Introduction to Molecular and Cell Biology, Biol. 220 Lecture 24: Organelle Transport
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Overview of sorting of nuclear-encoded proteins in eukaryotic cells (movie).

• Possible destinations of cytoplasmically translated proteins.
• Cytoplasm (no signal)
• Nucleus (nuclear localization signal, not cleaved)
• Chloroplast (chloroplast localization signal, cleaved)
• Mitochondria (mitochondrial localization signal, cleaved)
• Peroxisome (peroxisome localization signal, not cleaved)

• Possible destinations of proteins translated on the RER. (An N-terminal signal is required for attachment to ER. The signal is cleaved upon entry into the ER.)
• ER
• Golgi
• Lysosome
• Plasma membrane
• Secreted
• Fig. 17.1  Overview of sorting of nuclear-encoded proteins in eukaryotic cells.
  

 

Types of signals associated with organelle targeting (Table 17.1).

Table 17.1  Properties of Uptake Targeting Signal Sequences that Direct Proteins from the Cytosol to Organelles.

Signals and combinations of signals can target proteins to different locations within the mitochondria (Fig. 17.2) (Fig. 17.6).

Fig. 17.2  Uptake-targeting sequences of imported mitocholdrial proteins.

Fig. 17.4  Protein import intothe mitochondrial matrix.

Fig. 17.6  Two pathways by which different proteins are transported from the cytosol to the mitochondrial intermembrane space.

A mitochondrial matrix bound protein (Fig 17.4):

• Cytosolic chaperones deliver unfolded proteins to receptors in the organelle membrane which transfer the protein to a membrane channel.
• The signal passes through the channel and is cleaved in the matrix.
• The remainer of the protein passes through the channel and is bound by matrix chaperones which assist in folding.
• Transport requires energy in the form of ATP and also as a constant H+ ion membrane potential between matrix and intermembrane space.

Chloroplast targeting is similar to the mitochondrion pathways

• Stoma targeted Rubisco small subunit (Fig. 17.7).
• Fig. 17.7  Import of rubisco small subunits into the chloroplast stroma and the assembly of active rubisco enzyme.
  
• Thylakoid targeted plastocyanin  and metal binding protein (Fig 17.8).
• Fig. 17.8  Two of the four known pathways for transporting proteins from the cytosol to the thylakoid.
  

Peroxisomal Proteins (Fig. 17.10).

Fig. 17.10  Synthesis of catalase and its incorporation into peroxisomes.

• Proteins fold in the cytoplasm.
• Import does not require an energy gradient across membrane.
• C-terminal SKL is signal for import into peroxisome and is not cleaved.
• A 26 amino acid N-terminal uptake signal is cleaved upon entering peroxisome.
• Peroxisomal membrane proteins do not contain the SKL sequence.