Protein AQ_1862 from the hyperthermophilic bacterium Aquifex aeolicus is a porin and contains two conductance pathways of different selectivity

¹ Max Planck Institute of Biophysics

^* To whom correspondence should be addressed. E-mail: hartung{at}mpibp-frankfurt.mpg.de.

Submitted on March 13, 2007
Revised on April 13, 2007
Accepted on 13 June 2007

	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 about 0.2 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^->gluconateacetatepropionate 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 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 (K_m 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.

Key Words: Aquifex aeolicus, Ion channel, Ion transport, Porin, Selectivity, membrane