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Why does insertion of transmembrane domains occur in the rough ER?

To elaborate on that question, why in the secretory pathway does the insertion of transmembrane domains into the membrane occur in the rough ER as the protein is translated and threaded across the ER membrane? This is the answer to a problem from a science textbook. I thought that integral proteins are first produced in the rough ER, modified in the Golgi apparatus, and then shipped in vesicles that fuse with the cell membrane. If that’s the case, wouldn’t insertion of the entire protein into the membrane happen when the vesicle fuses with the cell membrane?

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