EVOLUTION OF ARTHROPOD HEMOCYANINS AND INSECT STORAGE PROTEINS (HEXAMERINS)

Crustacean and cheliceratan hemocyanins (oxygen-transport proteins) an d insect hexamerins (storage proteins) are homologous gene products, a lthough the latter do not bind oxygen and do not possess the copper-bi nding histidines present in the hemocyanins. An alignment of 19 amino acid sequences of hemocyanin subunits and insect hexamerins was made, based on the conservation of elements of secondary structure observed in X-ray structures of two hemocyanin subunits. The alignment was anal yzed using parsimony and neighbor-joining methods. Results provide str ong indications for grouping together the sequences of the 2 crustacea n hemocyanin subunits, the 5 cheliceratan hemocyanin subunits, and the 12 insect hexamerins. Within the insect clade, four methionine-rich p roteins, four arylphorins, and two juvenile hormone-suppressible prote ins from Lepidoptera, as well as two dipteran proteins, form four sepa rate groups. In the absence of an outgroup sequence, it is not possibl e to present information about the ancestral state from which these pr oteins are derived. Although this family of proteins clearly consists of homologous gene products, there remain striking differences in gene organization and site of biosynthesis of the proteins within the cell . Because studies on 18S and 12S rRNA sequences indicate a rather clos e relationship between insects and crustaceans, we propose that hemocy anin is the ancestral arthropod protein and that insect hexamerins los t their copper-binding capability after divergence of the insects from the crustaceans.