FIGURE 7.

Experimental diffusion observations of gold-tagged transferrin receptor in the plasma membrane of the live PtK2 cells at various frame times reveal its rapid hop diffusion over small compartments. (A) Typical experimental trajectories obtained at frame times of 0.025, 0.22, and 33 ms. In the case of 0.025-ms trajectory, various plausible compartments detected by a computer program developed previously (Fujiwara et al., 2002) are shown in different colors in the order of purple, blue, green, yellow, and red. (B) Apparent microscopic diffusion coefficient, D_1–3, for transferrin receptor plotted against the frame time of the camera (red triangles). Also shown are the results of Monte Carlo simulation (blue squares) for 54-nm compartments with underlying diffusion coefficient of 9 μm²/s and probability of passing a barrier of 0.0045. The standard error of the mean for each point is within the extent of the data markers. Even in the shorter frame-time regime, the diffusion coefficient is changing rapidly, implying that even a frame time of 25 μs is not sufficient to obtain the intrinsic diffusion coefficient in such a small compartment. (C) Distribution of the compartment size as determined from the transferrin receptor trajectories observed at a frame time of 25 μs.