Fig. 2. The exit of TM1 from the ER translocon is facilitated by C-terminal TMs in the opsin nascent chain. A set of integration intermediates containing only opsin TM1 with a single cysteine probe [Cys56] were analysed by crosslinking with BMH as described for Fig. 1. The remainder of the opsin sequence including the C-terminal TMs was replaced with hydrophilic sequence derived from preprolactin. Nascent chain lengths identical to those studied with opsin were investigated, and all symbols are as defined in the legend to Fig. 1.