Protein AQ_1862 from the Hyperthermophilic Bacterium Aquifex aeolicus Is a Porin and Contains Two Conductance Pathways of Different Selectivity

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 ∼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−≈Br−≈NO3−>F−> gluconate ≈ acetate ≈ 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 (Km ≈ 20 mM). In contrast the fluctuating component is mainly cation selective with a conductance sequence K+≈Rb+>NH4+>Na+≈Li+≈Cs+. It saturates at low salt concentrations (Km ≈ 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.