Ser-322 is a critical site for PKC regulation of the MDCK cell taurine transporter (pNCT).

Previous studies have shown that the Madin-Darby canine kidney cell taurine transporter (pNCT) is downregulated by protein kinase C (PKC) activation. In this study, it is hypothesized that the highly conserved serine-322 (Ser-322) located in the fourth intracellular segment (S4) may play an important role in the function of taurine transporter, which is modulated by PKC phosphorylation. It is demonstrated that Ser-322 is the critical site of PKC phosphorylation, as determined by site-directed mutagenesis. When Ser-322 of pNCT was changed to alanine (S322A) and this mutant was evaluated in an oocyte expression system, taurine transport activity increased threefold compared with control (wild-type pNCT). Activation of PKC by the active phorbol ester 12-myristate 13-acetate did not influence taurine transport by mutant S322A. Kinetic analysis showed that the mutation of Ser-322 essentially changed the Vmax, rather than the Km, of the transporter. Mutation of all other PKC consensus sites did not affect transporter activity when expressed in the oocyte system. Western blot analysis showed that expression of taurine transporter protein was similar in oocytes injected with either wild-type or mutant pNCT cRNA, indicating that the enhanced taurine transport activity by mutant S322A was not caused by a greater amount of transporter expressed in the oocyte. Furthermore, this study demonstrated that the taurine transporter was phosphorylated after PKC activation, and this effect was not observed in mutant S322A. In conclusion, Ser-322 is critical in PKC regulation of taurine transporter activity. The steady-state taurine transporter activity is tightly controlled by endogenous PKC phosphorylation of Ser-322, which is located in the fourth intracellular segment of the taurine transporter.