Abstract
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.
