The function and evolution of insect storage hexamers.

The storage hexamers are a family of insect proteins with native molecular weights around 500,000 with six homologous subunits weighing generally between 70,000 and 85,000 daltons. They are synthesized and secreted by the fat body of feeding larvae and nymphs and reach extraordinary concentrations in the hemolymph just prior to metamorphosis. In holometabolous insects, they are partially recaptured by the fat body during the larval to pupal molt and stored in cytoplasmic protein granules, as well as in the hemolymph of the pupa. They disappear from these reservoirs during adult development; in many insects, both hemiand holometabolous, they are also utilized during the nymphal or larval molts. Their amino acids are primarily incorporated into new tissues and proteins during adult development, but they may also be incorporated into cuticle as intact protein, and in one case were found to be diverted to a small degree into energy metabolism. Finally, some storage hexamers have ligand binding and transport capabilities. Functionally, developmentally, and evolutionarily, they are a complex family of proteins whose analysis promises to be exceedingly useful in the study of many basic problems in the biology of insects.