Oocyte receptors for the specific accumulation of yolk proteins

A. S. Raikhel 1 , S. J. Seo 2 , H. M. Cheon 2 , J. Sun 1 , K. H. Cho 1 , & C. Y. Yun 1

1 Dept. Entomology, Michigan State Univ., East Lansing MI, USA; 2 Dept. Biology, Gyeonsang Natl. Univ., Chinju Gyeongnam, South Korea

Our ultimate goal is the elucidation of the molecular mechanisms underlying the specific accumulation of yolk protein precursors (YPP) in mosquito oocytes. We have previously characterized the vitellogenin receptor (VgR) as being responsible for specific internalization of the major yolk protein, vitellogenin (Vg). The mosquito VgR has been identified as a 205-kDa protein. Cloning of the cDNA encoding VgR has shown that it is a member of the low-density lipoprotein receptor (LDLR) family, sharing significant homology with the Drosophila yolk protein receptor. Insect Vg/YP receptors are unique in that they are twice as big as vertebrate VgR or VLDLR, carrying two clusters of cysteine-rich complement-type repeats (implicated in ligand recognition) instead of one as found in vertebrate VgR and LDLR. In the mosquito Aedes aegypti, several other YPP are synthesized in addition to Vg, in the vitellogenic fat body which are specifically accumulated by oocytes and deposited in mature yolk granules. These include two pro-enzymes, vitellogenic carboxypeptidase (VCP) and vitellogenic cathepsin B (VCB), and lipophorin (Lp). A specific Lp receptor (LpR) exists in the mosquito oocytes which is distinct from the Vg receptor and is homologous to the Locusta fat body LpR and to the vertebrate VLDL receptor. The cDNA encoding a putative ovarian mosquito lipophorin receptor (LpR) was cloned by a combination of PCR and cDNA library cloning. The deduced amino acid sequence has 64.3% homology with the locust fat body LpR and the structure of a receptor belonging to the LDL receptor family. The receptor consists of a typical cysteine-rich complement domain containing eight modules, an EGF homologous domain, a transmembrane domain and a cytoplasmic tail with a tyrosine internalization motif. Northern blot analysis revealed that the 4.1-kb transcript for this ovarian receptor is expressed at the previtellogenic and vitellogenic stages. In situ hybridization showed that LpR mRNA was present in the nurse cells and oocyte of developing primary follicles starting from early previtellogenic stages. We are presently investigating whether this newly discovered oocyte receptor is responsible for internalization of only Lp or if it serves as a multifunctional receptor binding, VCP and VCB as well.

Index terms: Aedes aegypti, vitellogenin receptor, lipophorin receptor, LDL receptor family.

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