Silkworm midgut proteins interacting with a hemolymph protease inhibitor, CI-8

CI-8 is the chymotrypsin inhibitor in hemolymph from the silkworm, Bombyx mori. It occurs in the midgut at the spinning stage of larva, but little information on the mechanism of its uptake in the midgut is available. We found that two polypeptides interacting with CI-8 are in the midgut membrane, and we purified them using a biotinylated CI-8, viz., p29 and p60, having molecular sizes of 29 kDa and 60 kDa respectively. The structures of p29 and p60 were examined by N-terminal amino acid sequencing and peptide mass mapping, including tryptic digestion. p29 was highly similar to the matured 19G1-30K lipoprotein from hemolymph, but p60 was novel. Purified p29 was recognized by anti-19G1-30K antibody, and was confirmed to be similar to 19G1-30K. The antibody also neutralized the CI-8 binding ability of p29 in the midgut membrane. p29 and p60 are perhaps proteinaceous factors involved in the uptake of CI-8 into the midgut through the membrane.