Complete cDNA-sequence of the receptor responsible for arylphorin uptake by the larval fat body of the blowfly, Calliphora vicina

Molecular Characteristics of Fat Body Protein 1 in the Oriental Fruit Fly, Bactrocera dorsalis

Simple Summary

Bactrocera dorsalis fat body protein 1 (Bdfbp1) cDNA was cloned. The deduced amino acid sequence contains three motifs: hemocyanin N, high molecular weight glutenin (gultenin hmw), and hemocyanin C from N to C termini. The glutenin hmw allows Bdfbp1 to fold into a compact form for storage. Bdfbp1 was highly expressed in the late third instar larvae and day 0 pupae. This suggests that Bdfbp1 is stored during larval stages as a storage protein for construction of adult tissues during pupal stages, and may be associated with adult eclosion.

Abstract

Bactrocera dorsails fat body protein 1 (Bdfbp1) cDNA was cloned (GenBank accession no. MT514270), and the complete 3,749-bp cDNA encoded a 1,152-amino acid protein. The phylogenetic relationship of dipteran fbp1s was analyzed. The sequence XP_028900815 from the insect genome project for Zeugodacus cucurbitae (LOC105219342) was proposed that two fbp1 genes were present in the sequence. The developmental transcriptional expression profiles were determined. In the larval stages, Bdfbp1 mRNA had significantly higher expression in the late third instar larvae compared with first, second, and early third instar larvae. In the pupal stages, the highest expression of Bdfbp1 mRNA was found in the newly pupated pupae and then decreased with age. In the fat body of female adults, Bdfbp1 was highly expressed in newly emerged samples and decreased rapidly over the following three days. In the fat body of male adults, Bdfbp1 was highly expressed in newly eclosed samples. RNAi treatment decreased the expression level of Bdfbp1 without statistical difference. However, RNAi treatment significantly decreased the rate of eclosion. These results suggest that Bdfbp1 may function as a storage protein and be associated with adult eclosion.

Molecular characterization of Antheraea mylitta arylphorin gene and its encoded protein

Antheraea mylitta arylphorin protein was extracted from the silk gland of fifth instar larvae and purified by ammonium sulphate precipitation, ion-exchange, and gel filtration chromatography. The N-terminal sequencing of ten amino acids (NH₂-SVVHPPHHEV-COOH) showed similarity with Antheraea pernyi arylphorin. Based on N-terminal and C-terminal A. pernyi arylphorin sequences, primers were designed, and A. mylitta arylphorin cDNA was cloned by RT-PCR from silk gland mRNA. Sequencing of complete cDNA including 25 nucleotides at 5' UTR (obtained by 5' RACE) showed that it consisted of an ORF of 2115 nucleotides which could encode a protein of 704 amino acids (predominantly aromatic residues) having molecular weight 83 kDa. Homology modelling was done using A. pernyi arylphorin as a template. Cloned arylphorin cDNA was expressed in E. coli and recombinant His-tagged protein was purified by Ni-NTA affinity chromatography. Analysis of tissue-specific expression of arylphorin by real-time PCR showed maximum expression in the fat body followed by silk gland and integument. 5' flanking region (759 bp) of arylphorin gene was amplified by inverse PCR and the full length gene (5359 nucleotides) containing five exons and four introns was cloned from the A. mylitta genomic DNA and sequenced. Polyclonal antibody was raised against purified arylphorin and more native arylphorin protein (500 kDa) was purified from the fat body by antibody affinity chromatography. Study of mitogenic effect of native and chymotrypsin hydrolysate of arylphorin on different insect cell lines showed that arylphorin could be used as serum substitute for in vitro cultivation of insect cells.

Ecdysteroid-mediated expression of hexamerin (arylphorin) in the rice moth, Corcyra cephalonica

The insect development is intricately controlled by morphogenetic hormones, juvenile hormone (JH) and 20-hydroxyecdysone (20E) through the regulation of gene/protein expression. The role of hexamerins in the metamorphosis of insects and reproduction and their control by 20E at the gene level has been widely reported in insects. In the present study we for the first time report the role of ecdysteroids in the regulation of hexamerin synthesis in a lepidopteran insect Corcyra cephalonica. The hormonal studies were carried out using the normal and the thorax-ligated insects with both 20E and its non-steroidal agonist RH-5992. The in vitro as well as in vivo studies showed a stimulatory effect of 20E and its agonist on the hexamerin synthesis including arylphorin (Hex 2), whereas hormone blockade with azadirachtin caused a time dependent reduction in synthesis. The northern analysis using Hex 2b cDNA as probe too confirmed the above result. This was followed by the cloning of the Hex 2b gene. The full length of the genomic clone was found to be 3.5kb long and has four exons interspersed by three introns. The genome walking analysis revealed the presence of a steroid hormone binding sequence "Ecdysone response element" (EcRE) in the 5' untranscribed region (UTR) of the gene. The data presented in this paper clearly suggest that hexamerin synthesis in C. cephalonica is transcriptionally regulated by 20E.

