Chapter 8Intracellular 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.

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. 13

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Nuclear import receptors are specific to the signal proteins

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 > 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.

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

• 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