Cloning, tissue and ontogenetic expression of the taurine transporter 1323 Fig. 2 Metamorphosis pattern and expression of taurine transporter (TauT) in S. senegalensis larvae reared under standard procedures. DAH, days after hatching. Results for metamorphosis pattern are expressed as percentage of each metamorphic stage (0 dark bars; 1 dark grey bar; 2 light grey bar; 3 grey bar; 4 open bar) found at a certain age (n = 20). Results for expression of taurine transporter (ﬁlled circles) are shown as TauT copy:eEF1a1 copy (n = 3 pooled samples). Results are given as mean ± standard deviation. Different numbers of asterisks represents signiﬁcant differences for the mean metamorphic stage at a certain age. Different letters represent signiﬁcant differences for the expression of the taurine transporter throughout larval development dissected larvae, a similar trend was observed in all frac- tions (head, digestive tract and body), with TauT expres- sion levels signiﬁcantly increasing from 12 until 30 DAH (Fig. 3). TauT expression decreased within compartments according to the following order: body [ head [ digestive tract. In Senegalese sole juveniles, TauT was ubiquitously observed in all examined tissues (Fig. 4). The highest expression levels were found in brain, heart and eye. In the digestive tract, TauT was more expressed in hindgut and stomach. The expression of TauT was not signiﬁcantly different among midgut or hindgut sections. Discussion This study describes the cloning of the full coding cDNA sequence for Senegalese sole TauT. The putative protein was formed by 625 AA, being easily identiﬁed by three signatures (sodium/chloride-dependent neurotransmitter transporter superfamily, neurotransmitter symporter family and taurine transporters) and the prediction of 12 trans- membrane domains, a characteristic common in all TauT. Senegalese sole TauT exhibited a high similarity with other vertebrate TauT, sharing over 76% identity with mammals and more than 87% with other teleosts, which strongly suggests a high substrate speciﬁcity of transport. Accord- ingly, the phylogenetic tree constructed for vertebrate TauT grouped Senegalese sole among teleosts, indicating high evolutionary conservation of TauT in ﬁsh species. Several studies have shown that few AA residues in S4 of TauT seem to be involved in the control of taurine transport (Han et al. 2006). Compared to other vertebrates, two noteworthy replacements are found for these AA in Senegalese sole, in positions 319 (R) and 322 (C). In the remaining vertebrates, position 319 is occupied by lysine (K), a positively charged AA that seems to be important for maintaining the loop structure and controlling the gating function of TauT by attracting the negatively charged aspartic acid (D) in position 325 (Han et al. 2006). The replacement of lysine by arginine (R319) in Senegalese sole may not affect this function, since arginine is also a positively charged AA and ion attraction with D325 will possibly be maintained. On the other hand, position 322 is occupied by a cysteine (C322) in ﬁsh, reptiles and birds, while in mammals a highly conserved serine is found in this position (S322). This replacement is likely to inﬂuence taurine uptake, since gating of TauT activity seems to be controlled by protein kinase C (PKC) phosphorylation of S322. For instance, Han et al. (1999) have shown that the 123