Raymond L, Slatin S L, Finkelstein A
J Membr Biol. 1985;84(2):173-81. doi: 10.1007/BF01872215.
The E1 subgroup (E1, A, Ib, etc.) of antibacterial toxins called colicins are known to form voltage-dependent channels in planar lipid bilayers. The genes for colicins E1, A and Ib have been cloned and sequenced, making these channels interesting models for the widespread phenomenon of voltage dependence in cellular channels. In this paper we investigate ion selectivity and channel size--properties relevant to model building. Our major finding is that the colicin E1 channel is large, having a diameter of at least 8 A at its narrowest point. We established this from measurements of reversal potentials for gradients formed by salts of large cations or large anions. In so doing, we exploited the fact that the colicin channel is permeable to both cations and anions, and its relative selectivity to them is a function of pH. The channel is anion selective (Cl- over K+) in neutral membranes, and the degree of selectivity is highly dependent on pH. In negatively charged membranes, it becomes cation selective at pH's higher than about 5. Experiments with pH gradients cross the membrane suggest that titratable groups both within the channel lumen and near the channel ends affect the selectivity. Individual E1 channels have more than one open conductance state, all displaying comparable ion selectivity. Colicins A and Ib also exhibit pH-dependent ion selectivity, and appear to have even larger lumens than E1.
被称为大肠杆菌素的抗菌毒素的E1亚组（E1、A、Ib等）已知能在平面脂质双分子层中形成电压依赖性通道。大肠杆菌素E1、A和Ib的基因已被克隆和测序，这使得这些通道成为研究细胞通道中普遍存在的电压依赖性现象的有趣模型。在本文中，我们研究了与模型构建相关的离子选择性和通道大小特性。我们的主要发现是，大肠杆菌素E1通道很大，其最窄点的直径至少为8埃。我们通过测量由大阳离子或大阴离子的盐形成的梯度的反转电位来确定这一点。在此过程中，我们利用了大肠杆菌素通道对阳离子和阴离子都具有通透性，并且其对它们的相对选择性是pH的函数这一事实。该通道在中性膜中对阴离子具有选择性（Cl-优于K+），并且选择性程度高度依赖于pH。在带负电荷的膜中，在pH高于约5时它变为对阳离子具有选择性。用跨膜pH梯度进行的实验表明，通道腔内和通道末端附近的可滴定基团都会影响选择性。单个E1通道具有不止一种开放电导状态，所有这些状态都表现出相当的离子选择性。大肠杆菌素A和Ib也表现出pH依赖性离子选择性，并且似乎具有比E1更大的内腔。
