The signal sequence moves through a ribosomal tunnel into a noncytoplasmic aqueous environment at the ER membrane early in translocation.
The signal sequence is in an aqueous milieu at an early stage in the translocation of a nascent secretory protein across the endoplasmic reticulum membrane. This was determined using fluorescent probes incorporated into the signal sequence of fully assembled ribosome-nascent chain-membrane complexes: the fluorescence lifetimes revealed that the probes were in an aqueous environment rather than buried in the nonpolar core of the membrane. Since these membrane-bound probes were not susceptible to collisional quenching by iodide ions, the space containing the signal sequence is sealed off from the cytoplasm by a tight ribosome-membrane junction. The nascent chain inside the ribosome is also not exposed to the cytoplasm and apparently passes through an aqueous tunnel in the ribosome.
Department of Chemistry and Biochemistry, University of Oklahoma, Norman 73019.
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Crowley
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Reinhart
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Johnson
D000818:Animals; D001692:Biological Transport; D002417:Cattle; D002451:Cell Compartmentation; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005456:Fluorescent Dyes; D007425:Intracellular Membranes / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000378:metabolism*; D012270:Ribosomes / Q000502:physiology*
D005456:Fluorescent Dyes; D021382:Protein Sorting Signals