Deduced amino acid sequence identity among muTAUT and vertebrate taurine transporters is lower (47–51%) than that among vertebrate taurine transporters (>78%). muTAUT has a lower affinity and specificity for taurine and a requirement for higher NaCl concentration than vertebrate taurine

In aquatic invertebrates such as molluscs, not equipped with advanced mechanisms for regulating their internal environment, osmotic adaptation at a cellular level using an ‘osmolyte system’ plays an essential role. This osmotic adaptation involves accumulation and quantitative regulation of a large amount of organic solutes called ‘osmolytes’ in the intracellular space, to enable cells to respond to the high and variable salinity of the internal environment while maintaining the cell volume and low, stable intracellular salinity concentration (Lockwood, 1963; Somero and Bowlus, 1983). Whereas information on the osmotic adaptation and regulation at both the cellular and whole-body levels, and the related molecular mechanisms in teleost fish, has been collected (Karnaky et al., 1998; Wood et al., 1995), osmotic adaptation at a cellular level in aquatic invertebrates, which is probably a more vital process than that in teleost fish or mammals, has almost been ignored and is still poorly understood. Although the studies dealing with the biochemical analysis of enzymes related to osmolyte metabolism (Nomura et al., 2001; Yoshikawa et al., 2002), change in cell volume on exposure to salinity change (Neufeld and Wright, 1998, 1996a,b), and transport activity of osmolytes (Neufeld and Wright, 1995; Petty and Lucero, 1999; Silva and Wright, 1992) have been reported, the related molecular mechanisms have never been studied. Here, we report the molecular cloning, function and expression analysis of a taurine transporter involved in taurine uptake in the cell of the Mediterranean blue mussel. Most molluscs cannot entirely regulate their internal environments (Somero and Bowlus, 1983). Among these, brackish species, including mussels, have to adapt to a wide range of osmolality (almost 0–1000·mOsm·kg–1). Taurine is found to be the most abundant and thus, is an important osmolyte not only in mussels (Deaton et al., 1985a,b; Gills, 1972; Livingstone et al., 1979; Potts, 1954; Toyohara and Hosoi, 2004; Zurburg and DeZwaan, 1981) but also in numerous other invertebrates, fishes and mammals (Huxtable, 1992). Intracellular taurine concentration in one species of mussels was estimated to be approximately 200·mmol·l–1, corresponding to one-fifth of the total intracellular osmolality (Neufeld and Wright, 1995). The intracellular taurine content is mainly regulated via a transmembrane transport (Huxtable, 1992). Taking these facts The Journal of Experimental Biology 208, 4203-4211 Published by The Company of Biologists 2005 doi:10.1242/jeb.01868