CUTLER, C.; CRAMB, G.; Univ. of St Andrews, UK: A comparative analysis of CFTR in teleost fish.

The cloning of CFTR cDNA sequences from a number of teleost species allows an analysis of both sequence and expression data to be performed. Analysis of teleost CFTR sequences shows, that as a group,teleost CFTR orthologues are approximately 15% less homologous to other species than might be expected based on the elasmobranch CFTR sequence. This is intriguing considering that the sequence of other teleost ion transporter orthologues show slightly higher levels of nucleotide or amino acid homology to mammalian versions, than do elasmobranch homologues. This would suggest an alteration of the functional or regulatory phenotype of teleost CFTRs. However, the only published evidence to date suggests teleost CFTRs are phenotypically similar to mammalian CFTRs. Furthermore, a closer look at the distribution of amino acid differences between teleost and mammalian CFTR sequences reveals that these are spread throughout the protein, with no obviously different conserved motifs present in teleost CFTRs. One possible explanation as to why teleost CFTRs are more divergent than might be expected, is that the role of CFTR in teleost fish may be somewhat altered. This may be due to the fact that, the teleost osmoregulatory system (involving CFTR in the gill) is usually engaged in actively maintaining ion homeostasis as plasma osmolalities are usually in considerable dis-equilibrium with the external environment. Whereas, elasmobranchs usually maintain plasma at an essentially iso-osmotic level in comparison to the environment and CFTR in the rectal gland is involved in the production of an iso-osmotic fluid secretion. There are also differences in expression and regulation between different teleost species, and these may either be related to the speed necessary for acclimation to different osmotic environments or possibly due to the presence of different osmoregulatory mechanisms.