Excitation of fluorescent dyes inactivates the outer hair cell integral membrane motor protein prestin and betrays its lateral mobility. 
The outer hair cell motor protein, prestin, which resides exclusively in the cell's lateral membrane, underlies the mammal's exquisite sense of hearing.  Here we show that photoexposure of the commonly used dyes Lucifer yellow, 6-carboxy-fluorescein, and 4-(2-[6-(dioctylamino)-2-naphthalenyl]ethenyl)-1-(3-sulfopropyl)-pyridinium (di-8-ANEPPS), that are in contact with the cell's lateral membrane can photo-inactivate the motor irreversibly, as evidenced by reduction in prestin's gating charge displacement or non-linear capacitance.  Furthermore, utilizing restricted fiber optic illumination of the lateral membrane, we show that whole-cell, non-linear capacitance is depleted beyond that expected for an immobile population in the exposed area.  These data indicate that lateral diffusion of prestin occurs within the cell's lateral plasma membrane.