Molecular Biology of the Cell

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

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Nuclear Pore Complexes Perforate the Nuclear
Envelope
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Side view of two NPCs note that the inner and
outer nuclear membranes are continuous at the
edges of the pore.
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Gated Diffusion Barrier of the NPC Meshwork
blocks passive diffusion of large
macromolecules.
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Importing of Proteins to the Nucleus

• 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.

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Nuclear import receptors are specific to the
signal proteins
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Nuclear Export

• 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

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Access to the Transport Machinery

• Some proteins that transport molecules out of the
  nucleus also have nuclear localization signals.
• They are continually shuttled back and forth.
• Rate of import gt 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.

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Peroxisomes 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.
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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.

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• Breakdown of fatty acids
• 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.

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Biosynthesis of Plasmalogens Phospholipids in
myelin. Myelin deficiencies of the axons
Peroxisome disorders lead to neurological
disorders.
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Not Plants Too???!!! Where they are called
glyoxysomes. Seeds and fatty acids. Animals
cannot con- vert fatty acids to
carbohydrates
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• Import of Proteins Into Peroxisomes
• 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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And From Where Do These Rascals Arise? Still
debated