Storage proteins with different physico-chemical characteristics have been identified in the
hemolymph of Lepidoptera, most of them are hexamerins, namely, hexamers of subunits of ca. 80 kDa.
According to phylogenetic analyses of aligned nucleotide sequences, hexamerins are classified into at least
three sub-groups: arylphorins, which are rich in aromatic amino acids at over 15 mol %, highly methioninerich
hexamerins with methionine at 5.8-12 mol % (H-MtH) and moderately methionine-rich hexamerins
with methionine at 3.4-5.4 mole % (M-MtH). These are evolutionally related to arthropod hemocyanin, HMtH
and M-MtH being judged to be derived from the same branch, which is diverged from the branch to
arylphorin in the phylogenetic tree. The other type of storage protein is biliverdin-binding protein (BP), a
dimer or tetramer of ca. 150 kDa subunits, with high density (1.26 g/ml); it belongs to the vitellogenin
family, but studies on this type have been limited to only a few species.
In most species, two or three different types of storage proteins are present, but the stages of their syntheses
differ by species. In general, storage proteins are most actively synthesized by the fat body during the
feeding period in the last larval instar, are soon after secreted in the hemolymph, and then, during larvalpupal
development, are partly or totally sequestered by the fat body, being stocked in the protein granules.
Some of the methionine-rich hexamerins (MtHs) are synthesized specifically in females, or more actively in
females than in males, and in these cases, fluctuating profiles of MtHs and/or tracer surveys support their
being utilized for ovarian development after being hydrolyzed to amino acids.
New findings in the common cutworm, Spodoptera litura have been described, in which five storage
proteins, that is, three hexamerins and two BPs, are sequestered by the fat body during larval-pupal
development, but these proteins are immuno-histochemically detectable in other tissues like midgut,
Malpighian tube, integument, dorsal vessels and pericardial cells. Tissue distribution profiles of these
storage proteins greatly change during development in a manner specific to respective proteins; in the pupal
stage, all of them become distributed in the imaginal buds, which differentiate to adult tissues, and three
species of storage proteins are detectable in eggs. These results raise the possibility that some tissues other
than fat body are involved in the synthesis of these proteins, which should function as amino acid reserves in
a specific period of molting, metamorphosis or reproduction.
Keywords: Hexamerin, arylphorin, methionine-rich hexamerin, biliverdin-binding protein, vitellogenin, fat
body, hemolymph, midgut, ovarian development, tissue formation, immuno-histochemistry, phylogenetic tree.
