Abstract
Inward rectifier K+ channel(IRK) and G protein-gated inward rectifier K+ channel (GIRK) have been recognized as crucial key marker proteins for early differentiation in tunicate embryos, Halocynthias. GIRK is expressed mainly in neural region by in situ hybridization study, while IRK is expressed in epidermal region and suppressed in neural region at the gastrula stage. So, we cloned Halocynthia's TuIRK and TuGIRK genomic genes. Here, we will report phylogenetic analysis of all IRK gene families included in bacterial, fly, worm, and several vertebrate genomic databases. Total 130 IRK family genes were classified into G protein-gated, ATP sensitive, ATP regulated, plain, and bacterial IRK groups. Both tunicate IRK and GIRK amino acid sequences were closely homologous to those of vertebrates. While, Halocynthia IRK and GIRK genomic genes were separated by many introns, some of which sites were observed in the channel pore and the membrane-spanning regions. This feature revealed their marked similarity to those of Ciona and Caenorhabditis. The introns were scarcely found in vertebrate, especially mammalian, IRK and GIRK genomic genes. However, interestingly, exclusively in tunicate and vertebrate GIRK genomic genes, a strongly conserved intron intervening site is found in the region, in which G protein beta likely interacts with GIRK. In addition, there are also two exclusively conserved amino acid sites nearby this region. Altogether, those results suggest that one aspect of phylogenetic evolution in genomic level is closely related to the functional structures of protein. [Jpn J Physiol 55 Suppl:S80 (2005)]
