MBoC | ARTICLE Diffusion of GPI-anchored proteins is influenced

by the activity of dynamic cortical actin ABSTRACT Molecular diffusion at the surface of living cells is believed to be predominantly driven by thermal kicks. However, there is growing evidence that certain cell surface mole-cules are driven by the fluctuating dynamics of cortical cytoskeleton. Using fluorescence cor-relation spectroscopy, we measure the diffusion coefficient of a variety of cell surface mole-cules over a temperature range of 24–37°C. Exogenously incorporated fluorescent lipids with short acyl chains exhibit the expected increase of diffusion coefficient over this temperature range. In contrast, we find that GPI-anchored proteins exhibit temperature-independent dif-fusion over this range and revert to temperature-dependent diffusion on cell membrane blebs, in cells depleted of cholesterol, and upon acute perturbation of actin dynamics and myosin activity. A model transmembrane protein with a cytosolic actin-binding domain also exhibits the temperature-independent behavior, directly implicating the role of cortical actin. We show that diffusion of GPI-anchored proteins also becomes temperature dependent when the filamentous dynamic actin nucleator formin is inhibited. However, changes in corti-cal actin mesh size or perturbation of branched actin nucleator Arp2/3 do not affect this be-havior. Thus cell surface diffusion of GPI-anchored proteins and transmembrane proteins that associate with actin is driven by active fluctuations of dynamic cortical actin filaments in addi-tion to thermal fluctuations, consistent with expectations from an “active actin-membrane composite ” cell surface