Molecular Mechanisms of Membrane Proteins Studied by Infrared Spectroscopy

Abstract

Membrane proteins are indispensable for cell signaling, bioenergetics, and transportation. Atomic structures of membrane proteins have been elucidated by methods in structural biology (e.g. X-ray crystallography). Structural changes of them in action should be investigated to improve the understanding of the molecular mechanisms in more detail. Infrared spectroscopy is a powerful method to analyze structural changes of protein by combining it with various kinds of stimulus-induced difference spectroscopic techniques. In this review article, my recent studies on microbial rhodopsin, ion channel and ion pump proteins are summarized. Light-induced difference infrared spectroscopy revealed the hydrogen-bonding change of Thr204 in pharaonis phoborhodopsin upon photoisomerization of the retinal chromophore, which turned out to be a key residue for the signal transduction. Attenuated total reflection (ATR) method was applied on the studies of ion-protein interaction of a potassium channel, KcsA, and a sodium pump, V-type ATPase. Finally, development of a rapid-buffer exchange apparatus for time-resolved ATR infrared spectroscopy is introduced, which would be a powerful technique for investigating ion- and ligand-binding reactions of membrane proteins.