Sappington, Thomas

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Insect Vitellogenin/Yolk Protein Receptors

2005-01-01 , Sappington, Thomas , Raikhel, Alexander , Sappington, Thomas , Entomology

The protein constituents of insect yolk are generally, if not always, synthesized outside the oocyte, often in the fat body and sometimes in the foilicular epithelium (reviewed in Telfer, 2002). These yolk protein precursors (YPP's) are internalized by the oocyte through receptor-mediated endocytosis (Roth et al., 1976; Telfer et al., 1982; Raikhel and Dhaclialla, 1992; Sappington and Rajkhel, 1995; Snigirevskaya et al., I 997a,b). A number of proteins have been identified as constituents of insect yolk (reviewed in Telfer, 2002), and some of their receptors have been identified. The taxonomically most wide pread class of major YPP in insects and other oviparous animals is vitellogenin (Vg). Although several insect Vg receptors (VgR) have been characterized biochemically, as of this writing there are only two insects from which VgR sequences have been reported, including the yellowfevcr mosquito (A edes aegypt1) (Sappington et al., 1996; Cho and Raikhel, 2001 ), and the cockroach (Periplaneta americana) (Acc. no. BAC02725). A BLAST search of the recently published genome sequence of the malaria mosquito (Anopheles gambiae) (Holt et al., 2002) yielded a predicted amino acid sequence (Acc. no. EAA06264) with 54% identity to the A. aegypti VgR sequence, and will be referred to here as the A. gambiae VgR.

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Tissue- and Stage-specific Expression of Two Lipophorin Receptor Variants with Seven and Eight Ligand-binding Repeats in the Adult Mosquito

2003-10-24 , Seo, Sook-Jae , Cheon, Hyang-Mi , Sappington, Thomas , Sun, Jianxin , Sappington, Thomas , Raikhel, Alexander , Entomology

We identified two splice variants of lipophorin receptor (LpR) gene products specific to the mosquito fat body (AaLpRfb) and ovary (AaLpRov) with respective molecular masses of 99.3 and 128.9 kDa. Each LpR variant encodes a member of the low density lipoprotein receptor family with five characteristic domains: 1) ligand recognition, 2) epidermal growth factor precursor, 3) putative O-linked sugar, 4) single membrane-spanning domains, and 5) the cytoplasmic tail with a highly conserved internalization signal FDNPVY. Proposed phylogenetic relationships among low density lipoprotein receptor superfamily members suggest that the LpRs of insects are more closely related to vertebrate low density lipoprotein receptors and very low density lipoprotein receptor/vitellogenin receptor than to insect vitellogenin receptor/yolk protein receptors. Two mosquito LpR isoforms differ in their amino termini, the ligand-binding domains, and O-linked sugar domains, which are generated by differential splicing. Polymerase chain reaction and Southern blot hybridization analyses show that these two transcripts originated from a single gene. Significantly, the putative ligand-binding domain consists of seven and eight complement-type, cysteine-rich repeats inAaLpRfb and AaLRov, respectively. Seven cysteine-rich repeats in AaLpRfb are identical to the second through eighth repeats of AaLpRov. Previous analyses (1) have indicated that the AaLpRov transcript is present exclusively in ovarian germ-line cells, nurse cells, and oocytes throughout the previtellogenic and vitellogenic stages, with the peak at 24–30 h after blood meal, coincident with the peak of yolk protein uptake. In contrast, the fat body-specific AaLpRfb transcript expression is restricted to the postvitellogenic period, during which yolk protein production is terminated and the fat body is transformed to a storage depot of lipid, carbohydrate, and protein.