Mosquito vitellogenin subunits originate from a common precursor

Analysis of the Vitellogenin gene of rice moth, Corcyra cephalonica Stainton

Vitellogenin (Vg) is a precursor of the major yolk protein, an essential nutrient for the embryonic development of oviparous animals including insects. Here, the gene(CceVg [Corcyra cephalonica Vg] ) encoding the Vg (CceVg of moth, C. cephalonica, was cloned and sequenced. The gene sequence was 6,721-bp long and contained 5five introns and six exons that together formed a 5,382-bp open reading frame. The deduced protein (CceVg) consisted of 1,793 amino acid residues, including a 16-amino-acid signal peptide. The putative molecular weight of the primary Vg protein was 202.46 kDa. The CceVg contained all conserved domains and motifs that were commonly found in most insect Vgs except the presence of a polyserine tract at the C-terminal region, which had not been reported in other lepidopteran Vgs. The expression pattern showed that CceVg was first transcribed at a very low level in the early larval stage but disappeared in later stage larva. In female, the CceVg mRNA was detected in early pupal stage and throughout adult stage. Interestingly, the CceVg mRNA was detected only in mated males at low levels, not in the virgin ones. Injection of CceVg double-stranded RNA into early-emergent females caused severely abnormal ovaries.

The major yolk protein vitellogenin interferes with the anti-plasmodium response in the malaria mosquito Anopheles gambiae

Functional gene analysis in malaria mosquitoes reveals molecules underpinning the trade-off between efficient reproduction and the antiparasitic response.

When taking a blood meal on a person infected with malaria, female Anopheles gambiae mosquitoes, the major vector of human malaria, acquire nutrients that will activate egg development (oogenesis) in their ovaries. Simultaneously, they infect themselves with the malaria parasite. On traversing the mosquito midgut epithelium, invading Plasmodium ookinetes are met with a potent innate immune response predominantly controlled by mosquito blood cells. Whether the concomitant processes of mosquito reproduction and immunity affect each other remains controversial. Here, we show that proteins that deliver nutrients to maturing mosquito oocytes interfere with the antiparasitic response. Lipophorin (Lp) and vitellogenin (Vg), two nutrient transport proteins, reduce the parasite-killing efficiency of the antiparasitic factor TEP1. In the absence of either nutrient transport protein, TEP1 binding to the ookinete surface becomes more efficient. We also show that Lp is required for the normal expression of Vg, and for later Plasmodium development at the oocyst stage. Furthermore, our results uncover an inhibitory role of the Cactus/REL1/REL2 signaling cassette in the expression of Vg, but not of Lp. We reveal molecular links that connect reproduction and immunity at several levels and provide a molecular basis for a long-suspected trade-off between these two processes.

Malaria annually claims the lives of almost 1 million infants and imposes a major socio-economic burden on Africa and other tropical regions. Meanwhile, the detailed biological interactions between the malaria parasite and its Anopheles mosquito vector remain largely enigmatic. What we do know is that the majority of malaria parasites are normally eliminated by the mosquito's immune response. Mosquitoes accidentally acquire an infection by sucking parasite-laden blood, but this belies the primary function of the blood in the provisioning of nutrients for egg development in the insect's ovaries. We have found that the molecular processes involved in delivering blood-acquired nutrients to maturing eggs diminish the efficiency of parasite killing by the mosquito immune system. Conversely, molecular pathways that set the immune system on its maximal capacity for parasite killing preclude the efficient development of the mosquito's eggs. Our results reveal some of the molecules that underpin this example of the trade-offs between reproduction and immunity, a concept that has long intrigued biologists.

Molecular characterization and expression pattern of Spodoptera litura (Lepidoptera: Noctuidae) vitellogenin, and its response to lead stress

Vitellogenin (Vg) cDNA from Spodoptera litura Fabricius was cloned and sequenced. The open reading frame (ORF) of Vg cDNA was 5247 nucleotides in length (GenBank Accession no. EU095334), which encoded for a protein of 1748 amino acids. S. litura Vg comprised three conserved regions (Vitellogenin-N domain, DUF1943 and von Willebrand factor type D domain (VWD)), a 17 amino-acid signal peptide and a RXXR cleavage signal (RTIR). The highly conserved GL/ICG motif, the DGXR motif and cysteine residues were found in the C-terminus of the Vg. Vg mRNA was found specifically in the female fat body. Vg expression was first transcribed in 6th day female pupae and levels increased with insect development. The maximum level of Vg mRNA appeared in 24-h-old adults. When S. litura larvae were exposed to lead (Pb) (25-200 mg Pb/kg), there was a significant inhibition in Vg of female adults. The start of Vg expression was advanced ahead by Pb, from 6th day pupae to 3rd day or 4th day pupae. Low levels of Vg in male adults were also induced by low concentrations of Pb (12.5 and 25 mg Pb/kg). These data show that Pb stress elicits an important Vg response in S. litura.