Structural basis for the presence of a monoglucosylated oligosaccharide in mature glycoproteins

Arylphorin is an insect hexameric storage protein. The structures of the oligosaccharides attached to this protein have recently been determined. However, their precise functions remain to be established. Proteolysis and MALDI MS studies disclose that the amino acid residues Asn196 and Asn344 are N-glycosylated with Glc(1)Man(9)GlcNAc(2) and Man(5-6)GlcNAc(2) oligosaccharides, respectively. Interestingly, significant variations in the amounts of glycans involving Glc(1)Man(9)GlcNAc(2) are evident in arylphorins purified from larvae reared at different seasons. The data suggest that the metabolism of larvae and local protein structure contribute to glycan development. Three-dimensional model of the protein speculated that N-glycosidic linkage to Asn196 in the Glc(1)Man(9)GlcNAc(2) structure was buried inside the twofold axis of the hexamer, whereas oligosaccharide linkages to Asn344 were completely exposed to solvent. This finding is in agreement with previous biochemical data showing that limited Glc(1)Man(9)GlcNAc(2) was released by protein-N-glycosidase F under non-denaturing conditions, in contrast to Man(5-6)GlcNAc(2) oligosaccharides.

Functional dissection of the hexamerin receptor and its ligand arylphorin in the blowfly Calliphora vicina

The process of receptor-mediated uptake of hexamerin storage proteins from insect haemolymph by fat body cells is a unique feature of the class Insecta. We identified the binding domains of the hexamerin receptor and the hexamerin ligand arylphorin in the blowfly, by means of the yeast-two-hybrid-system. The receptor-binding domain of arylphorin was located within domain 3 of the arylphorin monomer. The ligand-binding domain of the hexamerin receptor was mapped to the extreme N-terminus of the receptor. The binding domains identified exhibit no similarity to any functional protein domains known to date. Additionally, we identified two previously unknown protein-interactors of the hexamerin receptor. The results of this study provide further insights regarding the mechanism of the receptor-mediated endocytosis of storage proteins in insects.

Interaction of the anterior fat body protein with the hexamerin receptor in the blowfly Calliphora vicina

In late larvae of the blowfly, Calliphora vicina, arylphorin and LSP-2 proteins, which belong to the class of hexamerins, are selectively taken up by the fat body from the haemolymph. Hexamerin endocytosis is mediated by a specific membrane-bound receptor, the arylphorin-binding protein (ABP). Using the two-hybrid technique, we found that the anterior fat body protein (AFP) interacts with the hexamerin receptor. AFP, a homologue of the mammalian calcium-binding liver protein regucalcin (senescence marker protein-30), exhibits a strong binding affinity for a naturally occurring C-terminal cleavage fragment of the hexamerin receptor precursor (the P30 peptide) and other receptor cleavage products that contain P30. Expression of AFP mRNA and protein is restricted to the anterior part of the fat body tissue and to haemocytes in last-instar larvae. AFP mRNA occurs in all postembryonic developmental stages. Our results suggest that AFP plays a role in the regulation of hexamerin uptake by fat body cells along the anterior-posterior axis.

Ecdysone-regulation of synthesis and processing of fat body protein 1, the larval serum protein receptor of Drosophila melanogaster

At the end of the third larval instar of Drosophila melanogaster, larval serum proteins 1 and 2 (LSP-1 and -2) are taken up by cells of the fat body. Here, we show that the product of the ecdysteroid-inducible gene Fbp-1 (Fat Body Protein 1) is the receptor that binds LSP-1. Transcription and translation of Fbp-1 is stage-specifically restricted to the end of the third larval instar, starting around 99 h after egg laying. Expression of Fbp-1 is induced by a low level of 20-hydroxy-ecdysone (>/= 10-7 m). After translation, the FBP-1 protein is thought to be proteolytically cleaved in three subsequent steps. The final cleavage step is delayed by 6 h and relies on a higher concentration of ecdysone (>/= 10-5 m). Therefore, 20-hydroxy-ecdysone regulates Fbp-1 expression and function at two different levels. To the best of our knowledge, this study is the first to date to demonstrate two distinct functions for different concentrations of a steroid hormone on a single biochemical process.

