the fibrils join into an 8-fiber basket that restricts the passage of large particles (>60 kDa)
How are porteins larger than 60 kDa moved in/out of the nucleus?
Nuclear localization signals (NLS) or Nuclear export signals (NES) trigger active energy dependent gated transport through the nuclear membrane
Why is [Ran-GTP] high in the nucleus, but low in the cytosol?
RanGEF is a nuclear protein that exchanges GDP bound to Ran for GTP to form Ran-GTP
This gradient drives cargo transport across nuclear pores
Describe the steps for nuclear export.
RanGTP promotes cargo with NES to bind to an export receptor (exportin) which binds to the NPC fibrils and travels through nuclear pores
RanGAP hydrolyzes the RanGTP to dissociate the cargo
The exportin enters back into the nucleus to repeat the cycle
Describe the steps for nuclear import
Cytosolic cargo with NLS binds to an importin which interacts with the NPC to be imported into the cell
In the nucleus, RanGTP binds the complex and stimulates dissociation
Importin-RandGTP exits back into cytoplasm
How is the nuclear uptake of NF-kB regulated?
NFkB is housed in the cytoplasm where it is bound to I-kB and inactivated.
Following signal transduction, I-kB is phosphorylated and dissociates from NFkB revealing an NLS
True or false: Nuclear import signals can be reused during repeated rounds of mitosis.
True.
Although the nucleus breaks down and reforms in each cell cycle, the NLS on proteins stay put and direct the proteins into the correct place during division
Describe the structure of the mitochondrion.
Has two membranes.
The inner membrane is very folded (large surface area) but the outer membrane is not.
There are four compartments: matrix, inner membrane, inter-membrane space, outer membrane
True or false: Mitochondrial proteins are coded for from mitochondrial DNA
False. Most mitochondrial proteins are nuclear gene products
Describe protein transport into the mitochondrion
Post-translational
Signal sequence binds to TOM (outer membrane) receptors and TIM (inner membrane) receptors lined up
Unfolded protein passes through aqueous pores in the membrane
What is required for proteins to be inserted into the mitochondrial matrix?
Cytoplasmic heat shock protein (hsp70) bind the unfolded cargo protein, push protein through pore
Electric potential across inner membrane is dissipated to electrophorese the unfolded protein into the matrix space
Chaperones (hsp70) in the matrix bind unfolded protein and pull through the pore
What are the two types of mitochondrial chaperones that may assist in protein folding?
Hsp70: binds to unfolded protein domains and protects them during folding process
Hsp60: “cage” in which unfolded protein is protected while it finds its way through the folding process
Both are ATP-dependent
What is the general function of peroxisomes?
Peroxisomes oxidize things like fatty acid chains, cholesterol (into bile acids), metabolic intermediates (acetyl CoA production)
Detoxify poisons (alcohol)
How are peroxisomes formed?
They bud off of the endoplasmic reticulum and can expand and divide by fission
True or false: Proteins are introduced into peroxisomes during translation.
FALSE
Proteins are introduced post-translationally into peroxisomes
What processes occur within the endoplasmic reticulum?
Membrane and secreted proteins are produced
Lipids are produced
Calcium is stored
Lipid soluble drugs/toxins are detoxified
When are most proteins transported into the ER?
As they are tranlated (“cotranslational”)
Signal peptides are produced at cytoplasmic ribosomes, which cause ribosomes to localize to the ER so that the protein can be pushed through a pore into the ER lumen
What does a signal recognition particle do?
They guide ribosomes to the ER
SRP bind to signal peptides and arrest translational elongation until the SRP is bound to an SRP receptor on the ER membrane near the Sec61 translocation channel
How are free, soluble polypeptides synthesized into the ER lumen
The signal sequence directs translation to the ER surface and the protein is synthesized directly into the ER lumen
Signal peptidase cleaves the signal peptide upon completion of translation leaving a free, soluble protein in the ER
How are membrane proteins produced?
Stop-transfer signals are regions are charged residues flanking hydrophobic stretches that will end up being the transmembrane spans
After a stop-transfer, the ribosome will finish translation cytosolically
Sec61 will be pried open to release the protein into the ER membrane
How are proteins N-glycosylated?
Asparagine residues in the sequence N-X-S/T can be glycosylated in the lumen of the ER
Dolichol lipids in the membrane are substrates for buildup of big sugars that can be transferred onto N by oligosaccharyltransferase
What amino acid residues other than arginine can be glycosylated?
In O-glycosylation, the hydroxy groups of Serine or Threonine are glycosylated
Describe the unfolded protein response
ER can sometimes be overwhelmed with unfolded proteins
Aggregation occurs, which can be recognized by sensors that dimerize into ribonucleases
The sensors trigger splicing of mRNA that encodes a gene regulator protein that drives transcription of ER chaperone genes
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