Term
| What are the three mechanisms by which membrane enclosed organelles import proteins? |
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Definition
| Gated transport, post-translational protein translocation, co-translational protein translocation |
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Term
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Definition
| There are nuclear pores which function as gates. This is active transport. Proteins are fully folded when they enter a gate. |
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Term
| Talk about post-translational protein translocation |
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Definition
| There are membrane bound translocators which transport proteins across the membrane to a topologically distinct space. Translocators usually require the protein to unfold to snake through. This is how proteins get into the mitochondria and peroxisomes. |
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Term
| Talk about co-translational protein translocation. |
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Definition
| There is a signal sequence of hydrophobic amino acids that signals a growing polypeptide to be bound to a signal recognition particle (SRP). The SRP consists of 6 proteins and 1 RNA. When the SRP is bound, translation halts and the ribosome, protein, SRP complex moves to the ER. There is a translocon protein (a port) in the ER membrane along with an SRP receptor. The translocon accepts the protein and the SRP binds its receptor and then dissociates. The growing protein snakes through the translocon into the ER as it elongates. Within the ER, the ER signal is cleaved by signal peptidase. The protein then will fold normally inside the ER. |
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Term
| For co-translational protein translocation, why would you want these proteins to get to the ER during translation? |
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Definition
| These proteins end up in vesicles and get ferried to other intra/extracellular compartments when the vesicles fuse with those compartments. |
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Term
| How do proteins get to the nucleus? |
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Definition
| There is a Nuclear Localization Signal (NLS) which is a string of 4-8 positively charged amino acid residues. This signal gets recognized by a cytosolic nuclear transport receptor which then transports the cargo through the nuclear pore. |
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Term
| What happens to proteins that don't have a signal sequence? |
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Definition
| They are destined to stay in the cytoplasm forever. |
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Term
| Are nuclear pore complexes uni-directional, bi-directional, or something else? |
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Definition
| Nuclear pore complexes are bi-directional. proteins can come in or out. |
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Term
| Where are ribosomes made? |
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Definition
| The nucleolus and the cytoplasm. This is referred to as ribosome "biogenesis" |
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Term
| What is special about the nucleus' membrane? |
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Definition
| It is called the nuclear envelope. It has 2 lipid bilayers!!!! It has big nuclear pores which can accomodate folded post-translational proteins. |
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Term
| Do proteins enter the nucleus by diffusion or active transport? |
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Definition
| Some enter by diffusion (small ones) and others are actively transported. |
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Term
| Why would the nucleus allow free diffusion and the mitochondria not? |
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Definition
| The nucleus doesn't need to have a different charge/potential than the cytoplasm, however the mitochondria does for oxidative phosphorylation and etc. |
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Term
| Can anything diffuse into the nucleus? |
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Definition
| Not exactly. There is some evidence that the nuclear pores have some sort of mesh that does filter things a little bit. |
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Term
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Definition
| Importin (a nuclear importing protein) floats around the cytoplasm and recognizes NLS and binds it. |
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Term
| How does importin get the NLS-protein into the nucleus? |
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Definition
| Importin binds something in the nuclear pore complex (fibrils?) that gets it into the nucleus. |
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Term
| How is importin then removed from the NLS-protein? |
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Definition
| Ran-GTP does this in the nucleus. It binds to importin, and then NLS-protein gets released from the importin-Ran-GTP complex. |
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Term
| How does importin rid itself of Ran-GTP after the NLS-protein is released? |
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Definition
| Ran-GTP-importin is exported from the nucleus, and as it is exiting the nuclear pore, there is a GAP that hydrolyzes the GTP to GDP, which causes the importin to be released from the Ran-GDP. |
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Term
| How is nuclear Ran-GTP replenished? |
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Definition
| There is an import protein for Ran-GDP in the cytoplasm which takes Ran-GDP into the nucleus. Then, there is a GEF which exchanges the GDP for GTP, so Ran-GTP is then ready to kick some proteins off of importins. yeah baby. |
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Term
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Definition
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Term
| What type of charge does the NLS have? |
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Definition
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Term
| What is unique about the mitochondrion that poses a challenge for importing proteins? |
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Definition
| It is a double membrane system plus a matrix. So you might need to tag things for either of the membranes, or the intermembrane space, or the matrix. |
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Term
| Talk about the mitochondrial outer membrane. What does it have? What does this mean for the intermembrane space? |
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Definition
| Mitochondrion outer membrane is permeable to small molecules and proteins because it has porins (which are channels). The intermembrane space thus has a similar composition to the cytoplasm. |
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Term
| Talk about the mito inner membrane. What does it have? |
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Definition
| The inner membrane is impermeable to most ions and small molecules. This is important for maintaining the proton gradient required for oxidative phosphorylation (ATP generation), which occurs in the inner membrane. The inner membrane contains a huge amount of proteins. |
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Term
| What does the mitochondrial matrix have? |
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Definition
| It has the mitochondrial DNA as well as enzymes for the krebs cycle, which occurs there. |
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Term
| Where are most mitochondrial proteins synthesized and by what? |
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Definition
| In the cytosol on free ribosomes |
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Term
| How many membranes must proteins targeted to the mitochondrial matrix cross? |
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Definition
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Term
| What are the two mitochrondial transport protein complexes? |
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Definition
| TOM and TIM. (Translocase of the outer membrane, and translocase of the inner membrane.) |
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Term
| How are mitochondrial proteins imported? |
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Definition
| in unfolded form. There is a mitochondrial signal sequence recognized by a receptor in the outer mitochondrial membrane. The complex of receptor and protein diffuses laterally until the protein is at the right spot to go through the translocator (TOM). If the protein is destined for the inner membrane or the matrix, the protein will then also go through TIM. Then the mitochondrial signal sequence is cleaved by signal peptidase. |
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Term
| Are mitochondrial signal sequences preserved? |
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Definition
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Term
| How would a mitochondrial protein be targeted to different regions in the mitochondria? |
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Definition
| Signaling sequences would be slightly altered. |
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Term
| Are proteins going into the mitochondria fully translated? |
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Definition
| Yes, they are completed translated, but get unfolded via translocation. So this is post-translational translocation. |
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Term
| What in the mitochondrion helps to make sure proteins fold properly, considering they get unfolded during translocation? |
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Definition
| The mitochondria has its own HSPs. |
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Term
| Are peroxisomal signal sequences cleaved? |
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Definition
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Term
| What are peroxisomes? What do they have? What do they do? |
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Definition
| They are single membrane small organelles. They have enzymes for oxidative reactions (catalases) which break down certain substrates such as fatty acids, uric acid, amino acids, and methanol. |
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Term
| What are peroxisomal proteins called? |
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Definition
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Term
| How are peroxins imported into peroxisomes? |
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Definition
| There is a simple 3AA target sequence that binds a receptor. The rest is unknown, except that the 3AA target sequence is not cleaved. |
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Term
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Definition
| On free cytosolic ribosomes. |
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Term
| Are proteins going to peroxisomes fully folded? Is transport to the peroxisome post-translational? |
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Definition
| Transport to the peroxisome is post-translational, but the proteins are not fully-folded. |
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