KcsA, a potassium channel fromStreptomyces lividans, was the first ion channel to have its transmembrane domain structure determined by crystal-lography. Previously we have shown that its C-terminal cytoplasmic domain is crucial for the thermostability and the expression of the channel. Expression was almost abolished in its absence, but could be rescued by the presence of an artificial left-handed coiled coil tetramerization domain GCN4.