Organization and developmental expression of the mosquito vitellogenin receptor gene.

Vitellogenin is a precursor of the major yolk protein, vitellin. It is internalized by developing oocytes via receptor-mediated endocytosis. Previously, we characterized the vitellogenin receptor (VgR) from oocytes of the mosquito Aedes aegypti [Sappington, T.W., Kokoza,V.A., Cho,W.L. and Raikhel,A.S. (1996) Molecular characterization of the mosquito vitellogenin receptor reveals unexpected high homology to the Drosophila yolk protein receptor. Proc Natl Acad Sci USA 93: 8934-8939]. The VgR receptor has a unique structure with two putative ligand-binding domains. In order to understand the regulation of this important molecule, we characterized the VgR gene structure and its expression during vitellogenesis in the mosquito A. aegypti. We report here that the VgR gene was separated by five introns that have an average length of 60 bp, except for the second intron which was more than 20 kb long. Most introns were located within the coding regions of the first protein domain. We isolated two allelic variations of the VgR gene, VgR1 and VgR2, the nucleotide sequences of which differing only in their 5'-flanking regions. Considering their frequency in the mosquito genome, VgR2 appeared to be a major allele. The expression of VgR mRNA was studied by the Northern blot analysis and in situ hybridization. The level of the VgR transcript started to rise in the ovary one day post-eclosion. It continued its dramatic rise during the vitellogenic period, reaching its peak at 24 h PBM. The VgR transcript was present exclusively in ovaries where it was seen in oocytes and nurse cells of primary follicles and germ-line cells of the germarium.