Mobility of Min-proteins in Escherichia coli measured by fluorescence correlation spectroscopy

In the bacterium Escherichia coli, selection of the division site involves pole-to-pole oscillations of the proteins MinD and MinE. Different oscillation mechanisms based on cooperative effects between Min-proteins and on the exchange of Min-proteins between the cytoplasm and the cytoplasmic membrane have been proposed. The parameters characterizing the dynamics of the Min-proteins in vivo are not known. It has therefore been difficult to compare the models quantitatively with experiments. Here, we present in vivo measurements of the mobility of MinD and MinE using fluorescence correlation spectroscopy. Two distinct time-scales are clearly visible in the correlation curves. While the faster time-scale can be attributed to cytoplasmic diffusion, the slower time-scale could result from diffusion of membrane-bound proteins or from protein exchange between the cytoplasm and the membrane. We determine the diffusion constant of cytoplasmic MinD to be approximately 16\mu^{2}/s, while for MinE we find about 10\mu^{2}/s, independently of the processes responsible for the slower time-scale. Implications of the measured values for the oscillation mechanism are discussed.