As discussed in the previous section, proteins are
synthesized on ribosomes which are located mainly in the cytosol. Only a
small number of ribosomes are located in mitochondria and chloroplasts.
Proteins synthesized on these ribosomes can be directly incorporated into the
compartments within these organelles. However, most mitochondrial and
chloroplast proteins are encoded by nuclear DNA and synthesized on cytosolic
ribosomes. These and all other proteins synthesized in the cytosol must be
transported to appropriate locations in the cell. This is made possible by
the specific signal sequence in the newly synthesized peptide.
Figure 5-D-1. Protein sorting.
- If the N terminus of the new peptide contains a stretch of hydrophobic
residues, it is sent to the rough endoplasmic reticulum (ER) for further
sorting. Otherwise, it goes to non-ER pathways.
- The new peptide is retained in the rough ER if its C-terminus contains the
sequence "Lys-Asp-Glu-Leu" (KDEL in one-letter code). Otherwise, it will move to the Golgi apparatus.
- Proteins containing a specific transmembrane α helix will be localized in the Golgi apparatus.
- After glycosylation at the Golgi apparatus, the modified protein
containing a mannose 6-phosphate (M6P) will be delivered to the lysosome.
- Proteins that can aggregate with chromogranin B (secretogranin I) or
secretogranin II will be sorted into regulated secretory vesicles
where proteins are released upon specific stimulation. Otherwise, they
are sorted into another type of vesicles which continuously move to the
plasma membrane or outside the cell.
Mitochondria and Chloroplast
and Targeting of Mitochondrial and Chloroplast Proteins - From Molecular Cell Biology by Lodish et al.
Signal sequence: "Ser-Lys-Leu" (SKL in one-letter code) at the very C-terminus.