Stage- and ribosome-specific alterations in nascent chain-Sec61p interactions accompany translocation across the ER membrane.

Near-neighbor interactions between translocating nascent chains and Sec61p were investigated by chemical cross-linking. At stages of translocation before signal sequence cleavage, nascent chains could be cross-linked to Sec61p at high (60-80%) efficiencies. Cross-linking occurred through the signal sequence and the mature portion of wild-type and signal cleavage mutant nascent chains. At later stages of translocation, as represented through truncated translocation intermediates, cross-linking to Sec61p was markedly reduced. Dissociation of the ribosome into its large and small subunits after assembly of the precursor into the translocon, but before cross-linking, resulted in a dramatic reduction in subsequent cross-linking yield, indicating that at early stages of translocation, nascent chain-Sec61p interactions are in part mediated through interactions of the ribosome with components of the ER membrane, such as Sec61p. Dissociation of the ribosome was, however, without effect on subsequent translocation. These results are discussed with respect to a model in which Sec61p performs a function essential for the initiation of protein translocation.