Endoplasmic Reticulum

ID #1272

I'm looking at a lecture slide that shows a transmembrane protein with the AA sequence at the N-terminus getting cleaved and inserted into the membrane. I have written that when a stop-transfer sequence is ecountered, the protein stops being threaded into the lumen and is ejected into the bilayer and the translocon is closed. I don't understand how the protein ends up embedded right next to the closed translocon. The picture makes it look as though the translocon would have to be permeable to the protein for it to sort of slide through? Ejecting makes me think of it just popping out and ending up outside of the ER in the cytoplasm.

If a translocon itself was composed of a single, hollow molecule, then it would indeed be impossible to squeeze past it -- you'd have to go up or down. But a translocon is actually composed of multiple parts, and it is possible to be shoved between two of the parts, "ejecting" you into the hydrophobic interior of the membrane.

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