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FIGURE 8.26 Standard Signal Sequence for Exported Proteins

FIGURE 8.26 Standard Signal Sequence for Exported Proteins

The signal sequence contains a positive charged domain (containing lysine and/or arginine), an a-helical hydrophobic domain (rich in alanine, leucine and valine) and a cleavage site preceded by a glycine or serine and followed by a proline. A reverse turn due to glycine is found approximately half way through the hydrophobic domain.

to the exterior of the cell must be exported through the cell membrane. Similar systems exist in bacterial and eukaryotic cells. Proteins destined for export are tagged at the N-terminus with a signal sequence. This is cut off after export, by proteases attached to the outside of the membrane, and is therefore not present in the mature protein. The signal sequence consists of approximately 20 amino acids that form an a-helix. There is little specific sequence homology between signal sequences from different exported proteins. A positively charged, basic N-terminus of two to eight amino acids is followed by a long stretch of hydrophobic amino acids. The amino acid just before the cleavage site has a short side chain (Fig. 8.26).

A polypeptide destined for export is recognized by its signal sequence. In bacteria, the signal recognition protein (SecA) binds the signal sequence and guides it to the translocase complex in the cell membrane. The rest of the protein being exported signal sequence Short, largely hydrophobic sequence of amino acids at the front of a protein that label it for export translocase Enzyme complex that transports proteins across membranes

After export of a protein across the cell membrane via the translocase, the signal sequence is cut off.

FIGURE 8.27 Cotranslational Export of Proteins

FIGURE 8.27 Cotranslational Export of Proteins

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