Complex Systems Seminar
Messages Diffuse Faster Than Messengers: Pitfalls Interpreting Estimates of Effective Diffusion Coefficients Using FRAP and FCS
Speaker: John E. Pearson (Los Alamos National Lab)
Signaling molecules often diffuse in the presence of nearly immobile binding sites (or traps) which decrease their effective diffusion coefficients. When traps are present a "single molecule diffusion coefficient" governs the time dependence of the mean square displacement of a collection of individual walkers while a "collective diffusion coefficient" governs the rate at which concentrations spread. The single molecule diffusion coefficient can be arbitrarily slow compared to the collective one: messages diffuse faster than messengers.
Fluorescence recovery after photobleaching (FRAP) gives the single molecule diffusion coefficient. Fluorescence correlation spectroscopy (FCS) yields a 3-component autocorrelation function with components that correspond to the collective, single molecule, and trap diffusion coefficients. These results will be discussed in terms of recent observations made by Gregor et al and Abu-Arish et al using both FCS and FRAP on the diffusion of fluorescently labeled bicoid during the formation of the bicoid gradient in the developing Drosophilla embryo. In contrast to Abu-Arish and Gregor we will argue that their measurements are consistent with each other and with the time scale for bicoid gradient formation.