The bacterial K + channel from Streptomyces lividans (KcsA) is one of the most widely used model for the homologous human Kir channels . KcsA is an homotetramer protein of 160 amino acids assembled in 4 transmembrane αhelices per monomer. According to the crystallographic structure of its 124 amino acid transmembrane fragment , KcsA contains the four monomers packed asymmetrically around a central hydrophilic pore that serves for the K + flow (Figure , top view). Its N and C-terminal sequences face the intracellular cytoplasm. Although less studied than the transmembrane α-helices, KcsA Cterminal sequences have been also implicated in controlling the K + flow . The KcsA transmembrane α-helices form an inverted truncated V cone accommodating ion selectivity filters at their extracellular top at the mouth of the cone . Such ion selectivity filters contain 4 K + binding sites of four highly conserved copies of TVGYG stretches that provide closed / open reversible conformations to stop / allow K + flows (Figure , green sticks).