Effects of energization on membrane organization in mycoplasma

Fluorescence polarization and ESR experiments using various probes demonstrated that addition of glucose to resting Mycoplasma capricolum and Mycoplasma mycoides subs capri had, if any, a very limited effect on the physical state of their membrane lipids. Under the same conditions the degree of exposure of primary amino groups of membrane proteins to the aqueous surrounding, estimated from fluorescence labeling by fluorescamine and the cycloheptaamylose-fluorescamine complex was significantly increased. This energy dependent increase was blocked by dicyclohexylcarbodiimide (DCCD), an inhibitor of the membrane bound Mg2+ stimulated ATPase of mycoplasma and by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) which, in mycoplasma, only affects the chemical component of the proton-motive force. Variations in the proton activity gradient across the membrane induced by changing the pH of the labeling medium resulted in parallel variations in the ratio of relative intensities of labeling of energized to resting cells. The values taken by this ratio were up to two for a maximal proton gradient of 0.9 pH unit and tended to unity when the intracellular and extracellular pH tended to equalize. It is concluded that, upon mycoplasma cell energization, membrane proteins undergo a conformational change resulting in the exposure of new free amino groups. This conformational change is primarily dependent on the existence of a delta ph across the membrane and occurs in the absence of important modifications in the physical state of membrane lipids.