Complete sequence, expression, and evolution of the hexamerin LSP-2 of Calliphora vicina

In cyclorraphan Diptera, two different types of hemolymph proteins exist which belong to the hexamerin family. During the last larval instar, Calliphora vicina synthesizes, besides the major fraction of arylphorin, a second hexameric protein, LSP-2. Here the developmentally regulated biosynthesis of this protein was analyzed. Western blot analyses showed that LSP-2 is not present in eggs, 1st, and 2nd instar larvae, whereas it can be detected in all tissues of last instar larvae. We report the characterization of the complete cDNA sequence that encodes a LSP-2 subunit, a nascent polypeptide of 701 amino acids with a molecular mass of 83.16 kDa. By Northern blotting, a mRNA of about 2.2 kb coding for LSP-2 is identified exclusively in the fat body of 3rd larval instars reflecting the stage and tissue specificity of LSP-2 gene expression. Phylogenetic analysis demonstrates the existence of two distinct groups of hexamerins in Diptera.

Distribution and accumulation of storage protein-1 in ovary of Hyphantria cunea Drury

Storage protein-1 (SP-1) is a major storage protein found in the hemolymph and fat body of Hyphantria cunea. In this study, the uptake and accumulation of SP-1 into the ovary of H. cunea was investigated using biochemical and immunocytochemical methods. SP-1 in H. cunea has a high methionine content (4.6%) but is not female-specific, like other high methionine storage proteins. In the 6-day-old pupal ovary, SP-1 was detectable in trace amounts but accumulated to significant levels toward the end of the pupal stage. After adult emergence, SP-1 rapidly decreased in the ovarian follicles and remained low in the egg. This suggest that SP-1 is either extensively modified or degraded, causing a loss of its antigenic property in the ovary after adult emergence. During vitellogenesis, SP-1 is present in the hemolymph and penetrates through the tunica propria to reach the perioocytic space. From there, SP-1 is incorporated into yolk bodies. These results clearly show that SP-1 is taken up by the developing oocyte. Its disappearance suggests that SP-1 might be an amino acid reservoir for providing precursors for egg formation, in contrast to yolk proteins, which are utilized during postembryonic development.

Differential accumulation and tissue distribution of mosquito hexamerins during metamorphosis

The pupal hexamerins were characterized for two mosquitoes representative of the culicine and anopheline families, Aedes aegypti and Anopheles gambiae. Like higher Diptera, both mosquito species express two types of hexamerins, Hex-1 and Hex-2, whose subunits are distinguished by different levels of methionine and aromatic amino acids. In A. aegypti there are two heterohexamers, AaHex-1 and AaHex-2. In A. gambiae there are two homohexamers, AgHex-1.1 and AgHex-1.2, and one heterohexamer, AgHex-2. These hexamerins are rich in aromatic residues, with 18-23% Phe + Tyr for Hex-1 subunits and 13-17% Phe + Tyr for Hex-2 subunits. In addition, both mosquito species synthesize methionine-rich Hex-1 subunits: Aedes AaHex-1 gamma (8% met) and Anopheles AgHex-1.1 (3.9% met). Aedes Hex-1 and Hex-2 proteins exhibit different, stage-specific tissue distributions: AaHex-2 is the primary hexamerin of late larval hemolymph whereas AaHex-1 is the most important non-hemolymph protein of early pupae. Although both proteins are stored in the pupal fat body, peak AaHex-1 levels are 2-fold higher. Both pupal protein levels decline rapidly between 25 and 36 h after pupation. Furthermore, AaHex-1 not only reaches peak values in female Aedes pupae later than in males, but the methionine-rich AaHex-1 gamma subunit level is specifically higher in females. These observations suggest different roles for Hex-1 and Hex-2 during mosquito development.

Developmentally controlled cleavage of the Calliphora arylphorin receptor and posttranslational action of the steroid hormone 20-hydroxyecdysone

In response to a rise in ecdysteroid titre, fat body cells of insect larvae take up storage proteins from the haemolymph by receptor-mediated endocytosis. Here we show that the receptor responsible for incorporation of the major haemolymph protein arylphorin of the blowfly, Calliphora vicina, is subject to an unusual posttranslational processing that involves three distinct cleavage steps. After the removal of a 17-amino-acid signal peptide, a receptor precursor of 141 kDa is released. Before reaching the cell surface, the precursor is cleaved a second time, giving rise to the active 92-kDa arylphorin receptor, plus a 48-kDa peptide. The function of this 48-kDa peptide may be the prevention of premature ligand-receptor interaction in the endoplasmic reticulum. 20-Hydroxyecdysone initiates a third cleavage step of the arylphorin receptor, which results in a 62-kDa arylphorin binding protein and a 30-kDa peptide. Contrary to the standard model of steroid hormone action, the process which give rise to receptor cleavage can be induced by 20-hydroxyecdysone in vivo and in vitro even in absence of protein biosynthesis.

