Membranes in cells act not only as a structural element to separate compartments within the cell and to separate the cell from its environment, but also as a platform for assembly of complex and critical structures required for exchange of both nutrients and information between the interior and the surroundings. In general, we are interested in understanding the dynamic organization of the proteins and lipids in cellular membranes. While a pure lipid bilayer is a psuedo two dimensional fluid, the membranes surounding cells are much more complex due to their complex mixture of lipids, the addition of embedded protein and the possible presence of a sub-surface scaffolding known as the membrane skeleton. We use high time-resolution single molecule imaging techniques to investigate the mobilitiy of membrane and cytoplasmic molecules at rates up to 1000Hz and analyze their movements in terms of diffusion through media embedded with obstacles.

Critical signaling processes in live cells are fundamentally stochastic. This inherent noise can give rise to interesting characteristics in cellular responses to environmental cues. Specifically, we are investigating the phosphate deprivation response (PhoB/PhoR two component system) in live E. coli cells. A fluorescent reporter protein has been placed on the chromosome behind a propomter activated in this system, as a marker for gene expression as a result of phosphate starvation. We image fluctuations, among an ensemble of cells, in the response (protein production) to a sudden drop in environmental phosphate. The measureed mean, variance and modality of the response is compared to phenomenological models solved through stochastic simulation and Monte Carlo techniques.