Orientation of internal signal-anchor sequences at the Sec61 translocon.

Translocation and insertion of secretory and membrane proteins at the endoplasmic reticulum are mediated by the Sec61 translocon. Evidence from in vivo as well as in vitro experiments indicates that N-terminal signal-anchor sequences initially insert N-first before they invert their orientation to translocate the C-terminus. Inversion is driven by flanking charges according to the positive-inside rule and inhibited by increased signal hydrophobicity. Here, we show that upon extending the N-terminal hydrophilic domain preceding the signal core to more than ~20 residues, the insertion behavior changes. Apparent signal inversion and the effect of hydrophobicity are largely lost, suggesting that N-first insertion is limited to N-terminal signal anchors. Extended N-domains sterically hinder N-translocation in a length-dependent manner also for reverse signal anchors with inverted flanking charges. The results indicate a mechanistic difference in the insertion process of N-terminal and internal signal sequences.

The hydrophobic core of the Sec61 translocon defines the hydrophobicity threshold for membrane integration.

The Sec61 translocon mediates the translocation of proteins across the endoplasmic reticulum membrane and the lateral integration of transmembrane segments into the lipid bilayer. The structure of the idle translocon is closed by a lumenal plug domain and a hydrophobic constriction ring. To test the function of the apolar constriction, we have mutated all six ring residues of yeast Sec61p to more hydrophilic, bulky, or even charged amino acids (alanines, glycines, serines, tryptophans, lysines, or aspartates). The translocon was found to be surprisingly tolerant even to the charge mutations in the constriction ring, because growth and translocation efficiency were not drastically affected. Most interestingly, ring mutants were found to affect the integration of hydrophobic sequences into the lipid bilayer, indicating that the translocon does not simply catalyze the partitioning of potential transmembrane segments between an aqueous environment and the lipid bilayer but that it also plays an active role in setting the hydrophobicity threshold for membrane integration.