Haynes W John, Ling Kit-Yin, Saimi Yoshiro, Kung Ching
Laboratory of Molecular Biology, University of Wisconsin, Madison, Wisconsin 53706, USA.
Eukaryot Cell. 2003 Aug;2(4):737-45. doi: 10.1128/EC.2.4.737-745.2003.
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., approximately 80 in humans, approximately 30 in flies, and approximately 15 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.
钾离子选择性离子通道（K⁺通道）已在细菌、古细菌、真核生物和病毒中被发现。在草履虫和其他纤毛虫中，K⁺电流在基于纤毛的运动中起着至关重要的作用。我们利用先前报道的片段检索并测序了七个密切相关的草履虫K⁺通道基因（PAK）序列。另外八个独特的K⁺通道序列是从最近用于一个试点基因组测序项目的索引文库中检索到的。这些蛋白质翻译的比对表明，虽然这15个基因在不同时间发生了分化，但它们都保留了许多与后生动物K⁺通道的一个亚类（CNG/ERG型）相关的特征。我们的结果表明，大多数基因都有表达，因为所有预测的移码和同源比对中的几个缺口都包含从逆转录-聚合酶链反应产物中删除的草履虫内含子序列。15个基因组核苷酸序列中的一些变异涉及内含子的缺失，即使在非常密切相关的序列之间也是如此，这表明过去可能发生了逆转录。从可用的基因组序列推断，草履虫含有多达数百个这种类型的K⁺通道基因。这个数量比任何后生动物中已知的所有类型的K⁺通道基因数量都要多得多（例如，人类约有80个，果蝇约有30个，拟南芥约有15个）。为了理解这种多样性，我们讨论了它们得以保留的几个可能原因，包括响应淡水环境变化时表达水平的变化，就像在草履虫中其他主要质膜蛋白所看到的那样。
