FIGURE 5.

Estimates of the 3′ PI diffusion coefficient, turnover rate constant, and other parameters in individual cells. Circle symbols mark the mean parameter value averaged over the limiting model fits, and the error bars signify the range of parameter values for each cell. Open symbols signify the four cells that yielded less ideal model fits as described under Fig. 4. (a) The estimated diffusion coefficient D (μm²/s) in each cell is plotted versus the square of the line scan radius (μm²), demonstrating that the apparent lipid mobility is independent of cell size. The dotted line indicates the mean of the 32 cells (0.374 μm²/s). (b) The average turnover rate constant, k, is correlated versus the associated k_app, from a fit to Eq. 3, for each cell. The solid line is the best fit of the data to y = m^*x, with m = 1.75; the dashed line is the y = x line. (c) The fluorescence gain, σ, the ratio of the cytosolic volume to the effective volume of TIRF excitation, is plotted versus the square of the line scan radius (μm²). (d) The estimated values of basal/steady-state 3′ PI ratio, X₀/X_ss (x axis), and the steady-state probe-binding fraction, p_ss (y axis), are shown.