Molecular cloning and expression of a hexamerin cDNA from the malaria mosquito, Anopheles gambiae

During the last larval instar, dipteran insects synthesize two hexamerins rich in aromatic residues, typified by the larval serum proteins 1 and 2 (LSP-1 and LSP-2) of Drosophila melanogaster. We report here the characterization of a complete cDNA sequence encoding a LSP-1-like protein from a lower dipteran insect, the malaria mosquito Anopheles gambiae. The cDNA encodes the subunit of a homohexamer, A. gambiae hexamerin-1.1 (AgHex-1.1), which is a major pupal protein but only a minor constituent of late larval hemolymph. AgHex-1.1 is moderately rich in methionine (3.9%) and particularly rich in aromatic residues (21% Phe+Tyr). Cytogenetic analysis reveals AgHex-1.1 to be encoded by a single-copy gene localized to division 22F within the proximal 2La inversion breakpoint of chromosome 2 of A. gambiae. The AgHex-1.1 transcript is first detected in fourth-instar larvae (L4) and disappears abruptly in early pupae. In situ hybridization shows accumulation of the transcript uniquely in the larval fat body. AgHex-1.1 mRNA is re-expressed in male and female adults at about 10% of the L4 level, with no effect of bloodfeeding in females. The potential roles of AgHex-1.1 in Anopheles development and reproductive maturation are discussed.

Conservation of hexamerin endocytosis in Diptera

In cyclorrhaphan Diptera at least two different types of haemolymph proteins exist which belong to the class of hexamerins. In the last larval instar of Calliphora vicina, the highly aromatic hexamerin, arylphorin, and the second hexamerin, PII, make up about 90% of haemolymph proteins. Both of these proteins are selectively taken up by the fat body cells at the end of larval life and share a common membrane-bound receptor. In addition, hexamerins and possible hexamerin receptors of Calliphora vicina, Calliphora vomitoria, Drosophila melanogaster, Ceratitis capitata, Sarcophaga bullata, Musca domestica and Protophormia terraenovae were investigated. Uptake of arylphorin by the larval fat bodies of Calliphora vicina as well asarylphorin-receptor binding can be competed in vitro by haemolymph from other Diptera. Therefore, hexamerin-receptor binding must be conserved among related cyclorrhaphan Diptera and between different types of hexamerins within a species. As the degree of competition is in good agreement with the presumed phylogenetic distances between these species, the method described here provides a simple tool to estimate evolutionary distances.

Insect storage proteins: gene families and receptors

The accumulation and utilization of storage proteins are prominent events linked to the metamorphosis of holometabolous insects. Storage proteins are synthesized in fat body, secreted into the larval hemolymph and taken up by fat body shortly before pupation. Within the pupal fat body, these proteins are initially stored in protein granules, and later proteolytically broken down to supply amino acid resources necessary for the completion of adult development. Most, but not all storage proteins belong to a superfamily of hexameric larval serum proteins that are evolutionarily related to hemocyanin. This article reviews the classification of these proteins, based on their amino acid sequences, and the current knowledge of the receptors that mediate their selective uptake into pupal fat body.

Common origin of arthropod tyrosinase, arthropod hemocyanin, insect hexamerin, and dipteran arylphorin receptor

Dipteran arylphorin receptors, insect hexamerins, cheliceratan and crustacean hemocyanins, and crustacean and insect tyrosinases display significant sequence similarities. We have undertaken a systematic comparison of primary and secondary structures of these proteins. On the basis of multiple sequence alignments the phylogeny of these proteins was investigated. Hexamerin subunits, hemocyanin subunits, and tyrosinases share extensive similarities throughout the entire amino acid sequence. Our studies suggest the origin of arthropod hemocyanins from ancient tyrosinase-like proteins. Insect hexamerins likely evolved from hemocyanins of ancient crustaceans, supporting the proposed sister-group position of these subphyla. Arylphorin receptors, responsible for incorporation of hexamerins into the larval fat body of diptera, are related to hexamerins, hemocyanins, and tyrosinase. The receptor sequences display extensive similarities to the first and third domains of hemocyanins and hexamerins. In the middle region only limited amino acid conservation was observed. Elements important for hexamer formation are deleted in the receptors. Phylogenetic analysis indicated that dipteran arylphorin receptors diverged from ancient hexamerins, probably early in insect evolution.