Should this occur, it is likely that your transporter strongly absorbs at either 403 or 460 nm (the excitation wavelengths of HPTS's protonated and deprotonated species, respectively), and the corresponding emission at 510 nm is significant. Since this effect is more problematic at higher concentrations, lower concentrations should be tested. If the problem persists, perhaps this assay is not suitable to analyse the activity of that transporter.