Protein Targeting in Eukaryotic Cells

Protein Targeting

• Cellular function is compartmentalized, and proteins have to be directed to different parts of the cell, including organelles, membrane proteins, secreted proteins, and cytosolic proteins.
• Protein sequences act as "postcodes" to direct proteins to the correct location.

Targeting Proteins to the Endoplasmic Reticulum (ER)

• Targeting proteins to the ER involves a signal peptide sequence that is recognized by the Signal Recognition Particle (SRP) complex.
• The SRP complex binds to the signal sequence and targets the ribosome to the ER membrane.
• The protein is then guided through the translocon (translocation channel) in the ER membrane into the ER lumen, and the signal sequence is cleaved.

Protein Synthesis at the ER

• Protein synthesis starts with a cytosolic ribosome, and the signal sequence is the first part of the protein synthesized.
• The signal sequence is recognized by SRP, and the complex binds to the SRP-receptor on the ER membrane.

Membrane Proteins and the RER

• Stop transfer sequences (20-22 amino acids) remain as transmembrane segments of proteins.
• Most proteins synthesized in the RER are glycosylated, and a common preformed 14-sugar oligosaccharide is added to asparagine residues.

ER is the Starting Point for Many Proteins

• ER is the starting point for many proteins, which then reach their final destination through a series of intracellular transport steps involving membrane vesicles.
• Proteins are often further processed in vesicles.

ER – Golgi Interactions

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

Targeting Vesicles to Specific Compartments

• 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 Proteins to Lysosomes

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

Targeting Proteins to Mitochondria

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

Targeting Proteins to the Nucleus

• Nuclear proteins contain a Nuclear Localization Signal (NLS), which is recognized by importin and transported through the nuclear pore.
• The process requires G-protein Ran and hydrolysis of GTP.

Targeting Proteins to Peroxisomes

• Peroxin proteins have a three-amino-acid sequence (Ser-Lys-Leu) at the C-terminus, 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 that facilitates entry into the ER/Golgi system, and transport involves vesicles.
• Nuclear/mitochondrial/peroxisome proteins have specific targeting sequences, and targeting occurs after translation is complete in the cytosol.