Molecular characteristics of insect vitellogenins

Vitellogenins (Vgs) are precursors of the major egg storage protein, vitellin (Vn), in many oviparous animals. Insects Vgs are large molecules ( approximately 200-kD) synthesized in the fat body in a process that involves substantial structural modifications (e.g., glycosylation, lipidation, phosphorylation, and proteolytic cleavage, etc.) of the nascent protein prior to its secretion and transport to the ovaries. However, the extent to which Vgs are processed in the fat body varies greatly among different insect groups. We provide evidence by cloning and peptide mapping of four Vg molecules from two cockroach species (Periplaneta americana and Leucophaea maderae) that, in hemimetabolous insects, the pro-Vg is cleaved into several polypeptides (ranging from 50-to 180-kD), unlike the holometabolans where the Vg precursor is cleaved into two polypeptides (one large and one small). An exception is the Vg of Apocrita (higher Hymenoptera) where the Vg gene product remains uncleaved. The yolk proteins (YPs) of higher Diptera (such as Drosophila) form a different family of proteins and are also not cleaved. So far, Vgs have been sequenced from 25 insect species; 9 of them belong to Hemimetabola and 16 to Holometabola. Alignment of the coding sequences revealed that some features, like the GL/ICG motif, cysteine residues, and a DGXR motif upstream of the GLI/CG motif, were highly conserved near the carboxy terminal of all insect Vgs. Moreover, a consensus RXXR cleavage sequence motif exists at the N-terminus of all sequences outside the Apocrita except for Lymantria dispar where it exists at the C-terminus. Phylogenetic analysis using 31 Vg sequences from 25 insect species reflects, in general, the current phylogenies of insects, suggesting that Vgs are still phylogenetically bound, although a divergence exists among them.

Sequence and the developmental and tissue-specific regulation of the first complete vitellogenin messenger RNA from ticks responsible for heme sequestration

The first full-length mRNA for vitellogenin (Vg) from ticks was sequenced. This also represents the first complete sequence of Vg from the Chelicerata and of a heme binding Vg. The Vg cDNA from the American dog tick, Dermacentor variabilis was 5744nt in length (GenBank Accession number AY885250), which coded for a protein of 1843 aa with a calculated molecular weight of 208 kD. This protein had an 18 aa signal sequence, a single RXXR cleavage signal that would generate two subunits (49.5 and 157K in molecular weight) and lipoprotein N-terminal and carboxy von Willebrand factor type D domains. Tryptic digest MS analysis of vitellin protein confirmed the function of the cDNA as the tick yolk protein. Apparently, vitellin in D. variabilis is oligomeric (possibly dimeric) and is comprised of a mixture of the uncleaved monomer and subunits that were predicted from the single RXXR cleavage signal. The highly conserved GL/ICG motif close to the C-terminus in insect Vg genes was different in the tick Vg message, i.e., GLCS. This variant was also present in a partial sequence of Vg from Boophilus microplus. Phylogenic analysis showed that the full length Vg cDNA from D. variabilis and the partial cDNA from B. microplus were distinct from insects and Crustacea. The Vg message was not found in whole body RNA from unfed or fed males or in unfed and partially fed (virgin) females as determined by Northern blotting. The message was found in replete (mated) pre-ovipositional females, increased to higher levels in ovipositing females and was absent after egg laying was complete. The endocrine regulation of the Vg mRNA is discussed. The tissue sources of the Vg message are both the gut and fat body. Tryptic digest MS fingerprinting suggests that a second Vg mRNA might be present in the American dog tick, which needs further study.

