A final interesting question is the ribosomal environment of the nascent peptide: in brief, is it a groove or a tunnel? Low-resolution crystallographic studies on the ribosome have suggested a tunnel in the 50S subunit. A. Johnson (University of Oklahoma) reported evidence of a different kind that also supports the notion of a tunnel. Using fluorescently labelled derivatives of lysine incorporated into aminoacyltRNA, and thence into preprolactin, coupled with an endoplasmic reticulum docking system, he has measured two relevant parameters. First, fluorescence lifetime distinguishe~ whether the nascent peptide is in an aqueous or nonpolar environment. Second, quenching by iodide distinguishes whether the nascent peptide is accessible to solvent.

The experiments showed that the nascent peptide is in an aqueous environment both in free ribosomes and after docking to the membrane. However, the peptide is accessible to quenching while in free ribosomes, but not after docking to the membrane. Thus, after docking, the peptide is in an aqueous but sealed environment preventing exchange with external solvent. Taken together, these findings indicate that the nascent peptide is in an aqueous tunnel rather than a surface groove on the ribosome, the tunnel becoming continuous via an externally sealed junction with a passage in the membrane after docking.