Phosphorylation of vitellin of the silkworm,Bombyx mori: identification of vitellin phosphorylation and protein kinase in the ovary

Abstract

Silkworm vitellin was radioactively labeled both in vivo upon injection of [³²P]phosphate and in vitro upon incubation of tissue extracts with [γ-³²P]ATP. Phosphorylation of vitellin was also demonstrated in the cultured ovary. [³²P]phosphate groups were added to serine residues of the peptide, indicating that silkworm vitellin is a phosphoprotein. SDS slab gel electrophoresis revealed three major labeled peptides in cases of in vitro and in vivo labeling methods. Two of them were identified, with specific anti-vitellin serum, as vitellin subunits, whereas another labeled peptide has not yet been characterized. The latter protein was not found in the hemolymph and fat body, suggesting that it is specifically localized in the ovary but immunologically different from vitellin. Protein kinase activities capable of phosphorylating vitellogenin and vitellin were identified in various tissues of the silkworm and the kinases from the ovary were partially characterized.