Lecture 8

Protein Trafficking/Targeting

Protein targeting is necessary for proteins that are destined to work outside the cytoplasm.

Protein targeting is more complex in eukaryotes because of the presence of many intracellular compartments.

Prokaryotic protein targeting (secretion)

The chaperone protein SecB binds to the nascent polypeptide chain to prevent premature folding which would make transport across the plasma membrane impossible. SecE and SecY are transmembrane components which form a pore in the membrane through which the still unfolded polypeptide is threaded. The translocation process is energy-dependent (ATP) and is driven by SecA. Once the protein has passed through the pore, the signal sequence is cleaved off by an extracellular, membrane-bound protease.

N-terminal signal sequences of representative secreted prokaryotic proteins.

Protein	`-20 -10 -1 +1`
Leucine-binding protein	`MKANAKTIIAGMIALAISHTAMA EE...`
Pre-alkaline phosphatase	`MKQSTIALALLPLLFTPVTKA RT...`
Pre-lipoprotein	`MKATKLVLGAVILGSTLLAG CS...`

Hydrophobic residues in red.

Next: Eukaryotic Protein Targeting

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