Biozentrum, University of Basel, 4056 Basel, Switzerland.
Institute of Biophysics, Johannes Kepler University Linz, 4020 Linz, Austria.
Proc Natl Acad Sci U S A. 2021 Aug 17;118(33). doi: 10.1073/pnas.2102675118.
The topology of most membrane proteins is defined by the successive integration of α-helical transmembrane domains at the Sec61 translocon. The translocon provides a pore for the transfer of polypeptide segments across the membrane while giving them lateral access to the lipid. For each polypeptide segment of ∼20 residues, the combined hydrophobicities of its constituent amino acids were previously shown to define the extent of membrane integration. Here, we discovered that different sequences preceding a potential transmembrane domain substantially affect its hydrophobicity requirement for integration. Rapidly folding domains, sequences that are intrinsically disordered or very short or capable of binding chaperones with high affinity, allow for efficient transmembrane integration with low-hydrophobicity thresholds for both orientations in the membrane. In contrast, long protein fragments, folding-deficient mutant domains, and artificial sequences not binding chaperones interfered with membrane integration, requiring higher hydrophobicity. We propose that the latter sequences, as they compact on their hydrophobic residues, partially folded but unable to reach a native state, expose hydrophobic surfaces that compete with the translocon for the emerging transmembrane segment, reducing integration efficiency. The results suggest that rapid folding or strong chaperone binding is required for efficient transmembrane integration.
大多数膜蛋白的拓扑结构是由α-螺旋跨膜结构域在 Sec61 转运蛋白中连续整合定义的。转运蛋白为跨膜转移多肽片段提供了一个孔，同时使它们侧向进入脂质。对于每个约 20 个残基的多肽片段，其组成氨基酸的组合疏水性先前被证明定义了膜整合的程度。在这里，我们发现一个潜在跨膜结构域之前的不同序列会显著影响其整合所需的疏水性。快速折叠结构域、内在无序或非常短的序列、或能够与伴侣蛋白高亲和力结合的序列，允许以低疏水性阈值在膜的两个方向上进行有效的跨膜整合。相比之下，长蛋白片段、折叠缺陷突变体结构域和不与伴侣蛋白结合的人工序列会干扰膜的整合，需要更高的疏水性。我们提出，后者的序列在其疏水性残基上聚集时，部分折叠但无法达到天然状态，暴露出与转运蛋白竞争新出现的跨膜片段的疏水面，降低整合效率。结果表明，快速折叠或与伴侣蛋白的强结合对于有效的跨膜整合是必需的。
