Signal Anchor Sequence Provides Motive Force for Polypeptide Chain Translocation through the Endoplasmic Reticulum Membrane

Many proteins are translocated across and integrated into the endoplasmic reticulum membrane. The type I signal anchor sequence mediates the translocation of its preceding region through the endoplasmic reticulum membrane, but the source of the motive force has been unclear. Here, we characterized the motive force for N-terminal domain translocation using two probes. First, an Ig-like domain of the muscle protein titin (I27 domain) or its mutants were fused to the N termini, and translocation was examined in a cell-free translation system supplemented with rough microsomal membrane. The N-terminal translocation efficiencies correlated with the mechanical instabilities of the I27 mutants. When the I27 domain was separated from the signal anchor sequence by inserting a spacer, even the most unstable mutant stalled on the cytoplasmic side, whereas its downstream portion spanned the membrane. Proline insertion into the signal anchor sequence also caused a large translocation defect. Second, a streptavidin-binding peptide tag was fused to the N terminus. Titration of streptavidin in the translation system allowed us to estimate the translocation motive force operative on the tag. The motive force was decreased by the proline insertion into the signal anchor sequence as well as by separation from the signal anchor sequence. When the streptavidin-binding peptide tag was separated from the signal anchor, the proline insertion did not induce further deficits in the motive force for the tag. On the basis of the findings obtained by using these two independent techniques, we conclude that the signal sequence itself provides the motive force for N-terminal domain translocation within a limited upstream region.