Rational Design of Calcium Biosensors

Understanding the temporal and spatial changes in calcium concentration has been a difficult endeavor for many years due to the relatively small changes in calcium concentration during messenging events, the rapid changes upon physiological messenging, and the unavailability of fast, efficient, and sensitive sensors to detect calcium changes. In addition, the key factors in calcium binding have yet to be determined due to the metal-metal interactions, cooperativity, and conformational change involved in calcium binding to natural calcium-binding proteins. To overcome these obstacles and to engineer calcium sensors for in vivo studies of calcium signaling events, calcium binding sites have been engineered into Green Fluorescent Protein. The engineered binding sites demonstrate terbium binding affinity from 2-30 μM and calcium binding affinity from 50-100 μM. Site 177 demonstrates green fluorescence when expressed in mammalian cells and produces a response to calcium concentration changes when expressed in the cytosol. Addition of the cycle 3 mutations (M153T, V163A, F99S) to Site 177 allowed for increased brightness in the emission of the chromophore but still exhibited calcium response. The second generation Site 1 demonstrates fluorescence response to calcium concentration changes when expressed both in the cytosol and in the endoplasmic reticulum. Addition of M153T and V163A to Site 1 allowed for expression of fluorescent protein at 37 °C in HeLa cells and at 30 °C in bacteria. Site 1-M153T/V163A exhibits chromophore fluorescence response to calcium with a Kd of 100 μM and competition with Rhodamine-5N produced a calcium Kd of 107 μM. This designed sensor, Site 1-M153T/V163A is the first demonstration of a designed calcium binding GFP with calcium response measured both in vivo and in