50S ribosomal subunit polypeptide exit tunnel (null)

The 50S ribosomal subunit polypeptide exit tunnel is an unbranched, curved channel that connects the peptidyl transferase center of the large ribosomal subunit to its exterior, allowing the newly formed polypeptide to move out of the ribosome into the cytoplasm. The tunnel is lined by both ribosomal RNA and specific proteins (notably L4, L22, L23, L24, L29 in bacteria). Its diameter ranges from 10–20 Å, sufficient for accommodating linear or α-helical structures but too narrow for tertiary folding within the tunnel. The geometric and electrostatic properties of the tunnel play critical roles in regulating translation rates, influencing secondary structure formation, and mediating interactions with translation-regulating molecules such as antibiotics. Many antibiotics target this tunnel, binding to its constriction sites and blocking nascent peptide exit—thereby arresting protein synthesis in bacteria. While the tunnel itself is not a drug target in the traditional sense (being a structural feature), its involvement in drug binding makes it central to the molecular pharmacology of ribosome-targeting antibiotics. Mutations at the tunnel-lining regions are a common mechanism of antibiotic resistance. The tunnel also supports cotranslational folding and can stabilize the ribosome during translation by the physical presence of the nascent chain bridging the subunits.

Other names

nascent polypeptide exit tunnelribosomal exit tunnelNPET

Drugs bind in the exit tunnel, blocking passage of the nascent polypeptide and inhibiting peptide elongation, resulting in cessation of protein synthesis