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荧光标记的功能性 nhTMEM16 翻转酶在脂质双层中的结构映射。

Structural mapping of fluorescently-tagged, functional nhTMEM16 scramblase in a lipid bilayer.

机构信息

Department of Biochemistry, Weill Cornell Medical College, New York, New York 10065.

Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York 12180.

出版信息

J Biol Chem. 2018 Aug 3;293(31):12248-12258. doi: 10.1074/jbc.RA118.003648. Epub 2018 Jun 14.

Abstract

Most members of the TransMEMbrane protein 16 (TMEM16) family are Ca-regulated scramblases that facilitate the bidirectional movement of phospholipids across membranes necessary for diverse physiological processes. The nhTMEM16 scramblase (from the fungus ) is a homodimer with a large cytoplasmic region and a hydrophilic, membrane-exposed groove in each monomer. The groove provides the transbilayer conduit for lipids, but the mechanism by which Ca regulates it is not clear. Because fusion of large protein tags at either the N or C terminus abolishes nhTMEM16 activity, we hypothesized that its cytoplasmic portion containing both termini may regulate lipid translocation via a Ca-dependent conformational change. To test this hypothesis, here we used fluorescence methods to map key distances within the nhTMEM16 homodimer and between its termini and the membrane. To this end, we developed functional nhTMEM16 variants bearing an acyl carrier protein (ACP) tag at one or both of the termini. These constructs were fluorescently labeled by ACP synthase-mediated insertion of CoA-conjugated fluorophores and reconstituted into vesicles containing fluorescent lipids to obtain the distance of closest approach between the labeled tag and the membrane via FRET. Fluorescence lifetime measurements with phasor analysis were used to determine the distance between the N and C termini of partnering monomers in the nhTMEM16 homodimer. We now report that the measured distances do not vary significantly between Ca-replete and EGTA-treated samples, indicating that whereas the cytoplasmic portion of the protein is important for function, it does not appear to regulate scramblase activity via a detectable conformational change.

摘要

大多数跨膜蛋白 16(TMEM16)家族成员都是 Ca 调节的翻转酶,可促进磷脂在膜之间的双向运动,这是多种生理过程所必需的。来自真菌的 nhTMEM16 翻转酶是一种同二聚体,具有较大的细胞质区域和每个单体中的亲水、膜暴露的槽。该槽提供了用于脂质的跨膜导管,但 Ca 调节它的机制尚不清楚。由于在 N 或 C 末端融合大的蛋白质标签会使 nhTMEM16 失活,因此我们假设其包含两个末端的细胞质部分可能通过 Ca 依赖性构象变化来调节脂质易位。为了验证这一假设,我们使用荧光方法来绘制 nhTMEM16 同二聚体内部以及其末端与膜之间的关键距离。为此,我们开发了具有一个或两个末端酰基辅酶 A(ACP)标签的功能性 nhTMEM16 变体。这些构建体通过 ACP 合酶介导的 CoA 缀合荧光团的插入进行荧光标记,并在含有荧光脂质的囊泡中进行重建,以通过 FRET 获得标记标签与膜之间的最近距离。使用相分析的荧光寿命测量来确定 nhTMEM16 同二聚体中配对单体的 N 和 C 末端之间的距离。我们现在报告说,在 Ca 充足和 EGTA 处理的样品之间,测量的距离没有显着差异,这表明尽管蛋白质的细胞质部分对于功能很重要,但它似乎不会通过可检测的构象变化来调节翻转酶活性。

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