INTERNALIZATION AND RECYCLING OF VITELLOGENIN RECEPTOR IN THE MOSQUITO OOCYTE

The major yolk protein precursor in mosquito oocytes, vitellogenin (Vg ), is internalized by a 205-kDa membrane-bound receptor (VgR). Recentl y, VgR has been isolated permitting the production of polyclonal anti- VgR antibodies. To elucidate the pathway of VgR internalization and re cycling in mosquito oocytes during Vg uptake, we carried out an immuno gold electron-microscopic study, labeling both Vg and VgR in ultrathin frozen sections of ovarian tissue. VgR immunolabeling demonstrated th at the oocyte plasma membrane was subdivided into microdomains, with V gR being located between and at the lower portions of the oocyte micro villi. During the early stages of internalization, Vg and VgR were obs erved together in coated pits, coated vesicles, and early endosomes. F usion of early endosomes created transitional yolk bodies (TYB) in whi ch Vg and VgR became segregated. VgR label was present in the numerous tubular compartments that protruded from the TYBs. These tubular orga nelles extended to and fused with the plasma membrane, suggesting that they represented the vehicle for VgR recycling. Vg label was not obse rved in the tubular compartments. Instead, Vg accumulated in the core of the TYB, a region free of VgR label. Mature yolk bodies (MYB) were heavily labeled for Vg, but completely lacked any VgR label, indicatin g that MYB are storage compartments that do not participate in recepto r recycling. Thus, our immunocytochemical data clearly visualize the s teps in Vg/VgR internalization, dissociation, sorting, and recycling o f the receptor to the plasma membrane.