MOLECULAR EVOLUTION OF THE Ca 2+ ‐BINDING PHOTOPROTEINS OF THE HYDROZOA

Alignment of the primary structures of the hydrozoan photoproteins, aequorin, mitrocomin, clytin and obelin showed very strong amino acid sequence identities. The Ca(2+)-binding sites of the proteins were found to be highly conserved. The Ca(2+)-binding sites were also homologous to the Ca(2+)-binding sites of other Ca(2+)-binding proteins. However, aequorin, mitrocomin, clytin and obelin differed from other Ca(2+)-binding proteins in that they contained a relatively large number of cysteine, tryptophan, histidine, proline and tyrosine residues, suggesting that these residues may have evolved as part of the light-emitting mechanism. Construction of a phylogenetic tree showed that aequorin, mitrocomin, clytin and obelin form a closely related group of proteins.