Storage proteins in the silkworm,Bombyx mori

In the silkworm, Bombyx mori, two storage proteins named SP-1 and SP-2 were shown to decline in concentration in the haemolymph and increase in the fat body during the larval-pupal transformation, when protein granules are formed in the fat body at the same time as the degeneration of mitochondria and endoplasmic reticulum. At the larval-pupal ecdysis, in females the two proteins account for 60% of total fat body protein (80% of the soluble protein), while males have very little SP-1 and SP-2 comprises only 20% of the total fat body protein. The concentration of protein granules in the fat body cytoplasm is much greater in females than in males, and the granules in females have partially crystalline inner zones. This is different from males where granules with non-crystalline structure are most numerous.The properties of these proteins purified from pupal fat body are similar to those of Cecropia storage proteins and calliphorin, all of which have molecular weights of around 500,000 and are composed of subunits of mol. wt about 85,000. SP-1 differs from SP-2 by having an exceptionally high content of methionine, but much less glutamate, phenylalanine and tyrosine. SP-1 resembles another female-specific protein, vitellogenin and SP-2 resembles calliphorin in amino acid composition.From these results, it is concluded that SP-1 and SP-2 have storage roles and are deposited in protein granules.