Conformational change of the haemolymph juvenile-hormone-binding protein from Galleria mellonella (L)

A juvenile-hormone-binding protein (juvenile-hormone carrier), isolated from Galleria mellonella haemolymph, was treated with trypsin, chymotrypsin, carboxypeptidase A and subtilisin. Among these enzymes, only subtilisin was able to affect juvenile-hormone-binding activity of this protein. With SDS/PAGE it was shown that juvenile-hormone-binding protein, a 32-kDa peptide, is first slowly converted into a 30-kDa molecule, then into two or three smaller-molecular-mass species (20-25 kDa), which in turn were further digested to small peptides undetectable in PAGE. The 30-kDa peptide has a 2.4-times-higher dissociation constant for juvenile hormone than the native protein. No binding activity was detected for 20-25-kDa peptides. The rate of proteolysis of juvenile-hormone-binding protein was decreased by more than twofold in the presence of hormone, however, the overall cleavage pattern was unchanged. Under non-denaturing conditions, free binding-protein molecules could be separated from juvenile-hormone-binding-protein complex due to a slower electrophoretic mobility of the complex. As judged from ultracentrifugation and cross-linking experiments, binding of the hormone to its haemolymph carrier does not induce formation of oligomers, but shifts the sedimentation coefficient from 2.30S to 2.71S. It is concluded that juvenile-hormone binding induces a conformational transition of its carrier protein. This hormone-induced change might have a physiological significance for signal transmission.