Rectification properties and pH-dependent selectivity of Meningoccal class 1 (PorA) porin

¹ Universidad Jaume I
² Vollum Institute, Oregon Health & Science University

^* To whom correspondence should be addressed. E-mail: aguilell{at}fca.uji.es.

Submitted on July 9, 2007
Revised on August 21, 2007
Accepted on 25 September 2007

	Abstract

We studied current rectification properties and selectivity of class 1 porin (PorA) from Neisseria Meningitidis (strain H44/76 34) reconstituted in planar lipid membranes varying salt concentrations and pH. PorA channel shows voltage gating with characteristic time remarkably longer than other porins. Its current-voltage asymmetry, evaluated as current rectification ratio, changes non-monotonically with salt concentration. Interestingly, it reaches its maximum value at physiological concentration. Porin selectivity, quantified by reversal potential measurements, is also significantly asymmetric. Depending on the direction of the salt gradient the channel becomes more or less selective (10:1 vs. 5:1 Na⁺ over Cl^- ). Besides, the reversal potential measurements suggest that porin inserts directionally following the concentration gradient. Measurements over a wide range of pH show that while PorA is strongly cation selective at pH>6, its selectivity gradually changes to anionic in an acidic medium (pH< 4). We show that a continuum electrodiffusion model quantitatively accounts for conductance and reversal potential measurements at positive and negative applied voltages.

Key Words: bacterial porin, channel selectivity, current rectification, electrodiffusion modeling