Objective: UPlanSApo 100x oil/1.40	Exposure: 600 ms
Microscope: Olympus DSU/IX81	Gain: 3
Camera: Hamamatsu ImagEM	Interval: 500 ms

Fluorescent protein biosensors have found widespread utility in reporting on a diverse array of intracellular processes. By creatively fusing pairs of fluorescent proteins to biopolymers that perform critical functions involved in various aspects of physiological signaling, research scientists have developed a host of new molecular probes that are useful for optical live-cell imaging of important processes such as calcium wave induction, cyclic nucleotide messenger effects, pH, membrane potential fluctuations, phosphorylation, and intracellular protease action. An alternative, but quite useful, strategy to biosensor construction involves modifications to the fluorescent protein backbone structure itself, either to split the peptide into individual units that are combined in vivo to produce fluorescence or to join the natural amino and carboxy termini together and create an “insertion site” within the molecule for a sensor peptide. In this digital video, a cameleon biosensor composed of cyan and yellow fluorescent proteins sandwiching a calmodulin domain and the m13 domain is being expressed transiently in human cervical carcinoma cells (HeLa line).