Chapter 13 - Insights into the Pore of the Hair Cell Transducer Channel from Experiments with Permeant Blockers

Abstract

This chapter presents recent experiments designed to infer the properties of the ion-conducting pore of the mechanoelectrical transducer channel of sensory hair cells using permeant blockers. By combining results from experiments with three classes of large cationic permeant blockers, the fluorescent dye FM1-43, the aminoglycoside antibiotics, and the potassium-sparing diuretic amiloride, information has been obtained on the free energy profile along the transducer channel's pore as sensed by these blocker molecules. These energy profiles provide information about the position of the negatively charged binding site for the blockers as well as about positively charged barriers near the extracellular and intracellular faces of the channel that impede the blockers' permeation. The extracellular barrier is relatively modest and allows almost diffusion-limited entry of blockers from the extracellular side. A larger intracellular energy barrier effectively prevents exit of the blocking molecules from the intracellular side, trapping the blockers inside the hair cells. A putative geometrical model of the transducer channel pore is presented that draws on results from all the three classes of permeant blockers. The pore contains a large vestibule that is easily accessible from the extracellular side.