Intracellular Compartments and Protein Sorting

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Intracellular Compartments and Protein Sorting

• Haixu Tang
• School of Informatics

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The major intracellular compartments of an animal
cell
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Relative Volumes Occupied by the Major
Intracellular Compartments
INTRACELLULAR COMPARTMENT INTRACELLULAR COMPARTMENT PERCENTAGE OF TOTAL CELL VOLUME

Cytosol 54 54
Mitochondria 22 22
Rough ER cisternae 9 9
Smooth ER cisternae plus Golgi cisternae 6 6
Nucleus 6 6
Peroxisomes 1 1
Lysosomes 1 1
Endosomes 1 1
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An electron micrograph
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Development of plastids
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Hypothetical schemes for the evolutionary origins
of some organelles
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Four distinct families

1. the nucleus and the cytosol, which communicate
   through nuclear pore complexes and are thus
   topologically continuous (although functionally
   distinct)
2. all organelles that function in the secretory and
   endocytic pathways, including the ER, Golgi
   apparatus, endosomes, lysosomes, the numerous
   classes of transport intermediates such as
   transport vesicles, and possibly peroxisomes
3. the mitochondria
4. the plastids (in plants only).

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Secretory vs. endocytic pathways
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Protein traffic
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Protein traffic

• Gated transport
• Transmembrane transport
• Vesicular transport
• membrane-enclosed transport intermediates

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Sorting sequences
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Some sorting sequences
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Prediction of protein sorting

• Psort web server http//psort.nibb.ac.jp/
• prediction of protein localization sites in cells
  from their primary amino acid sequence

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Construction of Membrane-enclosed Organelles
Require Information in the Organelle Itself

• The information required to construct a
  membrane-enclosed organelle does not reside
  exclusively in the DNA that specifies the
  organelle's proteins. Epigenetic information in
  the form of at least one distinct protein that
  preexists in the organelle membrane is also
  required, and this information is passed from
  parent cell to progeny cell in the form of the
  organelle itself. Presumably, such information is
  essential for the propagation of the cell's
  compartmental organization, just as the
  information in DNA is essential for the
  propagation of the cell's nucleotide and amino
  acid sequences.

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Nuclear pore complexes
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Nuclear Envelope
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Nuclear lamina

• Consists of "intermediate filaments", 30-100 nm
  thick.
• These intermediate filaments are polymers of
  lamin, ranging from 60-75 kD.
• A-type lamins are inside, next to nucleoplasm
  B-type lamins are near the nuclear membrane
  (inner). They may bind to integral proteins
  inside that membrane.
• The lamins may be involved in the functional
  organization of the nucleus.

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Nuclear localization signals (NLSs)
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Protein import through nuclear pores
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Possible paths for free diffusion through the
nuclear pore complex
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Nuclear Import / Export Receptors
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The control of nuclear import during T-cell
activation
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The breakdown and re-formation of the nuclear
envelope during mitosis
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The subcompartments of mitochondria and
chloroplasts
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A signal sequence for mitochondrial protein
import
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Protein translocators in the mitochondrial membra
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Protein translocation depends on the temperature
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Protein import by mitochondria
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Energy required
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Two plausible models of how mitochondrial hsp70
could drive protein import
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Protein import from the cytosol into the inner
mitochondrial membrane or intermembrane space
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Translocation of a precursor protein into the
thylakoid space of chloroplasts
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The Endoplasmic Reticulum
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Free and membrane-bound ribosomes
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The signal hypothesis
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The signal-recognition particle (SRP)
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SRP direct ribosomes to the ER membrane
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Protein translocation
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Single-pass transmembrane protein
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Multipass membrane protein rhodopsin
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Protein glycosylation in the rough ER
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The export and degradation of misfolded ER
proteins
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The unfolded protein response in yeast
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Phospholipid exchange proteins