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Abstract

The "hypothetical protein" AQ_1862 was isolated from the membrane fraction of Aquifex aeolicus and identified as the major porin. In experiments with one conducting unit (molecule) a conductance of 1.4 nS was observed in 0.1 M KCl at pH 7.5. This stable (basic) conductance was superimposed by conductance fluctuations of approximately 0.25 nS. Because both events were always observed simultaneously, it is suggested that they are caused by the same molecular entity. Nonetheless they show very different properties. The basic conductance is anion selective at neutral pH with a conductance sequence Cl^- approximately Br^- approximately NO^-₃>F^->gluconate approximately acetate approximately propionate and does not saturate up to 0.5 M KCl. At alkaline pH and in the presence of large anions, it becomes unselective and the conductance saturates at low concentrations (K_m approximately 20 mM). In contrast the fluctuating component is mainly cation selective with a conductance sequence K⁺ approximately Rb⁺>NH⁺₄ >Na⁺ approximately Li⁺ approximately Cs⁺. It saturates at low salt concentrations (K_m approximately 15 mM) and is not affected by pH. In view of the diverging properties of both conductance components, it seems appropriate to assume that AQ_1862 has two different conducting pathways rather than one with two different open states.