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Chapter 8 Intracellular Compartments and Protein Sorting: Transport Between the Nucleus and the Cytosol. Relative Volumes Occupied by the Major Intracellular Compartments in a Liver Cell. Relative Amounts of Membrane Types in Two Kinds of Eukaryotic Cells. Some Signal Sequences.

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Chapter 8

Intracellular Compartments and Protein Sorting: Transport Between the Nucleus and the Cytosol




Some Signal Sequences Eukaryotic Cells


  • Inner Nuclear Membrane Eukaryotic Cells

  • Outer Nuclear Membrane

  • Bidirectional Traffic

    • Histones, DNA and RNA polymerases, gene regulatory proteins, RNA processing proteins are all selectively imported into the nuclear compartment from the cytosol

    • tRNAs, mRNAs are exported to the cytosol.



Side view of two NPCs; note that the inner and outer nuclear membranes are continuous at the edges of the pore.


Gated Diffusion Barrier of the NPC membranes are continuous at the edges of the pore.

Meshwork blocks passive diffusion of large macromolecules.


Importing of proteins to the nucleus
Importing of Proteins to the Nucleus membranes are continuous at the edges of the pore.

  • When nuclear proteins are extracted from the nucleus, then put into the cytosol, they find their way back to the nucleus.

    • Nuclear Localization Signals

    • How could you experimentally figure this out? That is, how could you even determine there was a nuclear localization signal?

    • Signals are short sequences rich in lysine and arginine and can be located just about anywhere on the protein.

  • Nuclear localization signals are recognized by nuclear import receptors.

    • Soluble in the cytosol and bind to both the signal on the protein to be transported and to the NPC proteins.

    • Fibrils

    • Hop through by repeated binding and dissociating and rebinding.



Nuclear export
Nuclear Export proteins

  • Large molecules such as rRNA subunits and RNA molecules are also selectively transported.

  • Nuclear Export Signals on these macromolecules.

  • Nuclear Export Receptors: bind to both signal and the NPC


Access to the transport machinery
Access to the Transport Machinery proteins

  • Some proteins that transport molecules out of the nucleus also have nuclear localization signals.

  • They are continually shuttled back and forth.

  • Rate of import > rate of export then the protein is mostly located in the nucleus.

  • Remember these localization signals control the flow of the protein, in and out of the nucleus. These signals are turned on and off by phosphorylation and amino acids close to the signal sequences.


Peroxisomes proteins

Bounded by single membrane

No DNA or ribosomes

Selective import

Presence of catalase

High usage of oxygen: may have

been an ancient organelle that

allowed cells to tolerate the

beginnings of an oxygen

environment.

Present day functions would be

those that have not been taken

over by mitochondria.


Peroxisomes use molecular oxygen and hydrogen peroxide to perform oxidation reactions
Peroxisomes Use Molecular Oxygen and Hydrogen Peroxide to Perform Oxidation Reactions

  • Catalase uses the hydrogen peroxide to oxidize a variety of substrates.

    • Liver and Kidney Cells where detox occurs.

    • 25% of any alcohol you drink is oxidized to acetaldehyde.

    • If excess hydrogen peroxide accumulates, catalase converts it to water and oxygen as we saw in the enzyme lab.


  • Breakdown of fatty acids Perform Oxidation Reactions

    • Beta Oxidation

      • Acetyl CoA

        • Which then must be transported out to be used in other reactions.

      • In mammalians, beta oxidation occurs in mitochondria and peroxisomes.


Biosynthesis of Plasmalogens Perform Oxidation Reactions

Phospholipids in myelin.

Myelin deficiencies of the axons

Peroxisome disorders lead to neurological

disorders.


Not Plants Too???!!! Where they are called glyoxysomes. Perform Oxidation Reactions

Seeds and fatty acids.

Animals cannot con-

vert fatty acids to

carbohydrates


  • Import of Proteins Into Peroxisomes Perform Oxidation Reactions

    • Shorts signal sequence of 3 amino acids can be at either end of the protein, the C- terminus or the N-terminus.

    • Soluble receptor proteins in the cytosol

    • Docking proteins on the peroxisome surface.

    • Requires ATP.

    • Zellweger Syndrome

      • Import process is dysfunction leading to empty peroxisomes causing abnormalities in the brain, liver and kidneys and thus die soon after birth.

      • One form of the syndrome is caused by a mutation in a gene coding for a peroxisomal integral membrane protein involved in import.



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