Prokaryotic K⁺ Channels

The roles of the K⁺ current in the action potential and resting potential are well known.  The first K⁺-channel gene (Shaker) was cloned in the Jan laboratory in the 70’s.  The first crystal structure of a K⁺ channel was solved 1998 by MacKinnon et al.  These studies are the corner stones of modern neurobiology.  The crystal structures readily explain how K⁺ channels can be highly selective and efficient at the same time.  They also explain clearly how channels can be gated by membrane voltage.  The most surprising aspect of the crystallographic work is that the structures are deduced largely from prokaryotic K⁺ channels even though K⁺ channels are historically and didactically the concern of neurobiology.  Ironically, the physiological roles of K⁺ channels in any bacteria or archaea are entirely unknown. 

Fig. 1.  A universal phylogenetic tree based on ssrRNA sequences showing the three domains of life.  Note that animals, plants, and fungi constitute only a small portion of biological diversity, even among the eukaryotes.  Crystal structures resolved to date were mostly of K+ channels or channel parts from prokaryotes, as marked.  From (6).

We have recently examined 270 prokaryotic genomes for their K⁺-channel genes (1).  Since microbes are the great majority of life’s diversity (Fig. 1), 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. (Fig. 2)  Parasitic or symbiotic bacteria tend not to have K⁺ channels, while those showing lifestyle versatility often have more than one K⁺-channel gene.   We speculate that prokaryotic K⁺ channels function to allow adaptation to environmental and metabolic changes (1, 2).  See “K⁺-channel Survey” in this website for a comprehensive table on the K⁺-channels from the prokaryotic genomes we surveyed.

Fig. 2.  Types of prokaryotic K+-channel subunits.    These channels are sorted by the number of transmembrane helix (TM), the regulatory domain (highlighted in black), and by homology to the studied K+ channels (in parentheses).  Numbers indicate the total channel genes in that category identified from the 270 prokaryotic genomes.  The basic K+ channel cores, TM-P-TM, are highlighted in gray.  The channels with KTN (RCK) domains or charged S4 are the majority.  From (1).

Milkman encountered and recognized the K⁺-channel gene in E. coli, kch, in 1994 (3).  The crystal structure of Kch-channel’s  “RCK” C-terminal regulatory domain has been resolved (4).   However, knocking out the kch gene leads to no detectable laboratory phenotypes.  Patch clamp also has failed to detect Kch’s electric activities to date. 

To gain insights into the role(s) of Kch in E. coli physiology, we have isolated and examined “gain-of-function” mutants.   Expression of their mutant kch leads to growth stoppage, presumably due to unregulated “loose-cannon” channel openings. These mutants are specifically sensitive to K⁺; additions of Na⁺ or sorbitol do not stop growth (Fig. 3).  Thus, kch indeed form a K⁺-specific pathway in the live bacteria.  External H⁺ rescues the K⁺-sensitive phenotype is a Nerstian manner, suggesting that the Kch functions to regulate membrane potential (therefore the proton motive force) and not the cell K⁺ concentration.

Fig. 3.  Expression of the “gain-of-function” kch mutations selected after a random mutagenesis.  (A) The plasmid used, in which the kch gene is driven by its native promoter.  (B) Plate phenotype of these mutants (bottom three rows) showing growth inhibition upon the addition of 200 mM K+, but not Na+ or sorbitol.  (C) In a more well defined medium, the sensitivity can be shown at as low as 2.5 mM K+, (From (5).

1. Kou et al. (2005) FEMS Microbiol. Rev.29: 145.
2. Kou et al. (2005) in Bacterial Ion Channels and Their Eukaryotic Homologs, eds. Kubalski & Martinac, ASM Press, pp 1-20.
3. Milkman (1994) PNAS91: 3510.
4. Jiang et al. (2001) Neuron29: 593.
5. Kou et al. (2003) EMBO J. 22: 4149.
6. Loukin et al. (2005) J. Gen. Physiol.125: 521.

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