Protein Targeting in Eukaryotic Cells

Protein Targeting

• Cellular function in eukaryotic cells is compartmentalized, involving the plasma membrane, organelles, and cytosol.
• Proteins have to be directed to different parts of the cell, including organelles, membrane proteins, secreted proteins, and cytosolic proteins.

Signal Sequences

• Protein sequences act as "postcodes" to direct proteins to their correct locations.
• Signal peptides are recognized by the Signal Recognition Particle (SRP) protein complex.

Targeting to ER

• Targeting proteins to the ER via signal peptide involves secreted proteins, membrane proteins, and lysosomal proteins.
• Protein synthesis starts with a cytosolic ribosome, and the signal sequence is recognized by SRP, which binds to the SRP-receptor on the ER membrane.
• The ribosome is bound to the ER membrane, and the new protein is guided through the translocon into the ER lumen.

Membrane Proteins

• Stop transfer sequences (20-22 amino acids) remain as transmembrane segments of proteins.
• Transmembrane proteins remain in the ER membrane, while other proteins are in the ER lumen.

Protein Folding and Transport

• Most proteins synthesized in the RER are glycosylated, and a common preformed 14-sugar oligosaccharide is added to asparagine residues.
• Protein folding takes place, and improperly folded proteins are exported from the ER and degraded by the proteasome in the cytosol.
• The ER is the starting point for many proteins, which reach their final destination through a series of intracellular transport steps involving membrane vesicles.

ER-Golgi Interactions

• Protein-containing vesicles bud off from the ER, migrate, and merge with the cis-Golgi.
• Further protein maturation occurs in the Golgi, and movement between cisternae of the Golgi occurs via vesicles.
• In the trans-Golgi, mature proteins are packaged into vesicles for specific compartments.

Targeting Vesicles

• Trans-Golgi vesicles are coated with specific v-SNARE proteins that allow them to be targeted to specific compartments.
• A complementary t-SNARE protein at the intended destination facilitates the correct targeting of the proteins.

Targeting to Lysosome

• Lysosomal proteins are synthesized in the ER and transported to the trans-Golgi network.
• Lysosomal proteins are packaged into vesicles targeted to endosomes and phagosomes, which convert them into lysosomes.
• Lysosomal proteins are tagged with mannose-6-phosphate in the Golgi, which directs them into correct transport vesicles.

Targeting to Mitochondria

• Targeting occurs after translation is completed but before complete protein folding occurs.
• The protein is complexed with a chaperone (HSP70), and a specific signal sequence is recognized by a receptor in the outer membrane of the mitochondria.

Targeting to Nucleus

• Targeting occurs after translation and folding are completed in the cytoplasm.
• Nuclear proteins contain a Nuclear Localization Signal (NLS), which binds to importin and is transported through the nuclear pore.
• Requires G-protein Ran and hydrolysis of GTP.

Targeting to Peroxisome

• Peroxin proteins have a three-amino acid sequence at the C-terminus (Ser-Lys-Leu), which is recognized by PEX5 and imported into the peroxisome.

Summary

• Specific sequences in proteins allow for targeting to specific compartments.
• Specific proteins bind to each targeting peptide sequence.
• Secreted/membrane/lysosomal proteins have a Signal Sequence, while nuclear/mitochondrial/peroxisome proteins have specific targeting sequences.
• Targeting occurs after translation is complete in the cytosol.

Learn about the ways proteins are targeted to organelles or secreted in eukaryotic cells, including the role of signal peptides and 'postcode' sequences. Test your understanding of protein targeting and its functions.