Drosophila yolk protein produced in E. coli is accumulated by mosquito ovaries

Despite similar functions, the yolk proteins of the higher dipteran flies and the vitellogenins found in other insects are unrelated at the sequence level and have evolved from different genes. Both are selectively endocytosed into the ovary via receptors belonging to the LDLR receptor subfamily. We cloned the Drosophila yp1 gene into an E. coli expression vector and showed that the yolk protein produced by E. coli is taken up into ovaries of both Drosophila melanogaster and the malaria mosquito Anopheles gambiae, which normally uses vitellogenin.

Two isoforms of the early E74 gene, an Ets transcription factor homologue, are implicated in the ecdysteroid hierarchy governing vitellogenesis of the mosquito, Aedes aegypti

In the anautogenous mosquito, Aedes aegypti, vitellogenesis is under the strict control of 20-hydroxyecdysone (20E), which is produced via a blood meal-activated hormonal cascade. Several genes of the ecdysteroid-regulatory hierarchy are conserved between vitellogenesis in mosquitoes and metamorphosis in Drosophila. We report characterization of two isoforms of the mosquito early E74 gene (AaE74), which have a common C-terminal Ets DNA-binding domain and unique N-termini. They exhibited a high level of identity to Drosophila E74 isoforms A and B and showed structural features typical for Ets transcription factors. Both mosquito E74 isoforms bound to an E74 consensus motif C/AGGAA. In the fat body and ovary, the transcript of AaE74 isoform homologous to Drosophila E74B was induced by a blood meal exhibiting its highest level coinciding with the peak of vitellogenesis. In contrast, the transcript of AaE74 isoform homologous to Drosophila E74A was activated at the termination of vitellogenesis. These findings suggest that AaE74A and AaE74B isoforms play different roles in regulation of vitellogenesis in mosquitoes.

Hepatopancreas is the extraovarian site of vitellogenin synthesis in black tiger shrimp, Penaeus monodon

The site of yolk protein synthesis in crustaceans has long been a subject of controversy. The vitellogenin gene structure was partially reported only very recently in Macrobrachium rosenbergii, after which the hepatopancreas was confirmed as the extraovarian site of vitellogenin synthesis in that species. Ovaries are the most frequently reported as the site of yolk protein synthesis in penaeid shrimp. Using cDNA reversed-transcribed from mRNA isolated from the hepatopancreas of vitellogenic female shrimp, Penaeus monodon, we found that its deduced amino acid sequence had high identity of 48% with that from M. rosenbergii vitellogenin. A similar location of the intron in the sequenced region of genomic DNA was also found between these two species. We therefore concluded that the hepatopancreas the extraovarian site of vitellogenin synthesis in P. monodon in vivo. The partial structure of vitellogenin gene is presented in this study.

Conserved molecular mechanism for the stage specificity of the mosquito vitellogenic response to ecdysone

In the mosquito Aedes aegypti, the adult female becomes competent for a vitellogenic response to ecdysone after previtellogenic development. Here, we show that betaFTZ-F1, the nuclear receptor implicated as a competence factor for stage-specific responses to ecdysone during Drosophila metamorphosis, serves a similar function during mosquito vitellogenesis. AaFTZ-F1 is expressed highly in the mosquito fat body during pre- and postvitellogenic periods when ecdysteroid titers are low. The mosquito AaFTZ-F1 transcript nearly disappears in mid-vitellogenesis when ecdysteroid titers are high. An expression peak of HR3, a nuclear receptor implicated in the activation of betaFTZ-F1 in Drosophila, precedes each rise in mosquito FTZ-F1 expression. In in vitro fat body culture, AaFTZ-F1 expression is inhibited by 20-hydroxyecdysone (20E) and superactivated by its withdrawal. Following in vitro AaFTZ-F1 superactivation, a secondary 20E challenge results in superinduction of the early AaE75 gene and the late target VCP gene. Electrophoretic mobility-shift assays show that the onset of ecdysone-response competence in the mosquito fat body is correlated with the appearance of the functional AaFTZ-F1 protein at the end of the previtellogenic development. These findings suggest that a conserved molecular mechanism for controlling stage specificity is reiteratively used during metamorphic and reproductive responses to ecdysone.

