Zhang Yao, Kulp Daniel W, Lear James D, DeGrado William F
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, USA.
J Am Chem Soc. 2009 Aug 19;131(32):11341-3. doi: 10.1021/ja904625b.
The forces that define the interactions of transmembrane helices have been evaluated using a model membrane-soluble peptide (MS1), whose packing is modeled on the two-stranded coiled-coil from GCN4. The thermodynamic stability of water-soluble coiled-coils depends on the side chain at the buried "a" position of the repeat, favoring large hydrophobic residues over small side chains. Here we show that just the opposite is true for the membrane-soluble peptide. Analytical ultracentrifugation and equilibrium disulfide interchange show that the stability of MS1 is greatest when Gly is at each "a" position of the heptad repeat (MS1-Gly), followed by Ala > Val > Ile. Moreover, MS1-Gly has a strong tendency to form antiparallel dimers, MS1-Ala forms a mixture of parallel and antiparallel dimers, while MS1-Val and MS1-Ile have a preference to form parallel dimers. Calculations based on exhaustive conformational searching and rotamer optimization were in excellent agreement with experiments, in terms of the overall stability of the structures and the preference for parallel vs antiparallel packing. The MS1-Gly helices are able to achieve more favorable and uniform packing in an antiparallel dimer, while MS1-Val and MS1-Ile have more favorable van der Waals interactions in a parallel dimer. Finally, the electrostatic component arising from the partial charges of the backbones become significant in the antiparallel MS1-Gly and MS1-Ala conformations, due to close packing of the helices. Thus, van der Waals interactions and electrostatic interactions contribute to the stability and orientational preferences of the dimers.
利用一种模型膜溶性肽（MS1）评估了定义跨膜螺旋相互作用的力，该肽的堆积是基于GCN4的双链卷曲螺旋构建的。水溶性卷曲螺旋的热力学稳定性取决于重复序列中埋藏的“a”位置的侧链，相比于小侧链，大的疏水残基更受青睐。在这里我们表明，膜溶性肽的情况恰恰相反。分析型超速离心和平衡二硫键交换表明，当甘氨酸位于七肽重复序列的每个“a”位置时（MS1-Gly），MS1的稳定性最高，其次是丙氨酸>缬氨酸>异亮氨酸。此外，MS1-Gly有强烈的形成反平行二聚体的倾向，MS1-丙氨酸形成平行和反平行二聚体的混合物，而MS1-缬氨酸和MS1-异亮氨酸更倾向于形成平行二聚体。基于详尽的构象搜索和旋转异构体优化的计算在结构的整体稳定性以及平行与反平行堆积的偏好方面与实验结果高度一致。MS1-Gly螺旋在反平行二聚体中能够实现更有利和均匀的堆积，而MS1-缬氨酸和MS1-异亮氨酸在平行二聚体中有更有利的范德华相互作用。最后，由于螺旋的紧密堆积，主链部分电荷产生的静电成分在反平行的MS1-Gly和MS1-丙氨酸构象中变得显著。因此，范德华相互作用和静电相互作用有助于二聚体的稳定性和取向偏好。
