Molecular properties and biosynthesis of major plasma proteins in Bombyx mori

In the silkworm, Bombyx mori, a group of structurally related proteins referred to as ‘30K proteins’ comprises the major plasma proteins of the last instar larvae. Four protein components consisting of 30K proteins were purified to homogeneity from the larval hemolymph and designated Component 1, 2, 3 and 4, respectively. Close similarity in amino acid composition was noticed between Components 1 and 3, and between Components 2 and 4. Rabbit antibody prepared against Component 4 crossreacted with Component 2 as well as Component 4 but not with Components 1 or 3. In a cell-free translation system, RNA isolated from the fat body of the last instar larvae directed the synthesis of proteins reactive with anti-Component 4 antibody. Developmental change in the hemolymph concentration of 30K proteins well reflected the level of functional mRNA for these proteins in the fat body, indicating that the biosynthesis of 30K proteins is regulated during development at pre-translational level.