BIOL130 Chapter Notes - Chapter 7: Nuclear Localization Sequence, Nuclear Export Signal, Signal Recognition Particle

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5 Aug 2016
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Unit 7: Intracellular Compartments and Protein Sorting
3 ways to import proteins into organelles: nuclear import, nuclear pore complex, nuclear
localization sequence
 transport across membrances and by vesicles
Endomembrane system though to evolved from the folding of plasma membrane
Mitochondria and chloroplasts thought to have evolved as endosymbionts – organisms that live
within body or cells of another organisms
Cytoskeleton: intermediate filaments extend from one side of the cell to the other,
microtubules, and microfilaments lie on the bottom of the cell
 important in cell shape and motility, and movement of materials in cells, and movement of
chromosomes during mitosis
Signal sequence: 15-60 AAs long and directs proteins to particular organelles (like ER, nucleus,
mito/chloro, peroxisomes)
If the ER signal sequence of a protein were mutated or deleted, it would likely end up in the
cytosol
Synthesis of all proteins begins in cytosol on free ribosomes
Nucleus has double membrane – nuclear envelope and has a network of fibres inside
Nuclear pore complex is highly selective – molecules below a certain size index can diffuse
nonselectively through the nuclear pore
 has ring subunits with nuclear fibrils and a nuclear basket on the inside and cytosolic fibrils
 high traffic of molecules through nuclear pore – 500 molecules selectively flow through 3000-
4000 pores per second
In order for proteins to go through the selectively nuclear pore, they have to have a nuclear
localization sequence (localization sequence- protein complex forms in cytosol) which is
attached on the protein and is recognized by a nuclear transport receptor which “escorts” the
protein through the ring subunit
Nuclear localization signal: an amino acid sequence that ‘tags’ a protein for import into nucleus
Nuclear export signal: tags a protein for export
Proteins pass through nuclear pores without unfolding
Moves out of nucleus: mature, properly folded mRNA, and ribosomal RNA (both made in
nucleus but need to move out)
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