Applications of Calcium‐sensitive Photoproteins in Experimental Biology

Abstract—Calcium‐sensitive photoproteins are “precharged bioluminescent proteins that are triggered to emit light by binding Ca2+ or certain other inorganic ions. Neither molecular oxygen nor any organic cofactor is required. The first such protein to be described was aequorin, and for various reasons that has been the one most widely studied. Photoproteins have been used as Ca2+‐indicators both in vitro and in living cells. Their chief advantages for this are (1) ease of signal detection, (2) high sensitivity, (3) relative specificity for Ca2+, and (4) lack of toxicity. Difficulties in the experimental use of the photoproteins stem from (1) their one‐time reactivity, (2) their large molecular size, (3) their scarcity, (4) the influence of experimental conditions on the sensitivity to Ca2+, (5) the nonlinearity of the relation between [Ca2+] and light intensity, and (6) the limited speed with which light intensity follows sudden changes in [Ca2+]. Photoproteins have now been used as intracellular calcium indicators in more than two dozen types of cells, and experience with the method is rapidly growing. They are also useful in the determination of calcium binding constants for other substances in vitro, and as models for studies of receptor‐ligand interactions in general.