Expression of the early-late gene encoding the nuclear receptor HR3 suggests its involvement in regulating the vitellogenic response to ecdysone in the adult mosquito

The insect steroid hormone, 20-hydroxyecdysone (20E), is a key factor controlling critical developmental events of embryogenesis, larval molting, metamorphosis, and, in some insects, reproduction. We are interested in understanding the molecular basis of the steroid hormone ecdysone action in insect egg development. The yellow fever mosquito, Aedes aegypti, in addition to being an important vector of human diseases, represents an outstanding model for studying molecular mechanisms underlying egg maturation due to stringently controlled, blood meal-activated reproductive events in this insect. To elucidate the genetic regulatory hierarchy controlling the reproductive ecdysone response, we have investigated ecdysone-regulated gene expression in vitellogenic mosquito ovaries and fat bodies. We have previously demonstrated the conservation of a primary ecdysone-triggered regulatory hierarchy, implicated in development of immature stages of Drosophila, represented by the ecdysone receptor/Ultraspiracle complex and an early gene E75 during the reproductive ecdysone response (Wang, S.-F., Miura, K., Miksicek, R.J., Segraves, W.A., Raikhel, A.S., 1998. DNA binding and transactivation characteristics of the mosquito ecdysone receptor - Ultraspiracle complex. J. Biol. Chem. 273, 27531-27540; Pierceall, W. E., Li, C., Biran, A., Miura, K., Raikhel, A.S., Segraves, W.A., 1999. E75 expression in mosquito ovary and fat body suggests reiterative use of ecdysone-regulated hierarchies in development and reproduction. Mol. Cell. Endocrinol. 150, 73-89). The present paper demonstrates that conservation of the factors involved in the ecdysone-responsive genetic hierarchy regulating female reproduction extends beyond the early genes. Here, we identify AHR3, a highly conserved homologue of the Drosophila HR3 early-late ecdysone-inducible gene in the mosquito. We show that AHR3 is expressed in both vitellogenic tissues of the female mosquito, the fat body and the ovary. The expression of AHR3 correlates with the ecdysteroid titer, reaching a peak at 24 h after a blood meal. Moreover, in vitro fat body culture experiments demonstrate that the kinetics and dose response of AHR3 to 20-hydroxyecdysone (20E), an active ecdysteroid in the mosquito, is similar to those of the late vitellogenic genes rather than the early E75 gene. However, as shown for other early and early-late genes, the 20E activation of AHR3 is not inhibited by the presence of cycloheximide, a protein synthesis inhibitor. Taken together, these findings strongly suggest AHR3 involvement in regulating the vitellogenic response to ecdysone in the adult mosquito.

Differential expression and regulation by 20-hydroxyecdysone of mosquito ultraspiracle isoforms

Ultraspiracle (USP), the insect homologue of the vertebrate retinoid X receptor, is an obligatory dimerization partner for the ecdysteroid receptor (EcR). Two USP isoforms, USP-A and USP-B, with distinct N-termini, occur in the mosquito Aedes aegypti. In the fat body and ovary, USP-A mRNA is highly expressed during the pre- and late vitellogenic stages, corresponding to a period of low ecdysteroid titer, while USP-B mRNA exhibits its highest levels during the vitellogenic period, correlating with a high ecdysteroid titer. Remarkably, 20-hydroxyecdysone (20E) has opposite effects on USP isoform transcripts in in vitro fat body culture. This steroid hormone upregulates USP-B transcription and its presence is required to sustain a high level of USP-B expression. In contrast, 20E inhibits activation of USP-A transcription. Although EcR.USP-A recognizes the same ecdysteroid-responsive elements, EcR.USP-B binds them with an affinity twofold higher than that of EcR.USP-A. Likewise, EcR.USP-B transactivates a reporter gene in CV-1 cells twofold more strongly than EcR.USP-A. These results suggest that USP-B functions as a major heterodimerization partner for EcR during the vitellogenic response to 20E in the mosquito.

Site of vitellogenin synthesis determined from a cDNA encoding a vitellogenin fragment in the freshwater giant prawn, Macrobrachium rosenbergii

Vitellogenesis is an important part of reproductive process in crustaceans, and the process is characterized by the synthesis and accumulation of yolk protein in the developing oocytes. The yolk proteins in crustaceans mainly consist of vitellogenin (Vg) and vitellin (Vn), which are respectively present in extra-oocyte tissues and intra-oocytes. The site and the process of yolk protein synthesis in crustaceans are still controversial. The synthesis site of Vg in a crustacean species, Macrobrachium rosenbergii, is determined by immunological and immunohistochemical techniques, and molecular cloning of a cDNA encoding the primary structure of Vn in this study. The hepatopancrease is clearly shown to be the synthesis site of Vg in this species. The length of Vg mRNA was estimated as about 6 kb from Northern blotting analysis. The partial primary structure of Vg gene is presented, and the post-translational processing are further discussed. For the first time, the partial primary structure of Vg gene and the synthesis site of Vg approached by molecular cloning in crustaceans are presented.

