Girshman J, Greathouse D V, Koeppe R E, Andersen O S
Department of Physiology and Biophysics, Cornell University Medical College, New York, New York 10021, USA.
Biophys J. 1997 Sep;73(3):1310-9. doi: 10.1016/S0006-3495(97)78164-5.
In organic solvents gramicidin A (gA) occurs as a mixture of slowly interconverting double-stranded dimers. Membrane-spanning gA channels, in contrast, are almost exclusively single-stranded beta(6,3)-helical dimers. Based on spectroscopic evidence, it has previously been concluded that the conformational preference of gA in phospholipid bilayers varies as a function of the degree of unsaturation of the acyl chains. Double-stranded pi pi(5,6)-helical dimers predominate (over single-stranded beta(6,3)-helical dimers) in lipid bilayer membranes with polyunsaturated acyl chains. We therefore examined the characteristics of channels formed by gA in 1-palmitoyl-2-oleoylphosphatidylcholine/n-decane, 1,2-dioleoylphosphatidylcholine/n-decane, and 1,2-dilinoleoylphosphatidylcholine/n-decane bilayers. We did not observe long-lived channels that could be conducting double-stranded pi pi(5,6)-helical dimers in any of these different membrane environments. We conclude that the single-stranded beta(6,3)-helical dimer is the only conducting species in these bilayers. Somewhat surprisingly, the average channel duration and channel-forming potency of gA are increased in dilinoleoylphosphatidylcholine/n-decane bilayers compared to 1-palmitoyl-2-oleoylphosphatidylcholine/n-decane and dioleoylphosphatidylcholine/n-decane bilayers. To test for specific interactions between the aromatic side chains of gA and the acyl chains of the bilayer, we examined the properties of channels formed by gramicidin analogues in which the four tryptophan residues were replaced with naphthylalanine (gN), tyrosine (gT), and phenylalanine (gM). The results show that all of these analogue channels experience the same relative stabilization when going from dioleoylphosphatidylcholine to dilinoleoylphosphatidylcholine bilayers.
在有机溶剂中，短杆菌肽A（gA）以缓慢相互转化的双链二聚体混合物形式存在。相比之下，跨膜gA通道几乎完全是单链β(6,3)-螺旋二聚体。基于光谱学证据，此前已得出结论，gA在磷脂双层中的构象偏好随酰基链不饱和程度而变化。在具有多不饱和酰基链的脂质双层膜中，双链ππ(5,6)-螺旋二聚体占主导（超过单链β(6,3)-螺旋二聚体）。因此，我们研究了gA在1-棕榈酰-2-油酰磷脂酰胆碱/正癸烷、1,2-二油酰磷脂酰胆碱/正癸烷和1,2-二亚油酰磷脂酰胆碱/正癸烷双层膜中形成的通道的特性。在这些不同的膜环境中，我们均未观察到能够传导双链ππ(5,6)-螺旋二聚体的长寿通道。我们得出结论，单链β(6,3)-螺旋二聚体是这些双层膜中唯一具有传导性的物种。有点令人惊讶的是，与1-棕榈酰-2-油酰磷脂酰胆碱/正癸烷和二油酰磷脂酰胆碱/正癸烷双层膜相比，gA在二亚油酰磷脂酰胆碱/正癸烷双层膜中的平均通道持续时间和通道形成能力有所增加。为了测试gA的芳香族侧链与双层膜的酰基链之间的特异性相互作用，我们研究了由短杆菌肽类似物形成的通道的特性，其中四个色氨酸残基被萘丙氨酸（gN）、酪氨酸（gT）和苯丙氨酸（gM）取代。结果表明，从二油酰磷脂酰胆碱双层膜转变为二亚油酰磷脂酰胆碱双层膜时，所有这些类似物通道都经历了相同的相对稳定性变化。
