PAK Paradox: Paramecium Appears To Have More K ⁺ -Channel Genes than Humans

Abstract

K؉ -selective ion channels (K ؉ channels) have been found in bacteria, archaea, eucarya, and viruses. In Paramecium and other ciliates, K ؉ currents play an essential role in cilia-based motility. We have retrieved and sequenced seven closely related Paramecium K ؉ -channel gene (PAK) sequences by using previously reported fragments. An additional eight unique K ؉ -channel sequences were retrieved from an indexed library recently used in a pilot genome sequencing project. Alignments of these protein translations indicate that while these 15 genes have diverged at different times, they all maintain many characteristics associated with just one subclass of metazoan K ؉ channels (CNG/ERG type). Our results indicate that most of the genes are expressed, because all predicted frameshifts and several gaps in the homolog alignments contain Paramecium intron sequences deleted from reverse transcription-PCR products. Some of the variations in the 15 genomic nucleotide sequences involve an absence of introns, even between very closely related sequences, suggesting a potential occurrence of reverse transcription in the past. Extrapolation from the available genome sequence indicates that Paramecium harbors as many as several hundred of this one type of K ؉ -channel gene. This quantity is far more numerous than those of K ؉ -channel genes of all types known in any metazoan (e.g., ϳ80 in humans, ϳ30 in flies, and ϳ15 in Arabidopsis). In an effort to understand this plurality, we discuss several possible reasons for their maintenance, including variations in expression levels in response to changes in the freshwater environment, like that seen with other major plasma membrane proteins in Paramecium.

Comparative Genomic Analysis of Integral Membrane Transport Proteins in Ciliates

“…Most of these are cation‐specific channels of the VIC family, in agreement with an earlier report suggesting that ciliates have more K + channels than animals (Haynes et al. ). These channels play important roles in ciliates in generating action potentials that control exocytosis, motility, tactic, and tactile behavior (Kung and Saimi ; Mohamed et al.…”

In Silico Ionomics Segregates Parasitic from Free-Living Eukaryotes

“…tetraurelia which, with 443 predicted ion channels, possessed the largest number of different ion channel subunits of all the analyzed eukaryotes. This is in agreement with previous reports (Haynes et al 2003). Paramecium tetraurelia was followed by C. elegans (372 predicted ion channels) and Dan.…”

Ion Channels in Microbes

“…Ionic currents have been recorded from a number of microbial cells including Dictyostelium (slime mold) (256), Chlamydomonas (a green flagellate) (355, 399) Paramecium and Stylonychia (ciliates) (65, 67, 128, 262, 311), Neurospora (bread mold) (203, 323), Uromyces (a parasitic bean rust fungus) (412) and single celled fungi Schizosaccharomyces pombe (fission yeast) (410) and Saccharamyces cerevisiae (baker’s yeast) (114, 115). Although the functions of a number of ion channels in ciliates are well understood and interpreted using concepts developed in animal biology, the roles of most ion channels in eukaryotic microbes still remain to be elucidated.…”