FTIR spectroscopy of secondary-structure reorientation of melibiose permease modulated by substrate binding

Analysis of infrared polarized absorbance spectra and linear dichroism spectra of reconstituted melibiose permease from Escherichia coli shows that the oriented structures correspond mainly to tilted transmembrane α-helices, forming an average angle of ∼26° with the membrane normal in substrate-free medium. Examination of the deconvoluted linear dichroism spectra in H₂O and D₂O makes apparent two populations of α-helices differing by their tilt angle (helix types I and II). Moreover, the average helical tilt angle significantly varies upon substrate binding: it is increased upon Na⁺ binding, whereas it decreases upon subsequent melibiose binding in the presence of Na⁺. In contrast, melibiose binding in the presence of H⁺ causes virtually no change in the average tilt angle. The data also suggest that the two helix populations change their tilting and H/D exchange level in different ways depending on the bound substrate(s). Notably, cation binding essentially influences type I helices, whereas melibiose binding modifies the tilting of both helix populations.