Seoh S A, Busath D
Department of Physiology, Brown University, Providence, Rhode Island 02912, USA.
Biophys J. 1995 Jun;68(6):2271-9. doi: 10.1016/S0006-3495(95)80409-1.
Compared with alkali metal cations, formamidinium ions stabilize the gramicidin A channel molecule in monoolein bilayers (Seoh and Busath, 1993a). A similar effect is observed with N-acetyl gramicidin channel molecules in spite of the modified forces at the dimeric junction (Seoh and Busath, 1993b). Here we use electrophysiological measurements with tryptophan-to-phenylalanine-substituted gramicidin analogs to show that the formamidinium-induced channel molecule stabilization is eliminated when the four gramicidin tryptophans are replaced with phenylalanines in gramicidin M-. This suggests that the stabilization is mediated by the tryptophan side chains. Tryptophan residues 9, 13, and 15 must cooperate to produce the effect because replacement of any one of the three with phenylalanine significantly reduces stabilization; replacement of Trp-11 with phenylalanine causes negligible decrease in stabilization. In addition, formamidinium-related current-voltage supralinearity and open-channel noise are absent with gramicidin M-. When the lipid bilayer was formed with monoolein ether rather than monoolein ester, the channel lifetimes were reduced markedly and, at low voltage and relative to those in KCl solution, were decreased by a factor of 2, whereas the open-channel noise was unaffected and the current-voltage relation was only modestly affected. These results suggest that formamidinium modifies the state of the tryptophan side chains, which, in turn, affects channel lifetime, current-voltage supralinearity, and open-channel noise through interactions with water or lipid headgroup atoms including the lipid ester carbonyl.
与碱金属阳离子相比，甲脒离子可稳定单油精双层中的短杆菌肽A通道分子（Seoh和Busath，1993a）。尽管二聚体连接处的作用力有所改变，但在N-乙酰短杆菌肽通道分子中也观察到了类似的效应（Seoh和Busath，1993b）。在此，我们使用色氨酸到苯丙氨酸取代的短杆菌肽类似物进行电生理测量，结果表明，当短杆菌肽M-中的四个短杆菌肽色氨酸被苯丙氨酸取代时，甲脒诱导的通道分子稳定性消失。这表明这种稳定性是由色氨酸侧链介导的。色氨酸残基9、13和15必须协同作用才能产生这种效应，因为将这三个残基中的任何一个替换为苯丙氨酸都会显著降低稳定性；将Trp-11替换为苯丙氨酸对稳定性的降低可忽略不计。此外，短杆菌肽M-不存在与甲脒相关的电流-电压超线性和开放通道噪声。当脂质双层由单油精醚而非单油精酯形成时，通道寿命显著缩短，在低电压下且相对于KCl溶液中的通道寿命，缩短了2倍，而开放通道噪声未受影响，电流-电压关系仅受到适度影响。这些结果表明，甲脒改变了色氨酸侧链的状态，进而通过与水或脂质头基团原子（包括脂质酯羰基）的相互作用影响通道寿命、电流-电压超线性和开放通道噪声。
