CHARACTERIZATION OF THE SOLUBILIZED OOCYTE MEMBRANE RECEPTOR FOR INSECTICYANIN, A BILIPROTEIN OF THE HAWKMOTH, MANDUCA SEXTA

Abstract We report the solubilization and characterization of the oocyte membrane receptor for insecticyanin, a blue biliprotein of the hawkmoth Manduca sexta . The insecticyanin receptor was solubilized using 40 mM CHAPS. Strong binding affinity of [ 125 I]insecticyanin to its solubilized receptor was demonstrated to be heat-labile, pH-dependent, Ca 2+ -dependent, and saturable. The binding was inhibited by excess unlabeled insecticyanin, but not by two other major hemolymph and oocyte proteins, vitellogenin and lipophorin. The receptor for insecticyanin showed tissue specificity: it was present only in oocyte membranes, not in membranes of fat body, midgut or ovariole sheath. The equilibrium data for the solubilized receptor, K d and B max , were estimated to be 17 nM and 11.4 pmol/mg solubilized proteins, respectively. The results from co-immunoprecipitation showed that the apparent molecular mass for the insecticyanin receptor is approximately 185 kDa while chemical crosslinking of the insecticyanin–receptor complex revealed a product with a molecular mass near 10 3 kDa. This suggests that the insecticyanin receptor has a multimeric structure, or that four receptor molecules can bind to one insecticyanin tetramer.