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Rectification Properties and pH-Dependent Selectivity of Meningococcal Class 1 Porin

Javier Cervera ^*, Alexander G. Komarov  and Vicente M. Aguilella ^*

^* Departamento de Física, Universitat Jaume I, 12080 Castellón, Spain; and Vollum Institute, Oregon Health and Science University, Portland, Oregon

Correspondence: Address reprint requests to Vicente M. Aguilella, E-mail: aguilell{at}fca.uji.es.

We studied the 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 a characteristic time remarkably longer than other porins. Its current-voltage asymmetry, evaluated as the current rectification ratio, changes nonmonotonically 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⁺/Cl^–). Besides, the reversal potential measurements suggest that porin inserts directionally following the concentration gradient. Measurements over a wide range of pH show that although 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.