The prothoracicotropic hormone bombyxin has specific receptors on insect ovarian cells.

Bombyxin II, a product of the brain of the adult silkmoth, Bombyx mori, binds to ovarian cells of three different species of lepidoptera, i.e. B. mori (silkmoth), Samia cynthia ricini (ailanthus moth), and an ovarian cell line of Spodoptera frugiperda (Sf9) (fall armyworm). Crude Sf9 cell membrane preparations were used to show that the purported bombyxin receptor binds its ligand in a specific, saturable, and reversible manner. The dissociation constant of the bombyxin-receptor complex is 260+/-90 pM. Quantitative binding studies and Scatchard analysis suggest that every Sf9 cell displays 20000 receptors on the surface. The cross-linked bombyxin-receptor ligand complex has an apparent molecular mass of about 300 kDa as determined by SDS/PAGE. Reduction causes the bombyxin receptor to dissociate into two subunits with molecular masses of 90 kDa and 116 kDa. The size and subunit structure of the putative bombyxin receptor on Sf9 cells show some similarities to the mammalian insulin receptor.