Song J, Minetti C A, Blake M S, Colombini M
University of Maryland, Department of Biology, College Park, Maryland 20742, USA.
Biophys J. 1999 Feb;76(2):804-13. doi: 10.1016/S0006-3495(99)77244-9.
Class 1 porins (PorA/C1) from Neisseria meningitidis achieve both high selectivity and high conductance. The channel is highly selective (24:1 Na+ over Cl-), suggesting a highly negatively charged selectivity filter. The trimeric nature of PorA/C1 accounts for part of the enormous conductance in 200 mM NaCl (0.97nS). However, the currents that can be achieved exceed the simple infinite-sink calculation for a pore 0.7 nm in radius (estimated from nonelectrolyte permeability). The conductance is linear with salt activity from 20 mM to 2.0 M NaCl with no sign of saturation at low salt. Impermeant polymers reduce the conductance in a manner consistent with their ability to reduce bulk conductivity. Extrapolating from the known structure of homologous porins, the selectivity filter is likely to be small and localized. If small and highly negatively charged ( approximately 9 charges), the predicted conductance would be an order of magnitude higher than that observed. The rate at which ions reach the selectivity filter seems to limit overall ionic flux. PorA/C1 rectifies strongly, and this rectification can be accounted for by calculated differences in the voltage and concentration profiles in the access regions. Thus, it appears that the conductance of this channel is determined by the access resistance and the selectivity by a highly-conductive filter.
脑膜炎奈瑟菌的1类孔蛋白（PorA/C1）兼具高选择性和高电导率。该通道具有高度选择性（Na⁺对Cl⁻的选择性为24:1），表明存在一个带高度负电荷的选择性过滤器。PorA/C1的三聚体性质是其在200 mM NaCl中具有巨大电导率（0.97 nS）的部分原因。然而，所测得的电流超过了根据半径为0.7 nm的孔（由非电解质渗透率估算）进行的简单无限汇计算结果。在20 mM至2.0 M NaCl范围内，电导率与盐活度呈线性关系，低盐时无饱和迹象。非渗透性聚合物以与其降低整体电导率能力一致的方式降低电导率。根据同源孔蛋白的已知结构推断，选择性过滤器可能较小且位置固定。如果其较小且带高度负电荷（约9个电荷），预测的电导率将比观察到的高一个数量级。离子到达选择性过滤器的速率似乎限制了整体离子通量。PorA/C1具有强烈的整流作用，这种整流作用可以通过计算得出的进入区域电压和浓度分布差异来解释。因此，该通道的电导率似乎由进入电阻决定，而选择性则由高导电过滤器决定。
