Figure 4.

Spot variation FCS supports the view that upon presence of the actin mesh a transition from fast diffusion at a subdiffraction scale to slow macroscopic diffusion occurs. (A) Before actin linking, the diffusion time of the labeled lipid changes linearly with the spot size (open circles) and the extrapolation of the fitted line to zero spot size intersects at the origin (τ_d0 = 6 ± 209 μs). This is in agreement with a constant diffusion coefficient also at subdiffraction scales. In the presence of actin (solid circles), the extrapolation to the origin intersects at a slightly negative diffusion time (τ_d0 = −501 ± 241 μs). This could indicate a transition from fast diffusion (similar to the diffusion without actin) at a subdiffraction scale to slower diffusion on a larger scale (due to the presence of actin). (B) Furthermore, for the protein (open squares) the diffusion time in the membrane without actin scales linearly with the spot size and the fit intersects at the origin (τ_d0 = 62 ± 329 μs). In the presence of the actin mesh (solid squares) the extrapolated diffusion time at zero spot size is strongly negative (τ_d0 = −7880 ± 2400 μs), consistent with a transition from faster to slow diffusion at a subdiffraction scale, due to the presence of the actin mesh. All errors are standard deviations.