Three isoforms of a hepatocyte nuclear factor-4 transcription factor with tissue- and stage-specific expression in the adult mosquito

We cloned three isoforms of hepatocyte nuclear factor-4 (HNF-4) from the mosquito Aedes aegypti, designated AaHNF-4a, AaHNF-4b, and AaHNF-4c. AaHNF-4a and AaHNF-4b are typical members of the HNF-4 subfamily of nuclear receptors with high amino acid conservation. They differ in N-terminal regions and exhibit distinct developmental profiles in the female mosquito fat body, a metabolic tissue functionally analogous to the vertebrate liver. The AaHNF-4b mRNA is predominant during the previtellogenic and vitellogenic phases, while the AaHNF-4a mRNA is predominant during the termination phase of vitellogenesis, coinciding with the onset of lipogenesis. The third isoform, AaHNF-4c, lacks part of the A/B and the entire C (DNA-binding) domains. The AaHNF-4c transcript found in the fat body during the termination of vitellogenesis may serve as a transcriptional inhibitor. Both AaHNF-4a and AaHNF-4b bind to the cognate DNA recognition site in electrophoretic mobility shift assay. Dimerization of AaHNF-4c with other mosquito HNF-4 isoforms or with mammalian HNF-4 prevents binding to the HNF-4 response element. In transfected human 293T cells, AaHNF-4c significantly reduced the transactivating effect of the human HNF-4alpha1 on the apolipoprotein CIII promoter. Electrophoretic mobility shift assay confirmed the presence of HNF-4 binding sites upstream of A. aegypti vg and vcp, two yolk protein genes expressed in the female mosquito fat body during vitellogenesis. Therefore, HNF-4, an important regulator of liver-specific genes, plays a critical role in the insect fat body.

Cloning of cDNA for vitellogenin of the parasitoid wasp, Pimpla nipponica (Hymenoptera: Apocrita: Ichneumonidae): vitellogenin primary structure and evolutionary considerations

The cDNA for vitellogenin (Vg) of the parasitoid wasp Pimpla nipponica (Hymenoptera: Apocrita) was cloned and sequenced. The deduced amino acid sequence with 1807 residues was obtained. The N-terminal 20 amino acids chemically determined for vitellin (Vn) agreed completely with the deduced 20 amino acids that follow the 16 amino acid residues for putative signal peptide. The cDNA clone for the Vg of the turnip sawfly Athalia rosae (Hymenoptera: Symphyta), previously obtained and partially sequenced, was also completely sequenced and the amino acid sequence deduced. Amino acid sequences were compared between these two species and also with known Vg sequences from other insects. Common to all these insects is the presence of two long regions with relatively well-conserved amino acid sequences, one near the N-terminal extending 267-282 residues (including two cysteines at conserved locations), and the other starting at position 450 to 655 and extending 279-283 residues, and of a region at the C-terminal extending some 200 residues (about 250 in Aedes aegypti due to the presence of a serine-rich stretch) with 10 cysteines at conserved locations. A molecular phylogenetic tree was constructed.

Analysis of the mosquito lysosomal aspartic protease gene: an insect housekeeping gene with fat body-enhanced expression

In the fat body of insects with cyclic egg maturation, lysosomes play a critical role in the termination of vitellogenesis by selectively degrading the secretory machinery involved in the massive production of yolk protein precursors. To investigate this fat body-specific lysosomal activity in the mosquito, a cathepsin D-like aspartic protease (LAP) was previously purified and its cDNA cloned. Here we report the isolation of the AaLAP gene from an Aedes aegypti genomic library. The transcribed region of the gene is comprised of five exons, spanning 1904 base pairs. Restriction fragment length polymorphism (RFLP) and genomic clone analyses show this gene to be single copy and polymorphic. Primer extension analysis revealed two putative transcription start sites (TSS). The extension products corresponding to the distal and proximal TSSs were present in both pre- and vitellogenic fat bodies, suggesting that both TSSs are involved in housekeeping as well as tissue-specific expression of this gene. TATA box-like and arthropod initiator sequences, hallmarks of regulated genes, were present near each putative TSS. Several sequences resembling binding sites for liver- and fat body-specific transcription factors were identified within 1 kb upstream and downstream of the gene. Significantly, direct binding for the C/EBP and GATA families of transcription factors was demonstrated in vitro by electrophoretic mobility shift assays (EMSA). Three sequences located upstream of AaLAP resembled the Drosophila melanogaster yolk protein fat body enhancer (Dm Yp FBE). Potential hormone-response elements were also recognized in the gene; however, they did not bind the mosquito ecdysteroid receptor/Ultraspiracle heterodimer in EMSA experiments, indicating that these sequences may interact with different nuclear receptors.

