BLAST searches for AQP0 sequences in the entire pool of genomes available at NCBI revealed that two
aqp0s are conserved between zebrafish (
Danio rerio) and puffer fish (
Tetraodron nigroviridis, Takifugu rubripes), most likely a product of the genomewide duplication that occurred at the base of the teleost fish lineage.
48 –51 Vihtelic et al.
52 identified both zebrafish
aqp0 genes in a lens library made from adults and named them MIP1 and MIP2; here we refer to them as
Aqp0a and
Aqp0b, in accordance with the aquaporin nomenclature. AQP0s from fish (zebrafish, puffer fish, puffer fish fugu, Mexican tetra, and killifish) are segregated into two distinct branches in the dendrogram: Aqp0as and Aqp0bs (
Fig. 1A). Zebrafish Aqp0a is 96% similar to the killifish AQP0 (MIPfun), which is a water channel.
44 Zebrafish Aqp0a and Aqp0b amino acid sequences share 85% identity (221 of 263 identical amino acids) and 95% similarity using the Gonnet similarity matrix.
53 We predicted the structures of the two zebrafish AQP0s based on their sequences and compared them to the published mammalian AQP0 structures.
21,54 Only 14 positions distinguish the dendrogram branch of Aqp0a sequences from the Aqp0b branch (
Fig. 1B, black arrows), but the analysis of the sequence alignment gives little direct insight into why both genes have been retained or how they might differ in function. Residues that influence the pH sensitivity of AQP0 (histidine [H] 39, H40, H43, H122)
25,26 differ both within and across species, suggesting variation in water-permeability regulation. The mammalian AQP0 C terminus presents several serines that are sometimes phosphorylated in the lens.
55 Three serines (S229, S231, S235) are particularly important because they are located within a consensus calmodulin-binding site, and their phosphorylation determines the calcium sensitivity of AQP0 water permeability.
29 Similarly, the C terminus of Aqp0a contains a potential calmodulin-binding site, but with only one serine residue (S231) instead of the three (S229, S231, S235). Aqp0b also presents a threonine at position 236. Both S231 and T236 of fish AQP0 are potential PKC phosphorylation sites (NetPhos). By analogy with bovine AQP0, this suggests that the water permeability of fish Aqp0s could be modulated by calcium by calmodulin binding and that phosphorylation could play a significant role.