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Chapter 1 : K Channels: a Survey and a Case Study of Kch of

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K Channels: a Survey and a Case Study of Kch of , Page 1 of 2

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Abstract:

K is apparently crucial in dealing with biological water on Earth, even today. Dehydration, i.e., external osmotic upshift, demands an adjustment of cellular osmolarity, lest the pressure difference break the membrane. In cases more thoroughly studied, uptake of K constitutes the first line of defense against de hydration. Cells from all animal tissues express K channels but not necessarily the Na and Ca channels. Thus, K channels seem to underlie some basic functions required by all cells, except a few extreme parasitic bacteria, and their variations far outnumber those of Na and Ca2 channels, which seem to have evolved by gene duplication and filter mutations of ancestral K channels. The cell physiology of is better documented than that of any other cells, and it is amenable to genetic or molecular biological manipulation. Flowering plants and frolicking animals are the minority, even among eukaryotes. The plant-like sp. and sp., for example, exhibit a depolarization-activated K current. It is therefore reasonable to focus attention on Kch in in the hope of gaining insights into the functions of prokaryotic K channels, even though the electric activities of Kch have not yet been reported. The topic of prokaryotic channels will likely not attract many microbiologists’ attention either, unless their biological roles are illustrated. That nearly every bacterial or archeal genome includes at least one K channel gene, however, leaves little doubt that these channels provide selective advantages.

Citation: Kuo M, Saimi Y, Kung C. 2005. K Channels: a Survey and a Case Study of Kch of , p 1-20. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch1

Key Concept Ranking

Bacteria and Archaea
0.65007025
Ion Channels
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Mechanosensitive Channels
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Figures

Image of Figure 1
Figure 1

Topology of known K channel pore-forming subunits. K channels are multimers of such subunits, often with other auxiliary subunits. The minimal structure of the pore-forming subunit comprises an N-terminal TM α-helical segment (S), a short pore helix (P), the canonical K filter sequence TXG(Y/F)GD, and the C-terminal TM segment (S) as seen in the viral form (upper left; see text). Such subunits form tetramers. The last TM segments from the four subunits converge to form the gate near the cytoplasmic side. Many prokaryotic or eukaryotic channels have this basic structure with variable lengths of peptide extending into the cytoplasm (top, center). Two such units are covalently joined to form the two-pore-domain-type K channels found in animals (upper right) that assemble as dimers. The well-known Shaker-type subunit has four additional TM segments preceding the basic structure. In many cases, the fourth segment has positively charged amino acid residues spaced regularly at three-residue intervals and is considered the sensor for voltage-sensitive channels. Subunits with eight TM segments and two pore domains are found in fungi (middle row). All the above structures are known to actually support K conductances in various experimental systems. There are genes in the genome that are conceptually translated into subunits with 12 TM segments and two pore domains. Their corresponding conductances have not been examined (bottom row).

Citation: Kuo M, Saimi Y, Kung C. 2005. K Channels: a Survey and a Case Study of Kch of , p 1-20. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch1
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Image of Figure 2
Figure 2

The diversity of prokaryotic K channel subunits. The putative prokaryotic K channel subunit proteins can be sorted into 10 types. All 10 types contain the K signature sequence with adjacent TM(s). These 10 types are different in the number of TM domains, the presence of a long tail at either or both the N and C terminus, and the presence of voltage-sensing charges in their S. Most of the long tails contain an RCK domain (oval), some of them contain CNBD (rectangle), and others do not match any known domain (line). See the text.

Citation: Kuo M, Saimi Y, Kung C. 2005. K Channels: a Survey and a Case Study of Kch of , p 1-20. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch1
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Image of Figure 3
Figure 3

The structure of the RCK domain of Kch (PDB ID: 1ID1). (A) This domain consists of seven alternative β-sheets (ribbons) and α-helices (black backbones). The first five (β-α) motifs form the Rossmann fold core. (β) In the crystal, two RCK domains are dimerized through the last (α-β-α) motifs as a handshake ( ). The backbone of the second domain is shown in thin lines. The figure was produced with the Ras- Mol program.

Citation: Kuo M, Saimi Y, Kung C. 2005. K Channels: a Survey and a Case Study of Kch of , p 1-20. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch1
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Figure 4

The K-sensitive phenotype of the Kch GOF mutants. (A) The GOF Kch mutants cannot grow on LB agar plates enriched with 200 mM K (K200). These were the conditions we used originally to screen for these GOF mutants. The genes are promoted by the native promoter of from pGEM vector. (B) The phenotyping was refined by using tryptone-agarose plates that have background [K] of ~0.5 mM (by flame photometry). A millimolar concentration of added K stops GOF mutant growth. (C) Top row, when inoculated into fresh tryptone-based media, the stationary-phase GOF mutant starts to grow like the wild type in media enriched with 5 mM Na (open circle) or 10 mM sorbitol (open diamond) but not in the one enriched with 5 mM K (open triangle). Bottom, adding 10 mM K (arrows) to the growing GOF mutant culture immediately suppresses growth (filled triangle). (Reprinted from )

Citation: Kuo M, Saimi Y, Kung C. 2005. K Channels: a Survey and a Case Study of Kch of , p 1-20. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch1
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Tables

Generic image for table
Table 1

Recognizable K channel genes in various genomes

Citation: Kuo M, Saimi Y, Kung C. 2005. K Channels: a Survey and a Case Study of Kch of , p 1-20. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch1

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