Microbial K+ Channels

A survey of the currently available genomes shows that K channels are found in most free-living bacteria, archaea, and protists, indicating early evolution before the divergences of the three major domains of life. Though vertically descended from the primordial K channels, more recent lateral transfers of K channel genes must also have occurred. Variations in the motifs of transmembrane or cytoplasmic domains, S4 features, gating mechanism, and even the putative K filter sequence are observed. Limited experimentation and sequence survey of free-ling versus parasitic bacteria show that K channels are not essential for survival in ordinary circumstances. The lone K channel in Escherichia coli is probably not for K uptake but for resting-potential regulation in vivo. Protists (eukaryotic unicells) vary greatly; the fission yeast has no recognizable K channel gene while paramecium has 298, some three times the number found in the human genome. JGP readers are familiar with the satisfaction of using choice microbial channels to resolve structures at atomic resolutions. However, cherry picking does not provide a view of the tree, let alone the expanding forest of “completed” genomes of 270 prokaryotes (bacteria and archaea), 15 fungi, and 25 protists searchable in the public domain at this writing. Plants and animals constitute only a very small slice of life’s diversity, as evident from objective analyses of molecular markers, such as the small subunit ribosomal RNAs (ssrRNAs). These ssrRNAs are universal and the degree of their nucleotide sequence similarity is used to gauge relatedness (Fig. 1). The following is a summary on microbial K channels, as gleaned from genome survey and limited experimentations.