The development of biosensors based on genetically engineered proteins offers many potential advantages to sensors that rely on natural proteins only. Here we present how protein engineering techniques can be used to integrate optical signal transduction functions directly into proteins by incorporating environmentally sensitive fluorescent groups that are allosterically linked to a ligand binding site, using a structure-based rational design approach. The location of potential fluorescent allosteric signal transduction (FAST) sites were predicted in the periplasmic Glucose Binding Protein of Escherichia coli. Single cysteine mutations were constructed at these locations, which allowed site-specific, covalent coupling of environmentally sensitive fluorophores. The fluorescence of these conjugates was shown to be hyperbolically dependent upon glucose concentration, indicating that the behavior of the fluorophore is allosterically coupled to the degree of occupancy at the glucose binding site, as predicted. Glucose Binding Proteins with engineered FAST functions may allow the development of a new class of optical glucose sensors.