Multi-colored homologs of the green fluorescent protein from hydromedusa Obelia sp.

The presence of green fluorescent protein (GFP) within the bioluminescent system of Obelia (Cnidaria, Hydrozoa, Campanulariidae) was inferred shortly after the discovery of GFP in Aequorea. Despite the enormous success of Aequorea GFP as a genetically encoded fluorescent label, Obelia GFP thus far has been defeating attempts to clone it from the hydroid life cycle stage. Here, we report cloning of three GFP-like fluorescent proteins (FPs) from Obelia medusa, representing cyan, green, and yellow spectral types. Such color diversity has never been detected outside class Anthozoa, suggesting a more general function for multi-colored fluorescence in cnidarians than has been previously hypothesized. An unusual property of the new FPs is the formation of large soluble complexes of well-defined sizes and molecular weights, corresponding to up to 128 individual polypeptides. This aligns well with the earlier observation that luminescence in Obelia, unlike in Aequorea, is localized within subcellular granules, which prompts further inquiry into the self-assembly properties of the new FPs and their interactions with the photoprotein. The discovery of Obelia FPs fills the four-decade-old gap in the knowledge of cnidarian bioluminescence and provides experimental material to further investigate the details of its molecular mechanism.