Ovarian- and somatic-specific transcripts of the mosquito clathrin heavy chain gene generated by alternative 5'-exon splicing and polyadenylation

Insect oocytes are extraordinarily specialized for receptor-mediated endocytosis of yolk protein precursors. The clathrin heavy chain (CHC) is the major structural protein of coated vesicles, the principal organelles of receptor-mediated endocytosis. To understand the role of clathrin in the development of the oocyte's powerful endocytotic machinery we determined the structure of the mosquito chc gene. The gene spans approximately 45 kilobases and its coding region is divided into seven exons, five of which encode the protein. Three distinct mature transcripts of this gene were identified in mosquito tissues. Two of them code isoforms of the CHC polypeptide differing in their NH2-terminal sequences, and are specifically expressed in female germ-line cells. The third transcript has a 3'-untranslated region about 1 kilobase longer than the other variants, and is found only in the somatic cells. Tissue-specific 5'-exon splicing and alternative polyadenylation of the primary transcript combine to give rise to these mRNAs. We identified two alternative promoters, distal and proximal, separated by approximately 10 kilobases involved in tissue-specific regulation of mosquito chc gene expression. Our data provide the first molecular evidence for complex structure and regulation of a chc gene, in this case occurring at both the transcriptional and post-transcriptional levels.

Molecular characterization of the mosquito vitellogenin receptor reveals unexpected high homology to the Drosophila yolk protein receptor

The mosquito (Aedes aegypti) vitellogenin receptor (AaVgR) is a large membrane-bound protein (214 kDa when linearized) that mediates internalization of vitellogenin, the major yolk-protein precursor, by oocytes during egg development. We have cloned and sequenced two cDNA fragments encompassing the entire coding region of AaVgR mRNA, to our knowledge the first insect VgR sequence to be reported. The 7.3-kb AaVgR mRNA is present only in female germ-line cells and is abundant in previtellogenic oocytes, suggesting that the AaVgR gene is expressed early in oocyte differentiation. The deduced amino acid sequence predicts a 202.7-kDa protein before posttranslational processing. The AaVgR is a member of the low density lipoprotein receptor superfamily, sharing significant homology with the chicken (Gallus gallus) VgR and particularly the Drosophila melanogaster yolk protein receptor, in spite of a very different ligand for the latter. Distance-based phylogenetic analyses suggest that the insect VgR/yolk protein receptor lineage and the vertebrate VgR/low density lipoprotein receptor lineage diverged before the bifurcation of nematode and deuterostome lines.

Cloning and characterization of cDNAs encoding the antibacterial peptide, defensin A, from the mosquito, Aedes aegypti

Insect defensins are cationic, inducible antibacterial peptides. Four full-length cDNAs encoding defensin A from the mosquito Aedes aegypti were cloned using polymerase chain reaction (PCR) and sequenced. All four cDNAs are 473 base pairs long, bearing an open reading frame of 98 amino acids with a few substitutions in the signal peptide domain. The deduced amino acid sequence of Aedes aegypti defensin (AaDef) contains a signal peptide sequence of 18 amino acids followed by a 40-amino acid putative propeptide domain and a 40-amino acid mature peptide domain. The mature peptide, with a predicted M(r) of 4148, shows 80% identity and 93% similarity to Phormia defensin A, and is identical to the peptide sequencing data for mosquito defensin A of Lowenberger et al. (1995) and B of Chalk et al. (1995). There are three potential phosphorylation sites but no glycosylation sites detected in AaDef. Three putative disulfide linkages between cysteines, characteristic of insect defensins, are conserved in AaDef. Aedes aegypti defensin mRNA is produced in response to a bacterial challenge; it is dramatically enhanced 6 h after bacterial injection, continues to increase through 24 h, and is maintained at high levels until at least 30 h post-bacterial injection.

