19G1-30 kDa proteins in the membrane of the pupal fat body of Bombyx mori interact with a hemolymph chymotrypsin inhibitor

In the silkworm, Bombyx mori, there are 30 kDa proteins that are expressed in the fat body and secreted into the hemolymph. A 19G1-30 kDa protein in the membrane fraction of the late larval midgut is a probable receptor for the hemolymph chymotrypsin inhibitor CI-8. Take-up of CI-8 into the pupal perivisceral fat body suggests that the 19G1-30 kDa proteins function as a receptor in the fat body. We used biochemical assays to investigate whether the 19G1-30 kDa proteins were the receptor for CI-8 in the pupal fat body. The results of affinity purification, far western blotting, and western blotting suggested that the 19G1-30 kDa proteins are the interacting factors of the membrane fraction of the pupal fat body. Tissue surface analysis indicated that these proteins were the canonical tissue-surface proteins. Immunological analysis showed that, after the take-up of CI-8 into the fat body, dissociation of the 19G1-30 kDa protein and CI-8 occurred followed by aggregation of the 19G1-30 kDa proteins to form protein granules. The results showed that the 19G1-30 kDa protein is a receptor of CI-8.