Can a single bacteriorhodopsin molecule change the structural state of one liposome?

Using ultrasonic velocity measurements the interaction of bacteriorhodopsin (BR) with large unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) was studied in gel (25 degrees C) and in liquid crystalline state (50 degrees C) of lipid bilayer. We could show that with the increasing BR concentration the increment of ultrasonic velocity increases and a saturation occur at a BR/Liposomes of ratio approximately 0.5 mol/mol. BR incorporation into the lipid bilayer in gel state leads to an increase of the increment of the ultrasonic velocity of the lipid to 9.51 +/- 1.47 ml/mol. This could be mainly attributed to a decrease in membrane compressibility or an increase in membrane volume or both. No changes of ultrasonic velocity increment were observed with the membrane in liquid crystalline state. In this case, BR probably is not able to change the mechanical properties of a considerably disordered membrane.