Lateral diffusion of an integral membrane protein: Monte Carlo analysis of the migration of phosphorylated light-harvesting complex II in the thylakoid membrane.

The lateral migration of the integral light-harvesting chlorophyll a/b protein complex of photosystem II, LHCII, has been studied in the undisturbed membranes of thylakoids without artificial probes. LHCII was phosphorylated at 0 degree C. The diffusion of the mobile phospho-LHCII from appressed grana to nonappressed membrane regions was induced by a temperature jump to 20 degrees C and analyzed by a rapid detergent fractionation of the two membrane areas. This long-range diffusion of the integral phospho-LHCII is analyzed by a Monte Carlo calculation which is based on a model of the thylakoid membrane and includes all integral proteins as mobile particles. A comparison of the calculation with the experimental time course indicates a diffusion constant of phospho-LHCII in the range of (2-4) x 10(-12) cm2 s-1. This value is evidence for a severe restriction of protein mobility in the appressed thylakoid membrane. From a statical point of view, the percolation theory predicts that the high protein density in the grana membranes is above the threshold of percolation and the long-range diffusion should be inhibited by finite clusters of lipids. However, the shape of the experimental time course is in favor of a lateral motion also of photosystem II and nonphosphorylated LHCII and not of a rigid lattice of these complexes. Our data and Monte Carlo analysis suggest a dynamic or fluid lattice of the protein complexes with a lifetime of the clusters in the millisecond time range. The consequences of these transient fluctuations on the long-range diffusion of plastoquinone are discussed.