Vitellogenin receptors: oocyte-specific members of the low-density lipoprotein receptor supergene family

Receptors that transport vitellogenin (VTG) into oocytes are of vital importance to egg-laying species, because they mediate a key step of oocyte maturation, a prerequisite to reproduction. Vitellogenins are lipophosphoglycoproteins that are produced under female hormonal control in large central organs (fat body in insects; liver in higher animals) and are transported in the circulation to the female gonads. VTG receptors localized in coated pits on the surface of growth-competent oocytes are able to accumulate in the yolk high concentrations of VTG and other ligands they recognize. The study of VTG receptors and their ligands has identified genes that specify related ligands, and a family of receptors. To date, all molecularly characterized VTG receptors belong to the low-density lipoprotein receptor supergene family, which ranges from a 600-kDa receptor in Caenorhabditis elegans to the 100-kDa so-called very-low-density lipoprotein receptors in mammals. These receptors, by and large, recognize ligands with similarities in structural elements first defined in the human apoplipoproteins B-100 and E. Recent studies on the receptor family have added VTG and lipoprotein lipase to the list of co-evolved ligands and have revealed that VTG receptors are able to interact with ligands other than VTG and also with some unrelated to lipoprotein metabolism. For example, the chicken VTG receptor also imports very-low-density lipoprotein, riboflavin-binding protein, and alpha-2-macroglobulin into growing oocytes. Such multifunctionality of receptors is likely the result of evolutionary pressure to provide the female germ cell with a highly economical machinery for vitellogenesis.

Analysis of a vitellogenin gene of the mosquito, Aedes aegypti and comparisons to vitellogenins from other organisms

A genomic clone of the Aedes aegypti vitellogenin A1 gene was sequenced including 2015 bp of 5' untranscribed sequence, 6369 bp of open reading frame interrupted by two introns, and a short 3' untranslated region. Primer extension was used to identify the transcription initiation site. The amino termini of the large and small subunits were located by N-terminal sequencing of vitellin purified from eggs. The length of the signal sequence and the position of the cleavage site between the two subunits were also determined. Three sequential imperfect repeats were found near the beginning of the small subunit. The sequence of the coding region appears to be polymorphic. Comparison of the signal sequences of seven insect vitellogenin genes revealed several conserved leucines, and a conserved position of an intron. However, the signal sequences are not conserved between these genes and the yolk protein genes of Cyclorraphid Dipteran insects. The cleavage sites between the small and large subunits in the vitellogenins of the mosquito, A. aegypti, sawfly, Athalia rosae, boll weevil, Anthonomus grandis, and silkworm, Bombyx mori are flanked by sequences rich in serine. Pairwise dot matrix analysis at the protein level showed that the mosquito, boll weevil and silkworm vitellogenins are significantly related with approx. 50% similarity. One region of the three insect vitellogenin genes, near the N-terminal of the large subunit, showed the highest levels of similarity, from 57.5 to 64.4%. The position of cysteines in insect vitellogenins is conserved, particularly in the C-terminus of the large subunit. Dot matrix comparison of the mosquito vitellogenin with that of Xenopus laevis and Caenorhabditis elegans showed much lower, but still significant degrees of relationship. Pairwise comparisons of the mosquito vitellogenin and the Drosophila melanogaster yolk proteins did not show significant similarities. Potential regulatory regions in the mosquito VgA1 gene were identified by comparison to regulatory elements known from other organisms, especially D. melanogaster, which could provide useful information for further functional analysis.

Structure and expression of mRNA for vitellogenin in Bombyx mori

Vitellogenin, a precursor of major yolk protein of the silkworm, Bombyx mori is a tetramer composed of each two molecules of heavy and light subunits. We cloned mRNA sequence for the B. mori vitellogenin and analyzed its structure. Sequence alignment of several overlapping cDNA clones indicated that the vitellogenin mRNA is approx. 5.7 kb, containing an open reading frame for a peptide with 1782 amino acid residues. By comparing the deduced amino acid sequence with the amino-terminal primary structures of vitellogenin subunits, it is suggested that the heavy and light subunits of the B. mori vitellogenin are encoded by a single contiguous mRNA. The primary translation product of the vitellogenin mRNA was detected in the microsomal fraction prepared from the fat body of vitellogenic females. Northern blot analysis of the fat body RNA demonstrated that the biosynthesis of vitellogenin in B. mori is regulated in a tissue-, sex- and stage-specific manner at the level of mRNA. Possible cause for discrepancy between the present results and our previous proposal (Izumi, S. and Tomino, S. (1983) Insect Biochem. 13, 81-85) on the biosynthesis of B. mori vitellogenin is also discussed.

