Expression of a poriferan potassium channel: insights into the evolution of ion channels in metazoans

Ion channels establish and regulate membrane potentials in excitable and non-excitable cells. How functional diversification of ion channels contributed to the evolution of nervous systems may be understood by studying organisms at key positions in the evolution of animal multicellularity. We have carried out the first analysis of ion channels cloned from a marine sponge, Amphimedon queenslandica. Phylogenetic comparison of sequences encoding for poriferan inward-rectifier K(+) (Kir) channels suggests that Kir channels from sponges, cnidarians and triploblastic metazoans each arose from a single channel and that duplications arose independently in the different groups. In Xenopus oocytes, AmqKirA and AmqKirB produced K(+) currents with strong inward rectification, as seen in the mammalian Kir2 channels, which are found in excitable cells. The pore properties of AmqKir channels demonstrated strong K(+) selectivity and block by Cs(+) and Ba(2+). We present an original analysis of sponge ion channel physiology and an examination of the phylogenetic relationships of this channel with other cloned Kir channels.