Rotational and Lateral Diffusion of Membrane Proteins and Lipids: Phenomena and Function

Motion of cell membrane components occurs over a wide range of time and distance scales. This chapter discusses the rotational and translational diffusion of membrane lipids and proteins and motions over a part of the time and distance scales. The short-range and rapid motions, especially of membrane lipids are summed up in terms of viscosity or fluidity. The chapter describes rotational motions in the range of 10⁶ to 10³ sec^-1 and with lateral movements in the range of 10^-12 to 10^-7 cm² sec^-1. Methods for measuring diffusion and some of the results obtained with these methods are described. It discusses the biological functions of diffusion in membranes. All the techniques for measuring diffusion depend upon some aspects of the spectra of selected probes. These probes may be roughly divided into optical probes and magnetic resonance probes. The methods used for measuring rotational diffusion are optical method and saturation-transfer electron-spin resonance (ESR) (S-T ESR). Rotation of erythrocyte band 3, bacterio- and vertebrate rhodopsin, calcium ATPase of sarcoplasmic reticulum, cytochromes and cytochrome oxidases, and acetylcholine receptor have been measured in native and synthetic membranes. These measurements yield information on protein–protein association as a function of membrane protein concentration, on coupling of integral membrane proteins with the cell cytoskeleton, and on coupling of enzymatically interacting components. All methods for lateral diffusion take one of the two approaches: measurement of the frequencies of molecular encounter or measurement of the time required for marked molecules to fill a defined area of membrane.