Molecular mobility on the cell surface.

Many measurements of lateral diffusion of proteins and lipids on cell membranes and lipid model membranes have become available through application of fluorescence photobleaching recovery methods. A puzzling aspect of these results is slow diffusion and partial immobilization of protein molecules on the cell surface. Observed protein diffusion coefficients on vertebrate structural tissue cells are consistently D less than or equal to 10(-10) cm2/s, while lipid analogues diffuse with D approx. 10(-8) cm2/s. Substantial fractions of the cell membrane proteins are not diffusible. In a pure viscous membrane, theoretical fluid dynamics has suggested only small differences between lipid and protein diffusion coefficients. Measurements of protein diffusion in model membranes recently showed D less than or equal to 10(-9) cm2/s, as expected. Recent experiments on cell membranes show that uncoupling of the membrane from the cytoskeleton by formation of blebs releases the membrane protein molecules so that diffusion is enhanced to D greater than or equal to 10(-9) cm2/s and the non-diffusible fraction is eliminated.