A site-specific cross-linking approach was used to study the integration of TM (transmembrane) segments 4-7 of the polytopic membrane protein, opsin, at the ER (endoplasmic reticulum). We found that although TM4 exits the ER translocon rapidly, TM segments 5, 6 and 7 are all retained at the translocon until opsin biosynthesis is terminated. Furthermore, although artificial extension of the nascent chain is not sufficient to release the C-terminal region of opsin from the translocon, substitution of the native TM segment 7 with a more hydrophobic TM segment results in its rapid lateral exit into the lipid bilayer. We conclude that the intrinsic properties of a TM segment determine the timing of its membrane integration rather than its relative location within the polypeptide chain. A pronounced and prolonged association of opsin TM5 with the translocon-associated component PAT-10 was also observed, suggesting that PAT-10 may facilitate the assembly of distinct opsin subdomains during membrane integration. The results of the present study strongly support a model in which the ER translocon co-ordinates the integration of selected TM segments in response to the specific requirements of the precursor being synthesized.