Summary
The ribosomal exit tunnel is a 100Å channel through which nascent polypeptides emerge. Its geometry permits α-helix formation and affects folding kinetics, while the vestibule region allows early tertiary contacts.
Key Points
- 1Tunnel is ~100Å long, accommodating 30-40 amino acids
- 2α-Helix formation occurs inside the tunnel
- 3Vestibule allows early tertiary contacts and small domain folding
- 4Stalling sequences enable regulatory checkpoints
- 5Exit site couples to chaperones and targeting machinery
The ribosomal exit tunnel shapes the earliest stages of protein folding, acting as a conduit that influences nascent chain conformation before the protein enters the cytosol.
Tunnel Architecture
Dimensions
- Length: ~100 Å (80-100 Å depending on species)
- Width: Varies from ~10 Å at constriction to ~20 Å at vestibule
- Volume: Accommodates 30-40 amino acids
Key Regions
1. Upper tunnel: Narrow, lined by ribosomal proteins L4 and L22
2. Constriction site: Narrowest point (~10 Å), regulatory checkpoint
3. Lower tunnel/vestibule: Widens near exit, allows secondary structure
Composition
- Primarily 23S rRNA (bacteria) / 28S rRNA (eukaryotes)
Folding Inside the Tunnel
α-Helix Formation
Tertiary Contacts
Folding Zones
- Zone 1 (PTC region): Extended chain only
- Zone 2 (upper tunnel): Helix nucleation possible
- Zone 3 (vestibule): Secondary structure, early tertiary contacts
- Zone 4 (exit): Full domain folding possible
Tunnel-Nascent Chain Interactions
- Electrostatic: Positively charged residues interact with rRNA
- Stalling sequences: Specific peptides (SecM, VemP) arrest translation
- Small molecules: Macrolide antibiotics bind tunnel, blocking transit