Fig. 3.

Diffusion of small organic dye labeled lipids in the plasma membrane of living mammalian cells. (A) Mean squared displacement (MSD) vs. time lag Δt along trajectories for Atto647N-labeled lipids diffusing in a supported lipid bilayer (SLB) and in the plasma membrane of live mammalian PtK2 cells. The examples show PE, SM and SM after COase treatment. The deviation from a linear dependence MSD(Δt), indicating non-Brownian motion, is lowest for PE and largest for SM. The deviation from linear behavior starting at ∼(95 nm)² for SLB is due to the finite observation area (of radius R = 110 nm in this case). (B) Typical examples of the cumulative probability P(Δr²,Δt) as a function of time lag Δt and squared displacement Δr² for all cases. Number of trajectories: 972 (PE), 2518 (SM), 2879 (SM-COase), and 1764 (SLB). The faster the lipid diffuses, the quicker it explores space, i.e., the more P(Δr²,Δt) flattens with longer lag time. SM shows a clear signature of dominant trapping, which is substantially reduced after COase treatment. (C) Results of global fits to P(Δr²) to a model containing a freely diffusing and a strictly stationary regime. The fraction of time α spent in the mobile regime differed substantially between PE and SM. PE was largely mobile, though did show short trapping, while SM was frequently trapped and remained in the stationary regime for most of the time. Cholesterol depletion by COase treatment led to higher mobility of SM. Values of SM-COase vary more strongly as the COase impact is somewhat variable.