SynK protein permits potassium flux, as revelaed by its expression in K<sup>+</sup>-uptake deficient <i>E. coli</i> strain LB2003.

<p><b>A</b>) SynK (<i>Synechocystis sp</i>. PCC 6803; gi:16331771) is characterized by selectivity filter sequence (bold characters) in pore region (brown characters) and by six predicted transmembrane segments (S1–S6 represented by different colours). ClustalW (1.83) alignments of SynK sequence with KvAP from <i>Aeropyrum pernix</i> (gi:14601099) and KvLm from <i>Listeria monocytogenes</i> (gi:16411529), two depolarization-activated prokaryotic potassium channels, is shown. “*” - identical residues in all aligned sequences; “:”–conserved and “.” - semi-conserved substitutions. Definition of S1–S6 segments in latter proteins is shown according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010118#pone.0010118-Ruta1" target="_blank">[19]</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010118#pone.0010118-Santos1" target="_blank">[20]</a>, in different colours. In SynK S1–S6 segments were defined according to secondary structure predictions (Porter, SPLIT4, TMHMM2 algorithms) and adjusted taking into account delimitation of α-helices as inferred from crystal structure of KvAP (according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010118#pone.0010118-Santos1" target="_blank">[20]</a>). Conserved residues, functional in Kv gating, are shaded grey. Please note the presence of some of the highly conserved residues in the sensor sequence of Kv channels, such as K63 in S2 and P86 in S3 in SynK. Polar residues (S and Q) in S4 are shaded yellow. <b>B</b>) Complementation growth test of <i>E. coli</i> LB2003 cells by <i>SynK</i>. <i>E. coli</i> LB2003 was transformed with plasmid harbouring pPAB404-<i>SynK</i> or empty vector. KAT1, an <i>Arabidopsis</i> K channel, was also included as a positive control. Transformants were grown on media supplemented with different concentration of KCl. <b>C</b>) Potassium uptake by K<sup>+</sup>-depleted <i>E.coli</i> containing <i>SynK</i> or empty vector. Net K<sup>+</sup> uptake by <i>SynK</i>-expressing <i>E. coli</i> LB2003 cells and control cells harbouring empty vector were measured at 20 mM KCl. Data are averages ±SD of results from four independent experiments.</p>