Rotational diffusion of bacteriorhodopsin in lipid membranes.

New techniques for measuring the diffusion of proteins in cell membranes have recently been reported [l-7] . Such measurements should in principle test to what extent proteins are freely floating in the fluid lipid bilayer, as envisaged in currently popular concepts of membrane structure [8]. They may also provide a method of investigating structural features of membranes which restrict or prevent diffusion. Before these aims can be fully achieved, it is necessary to have a sound basis for interpreting diffusion measurements in membranes. The familiar Stokes-Einstein equations are not applicable in two dimensional systems and indeed it is not certain that diffusion in lipid bilayers can in any case be treated by classical hydrodynamics. Model calculations of diffusion in membranes have recently been presented [9] but the results have yet to be critically tested. Experimental data which are sufficiently unambiguous to test theoretical predictions are most likely to be obtained with simple model systems rather than with cell membranes. Here we report the incorporation of bacteriorhodopsin, one of the best characterised membrane proteins, into phospholipid bilayers. We propose that this system should prove valuable for a detailed investigation of diffusion in membranes and give results of preliminary measurements of protein rotation.