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Updated: May 29, 2026

Reconstitution of Msp1 Extraction Activity with Fully Purified Components
Published on: August 10, 2021
Transmembrane segments of nascent polytopic membrane proteins control cytosol/ER targeting during membrane
Pen-Jen Lin1, Candice G Jongsma, Shuren Liao
1Department of Molecular and Cellular Medicine, Texas A&M Health Science Center, College Station, TX 77843, USA.
Abstract:
During cotranslational integration of a eukaryotic multispanning polytopic membrane protein (PMP), its hydrophilic loops are alternately directed to opposite sides of the ER membrane. Exposure of fluorescently labeled nascent PMP to the cytosol or ER lumen was detected by collisional quenching of its fluorescence by iodide ions localized in the cytosol or lumen. PMP loop exposure to the cytosol or lumen was controlled by structural rearrangements in the ribosome, translocon, and associated proteins that occurred soon after a nascent chain transmembrane segment (TMS) entered the ribosomal tunnel. Each successive TMS, although varying in length, sequence, hydrophobicity, and orientation, reversed the structural changes elicited by its predecessor, irrespective of loop size. Fluorescence lifetime data revealed that TMSs occupied a more nonpolar environment than secretory proteins inside the aqueous ribosome tunnel, which suggests that TMS recognition by the ribosome involves hydrophobic interactions. Importantly, the TMS-triggered structural rearrangements that cycle nascent chain exposure between cytosolic and lumenal occur without compromising the permeability barrier of the ER membrane.
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