Fluorescence energy transfer between ionophore, A23187, and membrane proteins of isolated outer and cytoplasmic membranes of a Gram-negative bacterium

Abstract

When trytophan residues of the proteins of outer and cytoplasmic membranes of a modeprately halophilic bacterium, Pseudomanas halosaccharolytica ATCC 29423, were excited at the wavelength 270 nm, tryptophan emission was observed at 330 nm. Adding the calcium ionophore, A23187, to both suspensions, the tryptophan emission at 330 nm decreased and the ionophore emission at 430 nm increased. Thus, when the calcium ionophore was increased in both suspensions, the ionophore emission increased with excitation of membrane tryptophan, that is, the fluorescence energy was transferred from tryptophan to the calcium ionophore. Using the Förster equation the critical distance was calculated to be 52 Å. As this distance is considerable compared with the diameter of the membrane protein molecules, the ionophore cannot be bound to the membrane proteins. It is probably located in the lipid bilayers.