Fat body protein granules and storage proteins in the silkmoth, Hyalophora cecropia.

Abstract

Fat body cells of silkmoth pupae (H. cecropia) contain granules with less dense outer zones and denser, often crystalline inner portions which appear after cocoon spinning and increase until the larval-pupal ecdysis. More granules form in females than in males. Urate granules, appearing fibrous in internal structure, first form about the same time, but their accumulation is more gradual, and continues in the pupa. Both types were isolated by centrifugation. Protein granules dissolve in buffers to yield proteins 1 and 2, with distinct electrophoretic and antigenic properties. These proteins were isolated individually from pupal fat body extracts by using their different thermal stabilities in phosphate buffer containing MgCl2 and (NH4)2SO4, respectively, and purification was completed by gel chromatography. Protein 1 had a MW of 480,000 and a subunit of 85,000 daltons, while protein 2 gave values of 530,000 and 89,000, respectively. Their amino acid compositions were similar but distinct. Proteins 1 and 2 accumulated in the hemolymph, beginning 3 days before spinning, reached maximal levels of spinning, and then declined in the hemolymph while granules were formed in the fat body, although the total hemolymph protein concentration did not decline at this time. The fat body of the late, feeding larva probably synthesized 2 related storage proteins and secreted them in partially crystalline granules as protein reserves for metamorphosis.