The deep roots and wide branches of the K(+)-channel family are evident from genome surveys and laboratory experimentation. K(+)-channel genes are widespread and found in nearly all the free-living bacteria, archaea and eukarya. The conservation of basic structures and mechanisms such as the K(+) filter, the gate, and some of the gate's regulatory domains have allowed general insights on animal K(+) channels to be gained from crystal structures of prokaryotic channels. Since microbes are the great majority of life's diversity, it is not surprising that microbial genomes reveal structural motifs beyond those found in animals. There are open-reading frames that encode K(+)-channel subunits with unconventional filter sequences, or regulatory domains of different sizes and numbers not previously known. Parasitic or symbiotic bacteria tend not to have K(+) channels, while those showing lifestyle versatility often have more than one K(+)-channel gene. It is speculated that prokaryotic K(+) channels function to allow adaptation to environmental and metabolic changes, although the actual roles of these channels in prokaryotes are not yet known. Unlike enzymes in basic metabolism, K(+) channel, though evolved early, appear to play more diverse roles than revealed by animal research. Finding and sorting out these roles will be the goal and challenge of the near future.

Although Kch of Escherichia coli is thought to be a K+ channel by sequence homology, there is little evidence that it actually conducts K+ ions in vitro or in vivo. We isolated gain‐of‐function (GOF) Kch mutations that render bacteria specifically sensitive to K+ ions. Millimolar added K+, but not Na+ or sorbitol, blocks the initiation or continuation of mutant growth in liquid media. The mutations are mapped at the RCK (or KTN) domain, which is considered to be the cytoplasmic sensor controlling the gate. Additional mutations directed to the K+‐filter sequence rescue the GOF mutant. The apparent K+‐specific conduction through the ‘loose‐cannon’ mutant channel suggests that the wild‐type Kch channel also conducts, albeit in a regulated manner. Changing the internal ATG does not erase the GOF toxicity, but removes kch's short second product, suggesting that it is not required for channel function in vivo. The mutant phenotypes are better explained by a perturbation of membrane potential instead of internal K+ concentration. Possible implications on the normal function of Kch are discussed.

The crystal structure of the RCK-containing MthK provides a molecular framework for understanding the ligand gating mechanisms of K ؉ channels. Here we examined the macroscopic currents of MthK in enlarged Escherichia coli membrane by patch clamp and rapid perfusion techniques and showed that the channel undergoes desensitization in seconds after activation by Ca 2؉ or Cd 2؉ . Additionally, MthK is inactivated by slightly acidic pH only from the cytoplasmic side. Examinations of isolated RCK domain by sizeexclusion chromatography, static light scattering, analytical sedimentation, and stopped-flow spectroscopy show that Ca 2؉ rapidly converts isolated RCK monomers to multimers at alkaline pH. In contrast, the RCK domain at acidic pH remains firmly dimeric regardless of Ca 2؉ but restores predominantly to multimer or monomer at basic pH with or without Ca 2؉ , respectively. These functional and biochemical analyses correlate the four functional states of the MthK channel with distinct oligomeric states of its RCK domains and indicate that the RCK domains undergo oligomeric conversions in modulating MthK activities.inactivation ͉ desensitization ͉ channel structure ͉ giant spheroplast ͉ patch clamp

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