Vitellogenin of the Tobacco Hornworm, Manduca Sexta: Properties and Endocytotic Incorporation into follicles

Abstract

Manduca sexta vitellogenin is a phosphoglycolipoprotein (Mr - 500,000) that contains two copies of the apoproteins (apovitellogenin-1, Mr 180,000 and apovitellogenin-11, Mr 45,000), 13 percent lipids, 3 percent carbohydrates and 0.6 percent phosphorus. The two apoproteins are immunologically distinct polypeptides and apovitellogenin-11 is not completely accessible to the aqueous environment in the intact molecule. The carbohydrate moiety located on apovitellogenin-1 has a high mannose structure (Man9 GlcNAc2). Sonicated follicle membranes bind 1251-labeled vitellogenin with high affinity and specificity (K0 = 1.3 x 10-8 M). Total binding sites were estimated at 4 x 1014 sites/ g of follicle membrane protein. The binding was sensitive to pH and calcium. Competition studies showed that binding of vitellogenin was blocked by vitellin and deglycosylated vitellogenin but not by lipophorin, microvitellogenin or apovitellogenin-11. These results suggest that the uptake of vitellogenin involves binding to specific receptors on follicle membranes and the carbohydrate moiety and apovitellogenin-11 are not involved in the interaction with the receptors.