SEQUESTRATION OF INSECTICYANIN, A BLUE HEMOLYMPH PROTEIN, INTO THE EGG OF THE HAWKMOTH MANDUCA-SEXTA - EVIDENCE FOR RECEPTOR-MEDIATED ENDOCYTOSIS

Sequestration of the blue biliprotein, insecticyanin, into developing oocytes of the hawkmoth, Manduca sexta was investigated. Immunodiffusi on assays revealed that insecticyanin concentration in mature eggs (29 .6 mu M) is slightly higher than that in hemolymph (25.8 mu M). The en docytotic uptake of insecticyanin was visualized at the light microsco pic level using autoradiography. Uptake of I-125-insecticyanin by isol ated oocytes was saturable. Analysis of in vitro uptake data estimated that the value of K-uptake (insecticyanin concentration at half-maxim al uptake rate) is 4.2 mu M and that the V-max (maximum rate of uptake ) is 1 pmol follicle(-1) h(-1). Labeled insecticyanin was shown to bin d to sonicated follicle membranes with high specificity and affinity. The K-D (equilibrium dissociation constant) and the B-m (total number of binding sites per follicle), were estimated as 4 x 10(-8) M and 8 x 10(7) respectively. Competition studies showed that binding of labele d insecticyanin to oocyte membranes was blocked by excess amounts of u nlabeled insecticyanin but not by lipophorin and vitellogenin of M. se xta. Additional membrane binding experiments demonstrated that recepto rs for insecticyanin are only present in the oocytes membranes, not in fat body or gut tissue.