CBNS 101 lecture8

CBNS 101 lecture8 - Intracellular Compartments and Protein...

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Intracellular Compartments and Protein Sorting
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n Signal sequences direct proteins to correct cellular address. n Recognized by specialized receptor proteins that mediate transport. In some cases, the localization of the receptor is restricted in the cell. Signal sequences on proteins act as "address tags z
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SIGNAL PEPTIDE SIGNAL PATCH Two arrangements for a signal sequence within a protein n Signal peptide - continuous amino acid sequence (15-60 aa). Located at ends or internally. Can be recognized either co- translationally or post-translationally . Often removed after transport. n Signal patch - signal comprised of amino acids found throughout the protein. Only recognized as signal after protein is translated and folded. Signal sequences on proteins direct the proteins to the correct cellular address
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All of these processes require energy. 1. Gated transport through nuclear pores. Transport occurs post-translationally; folded protein moves through nuclear l gate z 2.Transmembrane transport. a) Post-translational insertion of unfolded protein through lipid bilayer of mitochondria, chloroplasts, peroxisomes. b) Co-translational insertion of protein into ER. 3. Vesicular transport. After insertion into ER, proteins further transported by vesicles into Golgi, lysosomes, cell surface. Three ways in which address tags are used to direct protein transport.
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GATED TRANSPORT VESICULAR TRANSPORT TRANSMEMBRANE TRANSPORT The transport of molecules between the nucleus and the cytosol
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Nuclear Pore Complexes or nucleoporins: Gateway for Transport In and Out of the Nucleus n Huge complex, 100-nm wide circular openings in the nuclear envelope. n Can open to 25 nm in diameter. ~3000-4000 NPCs in typical mammalian cell. n Transport is not limited to proteins - includes ribonucleoproteins (RNPs). n Transport across the NPC is bidirectional . n Proteins do not need to be unfolded to pass through pore. n Passage through the NPC does not allow proteins to integrate into adjacent bilayer. n Assembled from ~30 (yeast) to over 50 (vertebrates) proteins called nucleoporins (nups).
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General mechanism of transport through nuclear pores Nuclear localization signal (NLS) proteins interact with a class of proteins that speciFcally bind to the NLS region in the cytoplasm: The nuclear import receptor The nuclear import receptor is localized to the pore by virtue of its ability to interact with ±G repeats on proteins that comprise the Fbrils of the nuclear pore. These interactions help transport the receptor:cargo protein complex through the pore. Once through the pore receptor dissociates from the cargo protein.
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Indirect: (receptor binding via an l adaptor z ) Direct: (different receptors-different cargos) Nuclear import Receptor bind Nuclear localization Signals and nucleoporins n Transport receptors bind to Nuclear Localization Signal on cargo protein in cytoplasm. n
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CBNS 101 lecture8 - Intracellular Compartments and Protein...

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