Associations between metals and the blue mesogleal protein of Cassiopea xamachana

The blue mesogleal pigment of the symbiotic jellyfish, Cassiopea xamachana Bigelow, 1882, is composed of two subunits, a larger glycosylated (35 kDa) moiety and a non-glycosylated (30 kDa) variant in lower concentration. In solution, the subunits assemble in large complexes of at least 10⁶ kDa. The pigment, known as Cassio Blue, appears to mitigate excessive solar radiation while allowing the passage of the wavelengths optimal for photosynthesis by the numerous algal symbionts in the mesoglea of the jellyfish. The pigment is an abundant protein comprising about 6% of all animal protein in the whole jellyfish and about 33% of all animal protein in the oral appendages. The protein also contains a diverse array of metals, notably Ag, Ca, Cu, Fe, Mg, and Zn, with traces of others. Metal stoichiometry varies among isolates averaging about 1 mol of all metals, taken together, for each mole of the pigment. Given the broad array of metals present, the pigment may also serve another purpose, for example, as a metal reservoir or trap. Few other proteins are associated with such a spectrum of metals. In addition, the amino acid sequences of the pigment tryptic peptides have no reasonable matches in any of the sequence databases. Our findings, taken as a whole, suggest that the Cassio pigment is indeed unusual and is likely a representative of a novel category of proteins, the original member of which is rpulFKz1, a chromoprotein endowed with Frizzled and Kringle domains.