The Sec61 translocon forms a pore to translocate polypeptide sequences across the membrane and offers a lateral gate for membrane integration of hydrophobic (H) segments. A central constriction of six apolar residues has been shown to form a seal, but also to determine the hydrophobicity threshold for membrane integration: Mutation of these residues in yeast Sec61p to glycines, serines, aspartates, or lysines lowered the hydrophobicity required for integration; mutation to alanines increased it. Whereas four leucines distributed in an oligo-alanine H segment were sufficient for 50% integration, we now find four leucines in the N-terminal half of the H segment to produce significantly more integration than in the C-terminal half, suggesting functional asymmetry within the translocon. Scanning a cluster of three leucines through an oligo-alanine H segment showed high integration levels, except around the position matching that of the hydrophobic constriction in the pore where integration was strongly reduced. Both asymmetry and the position effect of H-segment integration disappeared upon mutation of the constriction residues to glycines or serines, demonstrating that hydrophobicity at this position within the translocon is responsible for the phenomenon. Asymmetry was largely retained, however, when constriction residues were replaced by alanines. These results reflect on the integration mechanism of transmembrane domains and show that membrane insertion of H segments strongly depends not only on their intrinsic hydrophobicity but also on the local conditions in the translocon interior. Thus, the contribution of hydrophobic residues in the H segment is not simply additive and displays cooperativeness depending on their relative position.protein translocation | transmembrane helix T he conserved Sec61/SecY translocon provides a passage for hydrophilic polypeptide sequences across the membrane of the endoplasmic reticulum (ER) or the bacterial plasma membrane (1). It consists of Sec61α (Sec61p in yeast) with 10 transmembrane domains and the single spanning proteins Sec61β (Sbh1p) and Sec61γ (Sss1p), corresponding in bacteria to SecY/ SecG/SecE. The translocon forms a compact helix bundle that can open a pore across the membrane with a lateral gate toward the lipid membrane. In the idle state, the central pore is closed by a constriction of six, almost invariably hydrophobic side chains and a luminal plug helix. The plug closes and stabilizes the inactive translocon and is displaced by translocating peptides, while the constriction residues form a gasket around them, keeping the translocon sealed to ions and small molecules (2-4).The lateral gate allows transmembrane helices (TMs) to insert into the lipid phase. Systematic quantitative analyses of membrane integration of mildly hydrophobic sequences (H segments) defined the contribution of each amino acid to the probability of insertion into or transfer across the bilayer (5-9). These studies yielded "biological hydrophobicity scales," similar to scales determined b...

Unlike constitutively secreted proteins, peptide hormones are stored in densely packed secretory granules, before regulated release upon stimulation. Secretory granules are formed at the TGN by self-aggregation of prohormones as functional amyloids. The nonapeptide hormone vasopressin, which forms a small disulfide loop, was shown to be responsible for granule formation of its precursor in the TGN as well as for toxic fibrillar aggregation of unfolded mutants in the ER. Several other hormone precursors also contain similar small disulfide loops suggesting their function as a general device to mediate aggregation for granule sorting. To test this hypothesis, we studied the capacity of small disulfide loops of different hormone precursors to mediate aggregation in the ER and the TGN. They indeed induced ER aggregation in Neuro-2a and COS-1 cells. Fused to a constitutively secreted reporter protein, they also promoted sorting into secretory granules, enhanced stimulated secretion, and increased Lubrol insolubility in AtT20 cells. These results support the hypothesis that small disulfide loops act as novel signals for sorting into secretory granules by self-aggregation.

No abstract

Unlike constitutively secreted proteins, peptide hormones are stored in densely packed secretory granules, before regulated release upon stimulation. Secretory granules are formed at the trans-Golgi network (TGN) by self-aggregation of prohormones as functional amyloids. The nonapeptide hormone vasopressin, which forms a small disulfide loop, was shown to be responsible for granule formation of its precursor in the TGN as well as for toxic fibrillar aggregation of unfolded mutants in the endoplasmic reticulum (ER). Several other hormone precursors also contain similar small disulfide loops suggesting their function as a general device to mediate aggregation for granule biogenesis. To test this hypothesis, we studied the capacity of small disulfide loops of different hormone precursors to mediate aggregation in the ER and the TGN. They indeed induced ER aggregation although to different extents in Neuro-2a and COS-1 cells. Fused to a constitutively secreted reporter protein, they also promoted sorting into secretory granules, enhanced stimulated secretion, and increased Lubrol insolubility in AtT20 cells. These results support the hypothesis that small disulfide loops act as novel signals for secretory granule biogenesis and sorting by self-aggregation.