Mobility of bacteriorhodopsin in lipid vesicles.

BR* has been the subject of a wide variety of biophysical studies. Particular attractions are its ease of preparation in pure form, the availability of much structural information and its intrinsic chromophore, which facilitates a range of spectroscopic investigations (for review, see Stoeckenius et al., 1979). We have used BR to investigate various aspects of protein diffusion in membranes. In the native purple membrane, BR does not of course diffuse because it exists as a crystalline lattice. To observe diffusion, we have reconstituted BR into dimyristoylglycerophosphocholine vesicles (Cherry et al., 1978). This has provided a well characterized model system in which it has proved possible to make reasonably accurate measurements of rotational and lateral diffusion coefficients. Rotational diffusion of BR may be measured by exploiting the properties of its intrinsic chromophore, retinal. Excitation of BR by a brief laser pulse produces a transient decrease in absorption (depletion) at 570nm of milliseconds duration owing to molecules entering the photochemical cycle. When the exciting pulse is linearly polarized, the absorption transients are dichroic due to photoselection. From the decay of dichroism, or specifically the anisotropy r, rotational motion may be determined. The anisotropy at time t after excitation is defined by the following expression: