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.
Crowley
K S
KS
Department of Chemistry and Biochemistry, University of Oklahoma, Norman 73019.
Reinhart
G D
GD
Johnson
A E
AE
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