Evolution of the ribosomal exit tunnel through the eyes of the nascent chain.

The exit tunnel is a universally conserved feature of the ribosome that directs the nascent polypeptide into the cellular environment and is involved in co-translational folding, stalling, and antibiotic binding. While cryogenic electron microscopy has revealed variations in ribosome structure, tunnel definition and comparative analyses have largely relied on geometric algorithms. Here, we present a functional, nascent chain (NC)-centric characterization of the exit tunnel across the tree of life, derived from molecular dynamics simulations of 64 cytoplasmic ribosome structures. By mapping steric accessibility through the "eyes" of the NC at five distinct stages of translation, we reveal a topological and stage-dependent complexity invisible to geometric approaches, demonstrating how tunnel accessibility dynamically changes during biosynthesis. We identify transient, bacteria-specific lateral branches that, in archaeal and eukaryotic ribosomes, are structurally occluded by the eL39 protein and N-terminal extensions of the uL24 protein. These evolutionary "plugs" seal the tunnel wall and decrease its functional width. Collectively, our results demonstrate that the ribosome exit tunnel has a branched, lineage-specific topology where accessibility is temporally gated by NC length. This functional definition provides a new framework for understanding how ribosomal architecture modulates the early stages of protein biogenesis.