Mosquito vitellogenin receptor: Purification, developmental and biochemical characterization

Abstract

Vitellogenin receptors (VgRs) play a critical role in egg development of oviparous animals by mediating endocytosis of the major yolk protein precursor, vitellogenin. A modification of the method for extracting the mosquito (Aedes aegypti) VgR from ovary membranes resulted in an 11-fold higher yield and 56-fold increase in relative purity of the VgR, in turn permitting purification, antibody production, and microsequencing. A K d of 15 nM was estimated from binding assays for the enriched VgR, indicating a very high affinity for its ligand. Immunoprecipitation of [ 14C]VgR using anti-VgR polyclonal antibodies followed by SDS-PAGE under reducing conditions and fluorography demonstrated that the 205 kDa VgR does not consist of subunits held together with disulfide bonds. However, an immunoblot of the native VgR suggests that it exists as a ∼390 kDa noncovalent homodimer in its native state. Immunoblot assays confirmed that the VgR is present only in ovarian tissue. A quantitative immunoassay of VgR extracts showed that VgR was present in previtellogenic ovaries on the day of emergence, increasing from 2 ng to more than 10 ng per ovary by day 5. After initiation of vitellogenesis and onset of Vg uptake, VgR quantity increased rapidly between 8 and 24 h after a blood meal, then began to decline between 24 and 36 h. Immunocytochemistry confirmed the presence of substantial amounts of the VgR in 4-day-old previtellogenic oocytes. In both previtellogenic and vitellogenic ovaries, the VgR was present only in the oocyte, primarily in the cortex.