MOLECULAR EVOLUTION OF THE CA2-BINDING PHOTOPROTEINS OF THE HYDROZOA()

Alignment of the primary structures of the hydrozoan photoproteins, ae quorin, mitrocomin, clytin and obelin showed very strong amino acid se quence identities. The Ca2+-binding sites of the proteins were found t o be highly conserved. The Ca2+-binding sites were also homologous to the Ca2+-binding sites of other Ca2+-binding proteins. However, aequor in, mitrocomin, clytin and obelin differed from other Ca2+-binding pro teins in that they contained a relatively large number of cysteine, tr yptophan, histidine, proline and tyrosine residues, suggesting that th ese 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.