A novel ABC transporter: the first insight into zebrafish (Danio rerio) ABCH1.

Abch1 is a novel ATP binding cassette (ABC) transporter found in fish. This study represents an initial characterisation of this transporter using phylogenetic analyses, membrane topology prediction and determination of its tissue expression pattern in adult zebrafish (Danio rerio). Blast search showed that Abch1 orthologs are not present in genomes of other vertebrate taxa and similar genes are found only in invertebrate genomes. Abch1 is most closely related to the ABCG subfamily, although it shares only 12-14% of amino acid sequence identity with ABCG subfamily members. Topology analysis indicated that Abch1 is a half transporter that consists of six transmembrane domains with reverse domain arrangement (NBD-TMD), like ABCG subfamily members, but with differences in loop organization. Tissue distribution pattern revealed the highest Abch1 expression in brain, gills and kidney, followed by lower expression in intestine, gonads, skeletal muscle and liver. Considering moderate similarity in topology and tissue distribution pattern between Abch1 and ABCG subfamily members, we speculate that Abch1 is either involved in sterol transport similar to ABCG1, or is a part of the multidrug/multixenobiotic defence like ABCG2. These hypotheses remain to be addressed in further research.