cDNA cloning and expression of Rhodnius prolixus vitellogenin

It was shown that Rhodnius prolixus vitellogenin (Vg) is synthesized as precursors of 205 and 190 kDa. Each Vg subunit is antigenically related to a domain in the precursor molecules. Since Vg has been previously detected in R. prolixus male adults, protein synthesis by fat bodies from 5th instar male nymphs was investigated and no Vg synthesis could be detected. Also, a 6.1 Kb RNA is present in female adults but not in 5th instar male nymphs. Therefore, cDNAs from female adult and 5th instar male fat bodies were used for differential screening of a female fat body cDNA library leading to the isolation of several female specific clones. All the clones hybridizing to the female specific 6.1 Kb RNA species were identical. We also describe the construction of new expression vectors, pGex-A and pGex-B, derived from the previously described plasmid pGex-1N. The new vectors, together with pGex-3X, comprise a set of expression plasmids with cloning sites in all three possible reading frames that give a fusion polypeptide with the glutathione S-transferase. This carrier protein can be cleaved by digestion with factor Xa in all three plasmids; one of the Vg cDNA clones was subcloned in pGex-A. Antibodies affinity purified from the fusion protein Vg/glutathione S-transferase recognized both large Vg subunits, suggesting an antigenic relationship between them. Furthermore, the small Vg subunits were not recognized, indicating that they may be localized at the N-terminal region of Vg precursors.

Proteolytic processing of the vitellogenin precursor in the boll weevil, Anthonomus grandis

The soluble proteins of the eggs of the coleopteran insect Anthonomus grandis Boheman, the cotton boll weevil, consist almost entirely of two vitellin types with M(r)s of 160,000 and 47,000. We sequenced their N-terminal ends and one internal cyanogen bromide fragment of the large vitellin and compared these sequences with the deduced amino acid sequence from the vitellogenin gene. The results suggest that both the boll weevil vitellin proteins are products of the proteolytic cleavage of a single precursor protein. The smaller 47,000 M(r) vitellin protein is derived from the N-terminal portion of the precursor adjacent to an 18 amino acid signal peptide. The cleavage site between the large and small vitellins at amino acid 362 is adjacent to a pentapeptide sequence containing two pairs of arginine residues. Comparison of the boll weevil sequences with limited known sequences from the single 180,000 M(r) honey bee protein show that the honey bee vitellin N-terminal exhibits sequence homology to the N-terminal of the 47,000 M(r) boll weevil vitellin. Treatment of the vitellins with an N-glycosidase results in a decrease in molecular weight of both proteins, from 47,000 to 39,000 and from 160,000 to 145,000, indicating that about 10-15% of the molecular weight of each vitellin consists of N-linked carbohydrate. The molecular weight of the deglycosylated large vitellin is smaller than that predicted from the gene sequence, indicating possible further proteolytic processing at the C-terminal of that protein.

An extraovarian protein accumulated in mosquito oocytes is a carboxypeptidase activated in embryos

We report a phenomenon previously unknown for oviparous animals; in Aedes aegypti mosquitoes a serine carboxypeptidase is synthesized extraovarially and then internalized by oocytes. The cDNA encoding mosquito vitellogenic carboxypeptidase (VCP) was cloned and sequenced. The VCP cDNA hybridizes to a 1.5-kilobase mRNA present only in the fat body of vitellogenic females. The deduced amino acid sequence of VCP shares significant homology with members of the serine carboxypeptidase family. Binding assays using a serine protease inhibitor, [3H]diisopropyl fluorophosphate, showed that VCP is activated in eggs at the onset of embryonic development. Activation of VCP is associated with the reduction in its size from 53 kDa (inactive proenzyme) to 48 kDa (active enzyme). The active, 48-kDa, form of VCP is maximally present at the middle of embryonic development and disappears by the end.