The discovery of aequorin and green fluorescent protein.

We discovered aequorin and green fluorescent protein (GFP) in 1961 from the same species of jellyfish (Shimomura et al ., 1962). Our target was a luminescent substance, aequorin, and GFP was isolated as a by-product of aequorin owing to its bright conspicuous fluorescence. Both are unusual proteins but they had no particular importance when we first reported them. Their importance became apparent in the course of studies, and now, 40 years after their discovery, they are well known and widely used, aequorin as a calcium probe and GFP as a marker protein. In the characterization of these proteins, information obtained from the bioluminescence of the ostracod Cypridina played an extremely important role. Without the information gained on Cypridina luminescence, the characterization of aequorin would not have been possible. Because I had studied the biolu-minescence of Cypridina before I worked on aequorin, I would like to begin my story with my encounter with Cypridina. It was the spring of 1955. I was working as a teaching assistant at the Pharmacy School of Nagasaki University, and it was my fourth year in the role. My supervisor, Professor Shungo Yasunaga, was interested in my education and wanted me to broaden my knowledge. He kindly gave me a leave of absence for one year, and arranged for me to work at the laboratory of Professor Yoshimasa Hirata, at Nagoya University , as a visiting researcher. Professor Hirata was an expert in the chemistry of natural products. At my first meeting with Professor Hirata, he showed me dried Cypridina stored in a large vacuum desiccator and asked me to purify the luciferin and crystallize it. Cypridina is a tiny egg-shaped crustacean ostracod only 2– 3 mm in length (illustrated here in Fig. 1), and is abundant in shallow seas around Japan and southeast Asia. At night, the ostracod swims along and, upon encountering a predator, ejects luciferin and luciferase into the water, producing a cloud of blue luminescence. The animal then swims away into the darkness of the surrounding water. The luminescence is emitted by an enzyme reaction, commonly known as the luciferin–luciferase reaction, in which the reaction of luciferin with oxygen to produce light and oxyluciferin is catalysed by the enzyme luciferase. When Cypridina is dried, the specimen keeps its luminescence property almost permanently, and it will emit light again by simply wetting it with water. Because of this property, the Japanese military collected …