Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia.
Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, Russia.
Biophys J. 2021 Dec 7;120(23):5309-5321. doi: 10.1016/j.bpj.2021.10.030. Epub 2021 Oct 27.
Gramicidin A (gA) is a hydrophobic pentadecapeptide readily incorporating into a planar bilayer lipid membrane (BLM), thereby inducing a large macroscopic current across the BLM. This current results from ion-channel formation due to head-to-head transbilayer dimerization of gA monomers with rapidly established monomer-dimer equilibrium. Any disturbance of the equilibrium, e.g., by sensitized photoinactivation of a portion of gA monomers, causes relaxation toward a new equilibrium state. According to previous studies, the characteristic relaxation time of the gA-mediated electric current decreases as the current increases upon elevating the gA concentration in the membrane. Here, we report data on the current relaxation kinetics for gA analogs with N-terminal valine replaced by glycine or tyrosine. Surprisingly, the relaxation time increased rather than decreased upon elevation of the total membrane conductance induced by these gA analogs, thus contradicting the classical kinetic scheme. We developed a general theoretical model that accounts for lateral interaction of monomers and dimers mediated by membrane elastic deformations. The modified kinetic scheme of the gramicidin dimerization predicts the reverse dependence of the relaxation time on membrane conductance for gA analogs, with a decreased dimerization constant that is in a good agreement with our experimental data. The equilibration process may be also modulated by incorporation of other peptides ("impurities") into the lipid membrane.
短杆菌肽 A(gA)是一种疏水性十五肽,很容易整合到平面双层脂质膜(BLM)中,从而在 BLM 中诱导出大的宏观电流。该电流是由于 gA 单体的头对头跨膜二聚化形成离子通道引起的,单体-二聚体平衡迅速建立。任何平衡的干扰,例如,通过部分 gA 单体的敏化光失活,都会导致向新的平衡状态松弛。根据以前的研究,在膜中 gA 浓度升高时,gA 介导的电流的特征弛豫时间随着电流的增加而减小。在这里,我们报告了 N 端缬氨酸被甘氨酸或酪氨酸取代的 gA 类似物的电流弛豫动力学数据。令人惊讶的是,这些 gA 类似物引起的总膜电导升高时,弛豫时间增加而不是减小,这与经典动力学方案相矛盾。我们开发了一个通用的理论模型,该模型考虑了通过膜弹性变形介导的单体和二聚体的横向相互作用。改良的 gA 二聚化动力学方案预测了 gA 类似物的弛豫时间与膜电导的反向依赖性,二聚化常数降低,与我们的实验数据吻合良好。平衡过程也可能受到其他肽(“杂质”)掺入脂质膜的调节。