H+ PERMEATION AND PH REGULATION AT A MAMMALIAN SEROTONIN TRANSPORTER

The rat serotonin transporter expressed in Xenopus oocytes displays an inward current in the absence of 5-HT when external pH is lowered to 6.5 or below. The new current differs from the leakage current describ ed previously in two ways. (1) It is similar to 10-fold larger at pH 5 than the leakage current at pH 7.5 and reaches 1000 H+/sec per transp orter at extremes of voltage and pH with no signs of saturation. (2) I t is selective for H+ by reversal potential measurements. Similar H+-i nduced currents are also observed in several other ion-coupled transpo rters, including the GABA transporter, the dopamine transporter, and t he Na+/glucose transporter. The high conductance and high selectivity of the H+-induced current suggest that protons may be conducted via a hydrogen-bonded chain (a ''proton-wire mechanism'') formed at least pa rtially by side chains within the transporter. In addition, pH affects other conducting states of rat serotonin transporter. Acidic pH poten tiates the 5-HT-induced, transport-associated current and inhibits the hyperpolarization-activated transient current. The dose-response rela tionships for these two effects suggest that two H+ binding sites, wit h pK(a), values close to 5.1 and close to 6.3, govern the potentiation of the 5-HT-induced current and the inhibition of the transient curre nt, respectively. These results are important for developing structure -function models that explain permeation properties of neurotransmitte r transporters.