Sabirov Ravshan Z, Okada Yasunobu
Department of Cell Physiology, National Institute for Physiological Sciences, Okazaki 444-8585, Japan.
Biophys J. 2004 Sep;87(3):1672-85. doi: 10.1529/biophysj.104.043174.
The newly proposed function of the maxi-anion channel as a conductive pathway for ATP release requires that its pore is sufficiently large to permit passage of a bulky ATP(4-) anion. We found a linear relationship between relative permeability of organic anions of different size and their relative ionic mobility (measured as the ratio of ionic conductance) with a slope close to 1, suggesting that organic anions tested with radii up to 0.49 nm (lactobionate) move inside the channel by free diffusion. In the second approach, we, for the first time, succeeded in pore sizing by the nonelectrolyte exclusion method in single-channel patch-clamp experiments. The cutoff radii of PEG molecules that could access the channel from intracellular (1.16 nm) and extracellular (1.42 nm) sides indicated an asymmetry of the two entrances to the channel pore. Measurements by symmetrical two-sided application of PEG molecules yielded an average functional pore radius of approximately 1.3 nm. These three estimates are considerably larger than the radius of ATP(4-) (0.57-0.65 nm) and MgATP(2-) (approximately 0.60 nm). We therefore conclude that the nanoscopic maxi-anion channel pore provides sufficient room to accommodate ATP and is well suited to its function as a conductive pathway for ATP release in cell-to-cell communication.
新提出的大阴离子通道作为ATP释放的传导途径这一功能，要求其孔足够大以允许体积较大的ATP(4-)阴离子通过。我们发现不同大小的有机阴离子的相对渗透率与其相对离子迁移率（以离子电导率之比衡量）之间存在线性关系，斜率接近1，这表明半径达0.49 nm（乳糖酸盐）的测试有机阴离子通过自由扩散在通道内移动。在第二种方法中，我们首次在单通道膜片钳实验中通过非电解质排斥法成功测定了孔径。能够从细胞内（1.16 nm）和细胞外（1.42 nm）两侧进入通道的PEG分子的截止半径表明通道孔的两个入口存在不对称性。通过对称地从两侧施加PEG分子进行测量，得出平均功能孔径约为1.3 nm。这三个估计值远大于ATP(4-)（0.57 - 0.65 nm）和MgATP(2-)（约0.60 nm）的半径。因此，我们得出结论，纳米级的大阴离子通道孔提供了足够的空间来容纳ATP，非常适合其在细胞间通讯中作为ATP释放的传导途径的功能。
