뒤로Protein Sorting and Transport Inside the Eukaryotic Cell
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Inside the Cell – Protein Sorting
Introduction
Cells are highly organized structures, and their functions depend on the precise localization of proteins within specific compartments. Protein sorting is the process by which newly synthesized proteins are directed to their correct destinations within or outside the cell. This process is essential for maintaining cellular organization and function.
Cellular Organization and Internal Structures
Main Components of Eukaryotic Cells
Nucleus: Contains the cell's genetic material and is the site of transcription.
Cytoplasm: The region outside the nucleus, containing organelles and the cytosol.
Endomembrane System: Includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles, which work together to modify, package, and transport proteins.
Mitochondria and Chloroplasts: Organelles responsible for energy production (mitochondria) and photosynthesis (chloroplasts in plants).
Cytoskeleton: A network of protein filaments that provides structural support and facilitates intracellular transport.
Protein Sorting: How Cells "Know" Where to Send Proteins
Signal Sequences
Proteins contain specific amino acid sequences called signal sequences that act as molecular "zip codes" to direct them to their correct cellular locations.
Signal sequences are typically 15–60 amino acids long.
They are found only in proteins that are destined to be transported from the cytoplasm to other organelles or the cell membrane.
Removal or addition of signal sequences can redirect the destination of a protein.
Examples of Signal Sequences
"Zip Code" | Signal Sequence |
|---|---|
Nuclear Localization | N...-Pro–Pro–Lys–Lys–Lys–Arg–Lys–Val–... |
ER Signal | N–Met–Met–Ser–Phe–Val–Ser–Leu–Leu–Leu–Val–Gly–Ile–Leu–Phe–Trp–Ala–Thr–Glu–Ala–Glu–Gln–Leu–Thr–Lys–Cys–Glu–Val–Phe–Gln–... |
Experimental Evidence: Signal Sequence Relocation
Proteins normally destined for the cytosol remain there if they lack an ER signal sequence.
If an ER signal sequence is added to a cytosolic protein, it will be directed to the ER.
Conversely, removing the ER signal sequence from an ER protein causes it to remain in the cytosol.
Example: Swapping signal sequences between proteins can change their final cellular destinations, demonstrating the importance of these sequences in protein sorting.
Protein Synthesis and Initial Sorting
Ribosomes and Protein Synthesis
All protein synthesis begins on free ribosomes in the cytosol.
Depending on the presence of a signal sequence, the ribosome may remain free or become attached to the ER membrane.
Proteins destined for the cytosol, mitochondria, chloroplasts, or nucleus are synthesized on free ribosomes.
Proteins destined for the endomembrane system or secretion are synthesized on ribosomes that become attached to the ER.
Mechanisms of Protein Transport
Co-translational vs. Post-translational Transport
Co-translational Transport: The protein is transported into the ER while it is still being synthesized by the ribosome.
Post-translational Transport: The protein is fully synthesized in the cytosol and then transported to its final destination (e.g., mitochondria, chloroplasts, nucleus).
Summary Table: Co-translational vs. Post-translational Transport
Transport Type | When Transport Occurs | Destination Examples |
|---|---|---|
Co-translational | During protein synthesis | ER, Golgi, lysosomes, plasma membrane, secretion |
Post-translational | After protein synthesis is complete | Mitochondria, chloroplasts, nucleus |
Transport into Organelles
Nuclear Transport
Proteins needed in the nucleus contain a nuclear localization signal (NLS).
Transport occurs through nuclear pore complexes that span the nuclear envelope.
Import and export are mediated by transport receptors and require energy (often in the form of GTP hydrolysis).
Transport into Mitochondria and Chloroplasts
Proteins are synthesized in the cytosol and kept unfolded by chaperone proteins (e.g., Hsp70).
Signal sequences direct the protein to receptors on the organelle membrane.
The protein is threaded through translocation channels and folds once inside the organelle.
The Endomembrane System and Protein Trafficking
Overview of the Endomembrane System
Includes the ER, Golgi apparatus, lysosomes, endosomes, and vesicles.
Responsible for the synthesis, modification, sorting, and transport of proteins and lipids.
Vesicular Transport and Protein Sorting
Proteins are packaged into vesicles for transport between organelles.
Vesicles bud from one compartment and fuse with another, delivering their cargo.
Sorting signals ensure that proteins reach their correct destinations.
Protein Modification and Targeting
Proteins may be modified by the addition of carbohydrates (glycosylation), lipids, or other groups in the ER and Golgi.
Properly modified proteins are sorted and sent to their final destinations, such as the plasma membrane, lysosomes, or secretion outside the cell.
Summary
Protein sorting is essential for cellular function and organization.
Signal sequences act as molecular addresses, directing proteins to their correct locations.
Transport mechanisms include co-translational and post-translational pathways, involving various organelles and the endomembrane system.
Additional info: For further reading, see Becker's World of the Cell, p. 553, for more details on protein sorting mechanisms.
