Cholesterol depletion induces solid-like regions in the plasma membrane

Glycosylphosphatidylinositol-linked and transmembrane major histocompatibility complex (MHC) class II I-E-k proteins, as well as N-(6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (TritcDHPE), are used as probes to determine the effect of cholesterol concentration on the organization of the plasma membrane at temperatures in the range 22 degrees C - 42 degrees C. Cholesterol depletion caused a decrease in the diffusion coefficients for the MHC II proteins and also for a slow fraction of the Tritc-DHPE population. At 37 degrees C, reduction of the total cell cholesterol concentration results in a smaller suppression of the translational diffusion for I-E-k proteins ( twofold) than was observed in earlier work at 22 degrees C (five sevenfold) Vrljic, M., S. Y. Nishimura, W. E. Moerner, and H. M. McConnell. 2005. Biophys. J. 88: 334-347. At 37 degrees C, the diffusion of both I-E-k proteins is Brownian (0.9, alpha-parameter, 1.1). More than 99% of the protein population diffuses homogeneously when imaged at 65 frames per s. As the temperature is raised from 22 degrees C to 42 degrees C, a change in activation energy is seen at; 35 degrees C in the Arrhenius plots. Cytoskeletal effects appear to be minimal. These results are consistent with a previously described model of solid- like domain formation in the plasma membrane.