Department of Physiology, University of California, San Francisco, San Francisco, CA 94158, USA.
Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158, USA.
Cell Rep. 2019 Jul 9;28(2):567-579.e4. doi: 10.1016/j.celrep.2019.06.023.
As a Ca-activated lipid scramblase and ion channel that mediates Ca influx, TMEM16F relies on both functions to facilitate extracellular vesicle generation, blood coagulation, and bone formation. How a bona fide ion channel scrambles lipids remains elusive. Our structural analyses revealed the coexistence of an intact channel pore and PIP-dependent protein conformation changes leading to membrane distortion. Correlated to the extent of membrane distortion, many tightly bound lipids are slanted. Structure-based mutagenesis studies further reveal that neutralization of some lipid-binding residues or those near membrane distortion specifically alters the onset of lipid scrambling, but not Ca influx, thus identifying features outside of channel pore that are important for lipid scrambling. Together, our studies demonstrate that membrane distortion does not require open hydrophilic grooves facing the membrane interior and provide further evidence to suggest separate pathways for lipid scrambling and ion permeation.
作为一种钙激活的脂质翻转酶和离子通道，TMEM16F 通过这两种功能介导钙内流，促进细胞外囊泡的产生、血液凝固和骨形成。真正的离子通道如何翻转脂质仍然难以捉摸。我们的结构分析揭示了完整的通道孔和依赖于 PIP 的蛋白质构象变化的共存，导致膜的扭曲。与膜扭曲的程度相关，许多紧密结合的脂质呈倾斜状态。基于结构的突变研究进一步表明，中和一些脂质结合残基或那些靠近膜扭曲的残基特异性地改变脂质翻转的开始，但不改变 Ca 内流，因此确定了通道孔外对于脂质翻转很重要的特征。总之，我们的研究表明，膜的扭曲不需要亲水沟槽面对膜内部开放，并提供了进一步的证据表明脂质翻转和离子渗透的途径是分开的。
