Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093-0307, USA.
J Biomol NMR. 2014 Jan;58(1):69-81. doi: 10.1007/s10858-013-9806-y. Epub 2013 Dec 20.
Oriented sample solid-state NMR spectroscopy can be used to determine the three-dimensional structures of membrane proteins in magnetically or mechanically aligned lipid bilayers. The bottleneck for applying this technique to larger and more challenging proteins is making resonance assignments, which is conventionally accomplished through the preparation of multiple selectively isotopically labeled samples and performing an analysis of residues in regular secondary structure based on Polarity Index Slant Angle (PISA) Wheels and Dipolar Waves. Here we report the complete resonance assignment of the full-length mercury transporter, MerF, an 81-residue protein, which is challenging because of overlapping PISA Wheel patterns from its two trans-membrane helices, by using a combination of solid-state NMR techniques that improve the spectral resolution and provide correlations between residues and resonances. These techniques include experiments that take advantage of the improved resolution of the MSHOT4-Pi4/Pi pulse sequence; the transfer of resonance assignments through frequency alignment of heteronuclear dipolar couplings, or through dipolar coupling correlated isotropic chemical shift analysis; (15)N/(15)N dilute spin exchange experiments; and the use of the proton-evolved local field experiment with isotropic shift analysis to assign the irregular terminal and loop regions of the protein, which is the major "blind spot" of the PISA Wheel/Dipolar Wave method.
定向样品固态 NMR 光谱学可用于确定在磁或机械定向脂质双层中膜蛋白的三维结构。将该技术应用于更大和更具挑战性的蛋白质的瓶颈是进行共振分配,这通常是通过制备多个选择性同位素标记的样品并基于极性指数倾斜角(PISA)轮和偶极波分析常规二级结构中的残基来完成的。在这里,我们报告了全长汞转运蛋白 MerF 的完整共振分配,这是一个 81 个残基的蛋白质,由于其两个跨膜螺旋的 PISA 轮模式重叠,因此具有挑战性,通过使用固态 NMR 技术的组合来提高光谱分辨率并提供残基和共振之间的相关性。这些技术包括利用 MSHOT4-Pi4/Pi 脉冲序列提高分辨率的实验;通过异核偶极耦合的频率对准或通过偶极耦合相关各向同性化学位移分析转移共振分配;(15)N/(15)N 稀自旋交换实验;以及使用具有各向同性位移分析的质子演化局部场实验来分配蛋白质的不规则末端和环区,这是 PISA 轮/偶极波方法的主要“盲点”。