Protein Transport Overview and Transport into Organelles

(Lodish et al., 2000, Section )

Overview of sorting of nuclear-encoded proteins in eukaryotic cells (Fig. 17.1) (movie).

Possible destinations of cytoplasmically translated proteins.

Cytoplasm
Nucleus
Chloroplast
Mitochondria
Peroxisome

Possible destinations of proteins translated on the ER.

ER
Golgi
Lysosome
Plasma membrane
Secreted

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

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

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).
Thylakoid targeted plastocyanin and metal binding protein (Fig 17.8).

Peroxisomal Proteins (Fig. 17.10).

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.

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References:
Lodish, H., Berk, A., Zipursky, S.L., Matsudaira, P., Baltimore, D., Darnell, J., 2000, Molecular Cell Biology, 4th Ed., W.H. Freeman and Company, NY, New York. ISBN 0-7167-3136-3.