I. Technology Single molecule imaging in living cells. A strong barrage of single-molecule experimental work during the past decade has greatly expanded our capability in dissecting the real time dynamics of complicated molecular interactions. We are developing imaging technologies to apply single-molecule imaging to the studies of live cells. We do this for two purposes. (1) To have nanoscopic observations of the molecular localization or dynamics in cells. This can be done because single-molecule imaging is not constrained by the so-called “diffraction-limit” that makes normal fluorescence microscope have only micrometer-level spatial resolutions. (2) To follow real-time protein expression. We have developed assays to monitor the intracellular synthesis of individual protein molecules. This allows us to decipher the gene expression process with very high spatial and temporal resolution.II. Biology Intracellular Organization in Bacterial. Over the last decade, a common theme emerged from the studies of bacteria of many different species is that there is a great degree of order in the intracellular organizations, in contrast to conventional wisdom. We are particularly interested in two proteins FtsZ and MreB, which are prokaryotic analog of eukaryotic cytoskeleton proteins: tubulin and actin. We are trying to understand why do bacterial cells need these proteins, despite their small sizes, and how does these prokaryotic cytoskeletal protein interact with each other and other cellular components to fulfill their functions.Post-synaptic plasticity in Neuron. Neronal synapse function and plasticity depend on the physical structure of the dendritic spines which, in turn, responds to the synaptic activities. Identifying the mechanisms underlying this process is the key to understand neuron functioning. We are analyzing two aspects of this complex process: 1. the (re)organization of actin cytoskeleton in the spines; 2. the regulation of protein synthesis at dendrites in response to nearly synaptic activities.