Meningococcal PorA/C1, a channel that combines high conductance and high selectivity

Class 1 porins (PorA/C1) from Neisseria meningitidis achieve both high sele ctivity and high conductance. The channel is highly selective (24:1 Na+ ove r Cl-), suggesting a highly negatively charged selectivity filter. The trim eric 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 si mple 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 homo logous porins, the selectivity filter is likely to be small and localized. If small and highly negatively charged (similar to 9 charges), the predicte d conductance would be an order of magnitude higher than that observed. The rate at which ions reach the selectivity filter seems to limit overall ion ic flux. PorA/C1 rectifies strongly, and this rectification can be accounte d for by calculated differences in the voltage and concentration profiles i n the access regions. Thus, it appears that the conductance of this channel is determined by the access resistance and the selectivity by a highly-con ductive filter.