Flashcards in Cellular Organization (LE004) Deck (21):
What is an MTOC, what does it do, and where is it found?
MTOC – microtubule organizing center
These proteins stabilize the minus end of microtubules which are unstable and will depolymerize without the MTOC
How do actin filaments compare with microtubules in supporting the transport of cellular material?
Actin filaments also support the transport of cellular material like microtubules, but over much shorter distances.
Where in the cell might you find actin filaments localized?
Near the cell membrane where they provide structural support.
What are Class V Myosins?
Class V myosins are the class of myosins involved in the transport of organelles in several different types of cells. They contain a motor domain that binds actin filaments, and use the energy of ATP hydrolysis to walk along the filament.
The C-terminus of myosin V binds organelles.
Match the 2 to the correct terminus of the protein: (N or C terminus)
a. Signal Sequence
b. V myosin binding to organelles
The C-terminus of myosin V binds organelles
Signal sequences can be localized anywhere in a protein but are often found in the N-terminus.
Some membrane bound organelles have pores that only accommodate unfolded proteins, whereas other pores allow folded proteins to pass. Name some organelles for each of the above.
Only unfolded proteins: ER, mitochondria
Folded proteins as well: nucleus, peroxisome
What would be necessary to include in the protein sequence in order to translate a protein that spans the membrane several times?
To generate proteins that span the membrane several times, the protein would need several alternating stop and start transfer sequences.
What state is the small GTP-binding protein called “Ran” in, when it is in the cytoplasm? In the nucleus? What does it do in the nucleus?
The GTP binding protein “Ran” allows for the protein to differentiate their environment as nucleus or cytoplasm.
In the cytoplasm, Ran is in the Ran-GDP bound state.
In the nucleus, Ran is in the Ran-GTP bound state.
In the nucleus, Ran-GTP dissociates importins from the protein that the importin recently trafficked into the nucleus through the pore.
What is the receptor “importin”? What is it bind? What does it interact with and what do they do?
Importin binds nuclear import sequences in proteins. They interact with filaments that extend to the cytoplasmic side of the nuclear membrane. Together, they will bring the protein into the nucleus.
Ran-GTP, within the nucleus, will allow for the importin to dissociate from the cargo protein.
Which receptor is responsible for nuclear export of proteins?
“Exportin” is the receptor that interacts with the nuclear export sequence of proteins that are to be exported to the cytoplasm.
Ran-GTP will bind to this exportin-cargo complex and stabilize the interaction. All three of these will go through the pore.
In the cytoplasm, it will encounter Ran-GAP (GTP Activating Protein) which will convert the Ran-GTP to Ran-GDP (because now we are in the cytoplasm) and the Ran-GDP will cause exportin to dissociate from its cargo.
What are Pex proteins? What happens when you have mutations in Pex proteins?
Pex proteins are the family of proteins which will recognize the signal sequence for targeting a protein to a Peroxisome.
If there is a mutation in the Pex proteins, the cell cannot import proteins into peroxisomes and therefore, cells will lack peroxisomes.
→ Zelleweger’s Syndrome (set of diseases)
Lack muscle tone, ability to suckle.
Craniofacial abnormalities, enlarged liver
Not surviving beyond a year (prognosis)
Poor myelination of neurons because peroxisomes contribute to the synthesis of a lipid found in myelin.
Which motor protein is responsible for ie, bringing melanosomes back to the middle of the cell? Distributing them outward?
Dynein will walk the melanosomes back towards middle of cell (- part of Microtubules)
Kinesins will bring the melanosomes outward (+ part of the microtubules)
Peripheral neuropathy such as Charcot Marie Tooth will result in symptoms of: tingling of feet, hands, degeneration of motor neurons, etc. What is mutated in peripheral neuropathy? And what is happening to the motor neurons?
Kinesin motor proteins are mutated
Organelles are not sufficiently transported to the ends of cells – most affected in the longest neurons which are motor neurons.
As the organelles don’t make it to the end, the cell will start to degenerate.
This is lethal.
What is the structure of an actin filament? Are they polarized? Which direction does myosin go?
Polymer of a single actin molecule in helical structure.
Not much lateral interaction. They are not as stable as microtubules. Like cooked spaghetti, floppy. Not very good with long distance transfer. Leave that to the microtubules.
They are polarized.
Most myosin walk towards the (+) end.
How do microtubules and actin filaments work together to transfer material in the cell? (think about where actin filaments are found most in the cell)
Most cells will use a combination of motor proteins and a combination of microtubules and actin to transfer material.
You start with the microtubules which are long and do most of the transferring towards the cell membrane.
Actin filaments are mostly found near the cell membrane (they are also used for structure stabilizing underneath the cell membrane) and will deliver material the remainder of the way.
What is Fialuridine?
Why did it result in liver failure in patients during clinical trials? What does this tell us about transfer signaling sequences?
Fialuridine is a uridine analog that is a potent inhibitor of hepatitis B. When it gets incorporated into RNA, it destabilizes that RNA and prevents replication / division.
In mice, it successful in preventing the spreading of the virus in Hep B.
However in humans, it was successful at first, and then quickly resulted in liver failure because humans have a nucleoside transporter that allows for Fialuridine to enter the mitochondria and result in defected production of ATP and therefore, cell death.
This is further evidence that small changes in the signal sequence have profound effects on location of proteins.
What is translocon of the ER?
Translocon is a very narrow pore on the ER that only allows unfolded proteins to enter the ER.
What is the process of allowing a protein into the ER?
This happens co-translationally, meaning the protein is allowed into the ER through the translocon while it is being translated.
Because this protein is going through translocon in an unfolded state, it must fold INSIDE of the ER. This could lead to damage to cells.
What is a likely characteristic of the stop transfer sequence for transmembrane proteins?
What is the size limitation for allowing a molecule in and out of the nuclear pores?
Molecules smaller than 50 kD can pass through a nuclear pore through diffusion. Ie, 30 kD proteins can diffuse in.
Molecules larger than 50 kD need a mechanism to go through, via importins.
This pore is responsible for both import, export of all material (smaller or larger than 50 kD)