Voltage-dependent porin-like ion channels in the archaeon Haloferax volcanii

Membrane vesicles isolated from the cell envelope of the archaebacterium Haloferax volcanii were either reconstituted in giant liposomes and examined by the patch-clamp technique or were fused into planar lipid bilayers. In addition, cell envelope proteins were solubilized by detergent and reconstituted in azolectin liposomes, which were then fused into planar lipid bilayers. Independently of the technique used the predominant channel activity encountered exhibited the following characteristics. Channels were open at all voltages in the range approximately -120 to +120 mV and exhibited frequent fast transitions to closed levels of different amplitudes. At voltages greater than 120 mV the channels tended to close in a manner characterized by large, slow transitions of variable amplitudes. The tendency to close at high membrane potentials was much stronger at one polarity. The channel gating pattern was complex exhibiting a range of subconductances of 10-300 picosiemens in symmetric 100 mM KCl. These electrophysiological characteristics are comparable with those of bacterial and mitochondrial porins, suggesting that the archaeal channels may belong to the general class of porin channels. Some channels showed preference for K+, whereas the others preferred Cl-, suggesting the existence of at least two types of porin-like channels in H. volcanii.