1. Explain the three ways that proteins are imported into organelles and the sig
ID: 200954 • Letter: 1
Question
1. Explain the three ways that proteins are imported into organelles and the signal sequence. Include some examples of proteins would fit into each of these 3 categories.
2. Review the role of ubiquitin and proteasomes to regulate protein concentration.
3. What is a signal sequence and how does the cell use them?
4. Explain the structure of the nuclear envelope, lamina, and nuclear pore complex.
5. Explain the role of nuclear transport receptors and the nuclear localization signal.
6. Explain 2 ways that protein import into the nucleus is regulated.
7. Explain how proteins are imported into the mitochondria/chloroplast.
8. Explain the structure and types of ER and clarify the types of proteins made in the RER.
Explanation / Answer
1. There are three ways in which an proteins is imported into organelles.
a) Gated import for nuclear transport: Proteins are transported from the cytosol to the nucleus through nuclear pores in the nuclear envelope. Nuclear proteins have a nuclear localization signal in their N-terminal domain, which have lysine and arginine on protein surface. The pores act as gates that transport macromolecules through the nucleus.
Nuclear localization signals bind to the importin proteins and this complex will move through the nuclear pore. The Ran-GTP will bind to importin, reducing its affinity for the protein. The protein is released. The Ran-GTP/importin complex will move out of the nucleus through the pore. A GAP protein hydrolyzes Ran-GTP to Ran-GDP, causing a conformational change in Ran. Ran-GDP has less affinity for importin. Importin is released and Ran GDP is recycled to the nucleus where GDP is converted to GTP.
Example: Poly (ADP-ribose) polymerase 1, High mobility group box 1, RNA binding motif protein 25
b) Transport Proteins: Transport proteins are involved in transport to chloroplast, peroxisomes, and rER
across organelle membranes. Mitochondrial proteins are synthesized as precursor proteins in the cytosol . They are then translocated into mitochondria by a posttranslational mechanism. Mitochondrial precursor proteins have a signal sequence at their N terminus that folds in an amphipathic alpha helix with positively charged amino acids on one side. Heat shock proteins such as hsp70 bind to unfolded proteins in cytosol and transports them to mitochondria.
Protein translocators help transport the proteins to the different compartments of mitochondria. The TOM complex functions across the outer membrane, while Tim complexes TIM22 and Tim 23 work across the inner membrane. Tom imports the signal peptide to the intermembrane space and then integrates the protein into outer membrane. The TIM23 complex transports proteins into the matrix space, and inserts the transmembrane proteins into the inner membrane. The TIM22 inserts carrier protein for transport of ADP, ATP, and phosphate. The signal sequence is removed after import by a protease (the signal peptidase) present in the mitochondrial matrix.
Examples: apurinic/apyrimidinic endonuclease (APE1), NADH dehydrogenase (ubiquinone) flavoprotein 2 (NDUFV2).
c) Vesicle transport: Proteins move from the rough endoplasmic reticulum or Golgi to the other organelles which are membrane bound via transport vesicles. Lysosomal enzymes are transported from the Golgi apparatus to lysosomes.
In vesicle transport, there is formation of a vesicle by budding from the membrane. The cytoplasmic surface of transport vesicles are coated with proteins. There are clathrin coated, and non clathrin coated (COPI). COPI coated vesicles have transport resident ER proteins marked by the KDEL or KKXX retrieval signals. These proteins are transported back to the ER from the ER-Golgi intermediate compartment or the cis Golgi network.
There is blebbing of the plasma membrane and concentration of the proteins at the membrane. The vesicles with the cargo are pinched off from the vesicle. When the vesicle reaches the target organelle, the vesicle fused with the membrane and releases the content to the organelle.
Example: Erythropoietin, insulin like growth factor binding protein 5.
As per Chegg’s rules, only one question needs to be answered.
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