= Ribosomal exit tunnel
The <ribosomal exit tunnel> is the passage through the <large ribosomal subunit> leading from the <peptidyl transferase center> toward the exterior of the <ribosome>. The growing <polypeptide> enters this passage during <biological translation>. Its walls contain <ribosomal RNA> together with contributions from <ribosomal proteins>; the tunnel is part of the molecular machine, rather than an opening through the <cell membrane>.<The structural basis of ribosome activity in peptide bond synthesis by Poul Nissen and others>[Abstract, RNA and protein contributions to the polypeptide exit tunnel]{ref}
The tunnel can also help distinguish particular nascent sequences. A 2002 experiment identified a <SecM> peptide motif that arrests elongation. Mutations in <23S ribosomal RNA> or <ribosomal protein L22> could bypass this arrest; the affected residues faced the tunnel's constriction. Changing tunnel components changed the response to the nascent product, supporting a functional interaction between the <polypeptide> and its surroundings.<The ribosomal exit tunnel functions as a discriminating gate by Hitoshi Nakatogawa and Koreaki Ito>[Abstract, SecM motif and suppressor mutations]{ref}
Consider two chains containing the same number of <amino acid residues> in different orders. Equal length alone would not imply identical passage or stalling behaviour, because their side chains encounter different local environments. The SecM result makes that distinction experimentally meaningful: the <ribosomal exit tunnel> can participate in sequence-dependent regulation, while remaining the route followed by many unrelated <polypeptides>.
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