Bacillus stearothermophilus lctB gene gives rise to functional K+ channelsin Escherichia coli and in Xenopus oocytes

We have cloned a small Kt channel subunit (LctB) of the gram-positive bacte rium Bacillus stearothermophilus (B. stearo.). The B. stearo. LctB protein is only 134 amino acids long. The sequence contains a typical K+ channel P- domain with a K+ channel GYGD signature sequence and two hydrophobic, possi bly membrane-spanning segments M1 and M2. Unexpectedly, LctB K+ channels ex hibited properties which differed markedly from the ones reported for KcsA channels of the gram-positive bacterium Streptomyces lividans. LctB channel s, when expressed in E. coli, were targeted to the outer membrane and not l ike KcsA channels to the inner membrane. After reconstitution in black lipi d membrane, LctB channels mediated K+ currents at neutral pH. They were app arently not gated by pH like KcsA channels. Also, LctB cRNA produced functi onal LctB channels in the Xenopus oocyte expression system in marked contra st to KcsA. The results demonstrated that heterologous expression produced functional LctB channels both in E. coli and in Xenopus oocytes, It is prop osed that bacterial LctB subunits can be properly handled by the Xenopus oo cyte leading to the occurrence of functional LctB K+ channels in the oocyte plasma membrane.