Early fluorescence signals detect transitions at mammalian serotonin transporters

The mammalian serotonin transporters rSERT or hSERT were expressed in oocytes and labeled with sulforhodamine-MTS. The endogenous Cys-109 residue contributes most of the signal, and the labeled transporter shows normal function. The SERT fluorescence decreases in the presence of 5-HT and also depends on the inorganic substrates of SERT. The fluorescence also increases with membrane depolarization. During voltage-jump experiments, fluorescence relaxations show little inactivation or history dependence. The fluorescence signal has a voltage dependence similar to that of the prepriming step of the previously described voltage-dependent transient current. However, the fluorescence relaxations are the fastest voltage-dependent events yet studied at SERT; their time constants of approximately 8-30 ms are severalfold faster than the prepriming or inactivation phases of the transient currents. These fluorescence signals are interpreted within the framework of the gate-lumen-gate model. The signals may monitor initial events at the outer gate.