Overview
This lecture introduces protein trafficking in eukaryotic cells, focusing on how proteins are directed to the mitochondria, endoplasmic reticulum (ER), and nucleus through specific signal sequences.
Protein Trafficking Overview
- Protein trafficking directs proteins to their correct cellular locations via signal sequences.
- Most ribosomes begin translating in the cytosol; only some move to the ER membrane when translating proteins destined for the endomembrane system.
- Proteins use different pathways and mechanisms for localization to organelles like ER, mitochondria, and nucleus.
Signal Sequences and Sorting Mechanisms
- Signal sequences are stretches of amino acids within a protein acting as "zip codes" for cellular destination.
- ER signal sequence: N-terminal, hydrophobic stretch followed by charged amino acids; directs ribosome and protein to the ER (co-translational transport).
- ER retention signal (e.g., KDEL sequence) keeps proteins in the ER.
- Mitochondrial signal sequence: N-terminal, interspersed positively charged amino acids; directs proteins post-translationally in an unfolded state to mitochondria.
- Nuclear localization signal (NLS): Surface-exposed, rich in positively charged amino acids, directs fully folded proteins into the nucleus.
- Nuclear export signal allows proteins to exit the nucleus; distinct from import signals.
- Peroxisome targeting signal: Serine-lysine-leucine (not discussed in detail).
Protein Translocation Pathways
- Proteins for mitochondria and ER move into organelles unfolded through translocation channels and have their N-terminal signals cleaved after import.
- Nuclear pores allow fully folded proteins to pass; import/export signals remain part of the folded protein and are not cleaved.
- Proteins destined for the endomembrane system must enter the ER first; no retention signal means they are secreted by default.
Mechanism of Signal Sequence Action
- Signal sequences serve as binding sites for carrier proteins (e.g., SRP for ER, importin for nucleus).
- These carrier proteins recognize the signal and guide the protein to the correct cellular compartment.
Experimental Visualization
- Fluorescence microscopy (e.g., GFP-tagged proteins) enables visualization of protein localization and movement in live cells.
Key Terms & Definitions
- Signal Sequence — amino acid motif directing a protein's cellular location.
- SRP (Signal Recognition Particle) — binds ER signal sequence, guides ribosome/protein to ER membrane.
- KDEL Sequence — ER retention signal (Lys-Asp-Glu-Leu).
- NLS (Nuclear Localization Signal) — directs proteins into the nucleus.
- Importin/Exportin — proteins that mediate nuclear import/export via NLS and export signals.
- Co-translational Transport — protein import into ER during translation.
- Post-translational Transport — protein import into organelles after translation, typically unfolded.
Action Items / Next Steps
- Watch the recommended movies on ER and mitochondrial import and fluorescence microscopy.
- Review posted slides on Moodle for diagrams and summary tables.
- Prepare for the next lecture focusing on the detailed mechanism of nuclear import and export.