Ligand-binding domains in vitellogenin receptors and other LDL-receptor family members share a common ancestral ordering of cysteine-rich repeats

The DUF1943 and VWD domains endow Vitellogenin from Crassostrea gigas with the agglutination and inhibition ability to microorganism

Vitellogenin (Vg) is the major precursor of the egg-yolk proteins, which mainly acts as an energy reserve molecule for providing nutrients during embryonic development. Vg also plays an immune function in vertebrates such as fish, but there are few studies on the immune function of Vg in invertebrates. In the present study, a Vg homologue (CgVg) was identified and characterized in oyster Crassostrea gigas. There are three domains in the CgVg protein, including a Vitellogenin_N domain, a domain of unknown function 1943 (DUF1943) and a von Willebrand factor type D domain (VWD). The mRNA transcripts of CgVg were detected in all tested tissues with high expression in the gonad, hepatopancreas and haemocytes, which was 466.29-, 117.15- and 57.49-fold (p < 0.01) of that in adductor muscle, respectively. After Vibrio splendidus stimulation, the mRNA expression level of CgVg in haemocytes increased significantly at 6, 12 and 24 h, which was 1.97-, 3.58- and 1.3-fold (p < 0.01) of that in the seawater group, respectively. The immunofluorescence assay showed that positive signals of CgVg protein were mainly located at the cytoplasm of haemocytes. The recombinant protein of DUF1943 domain (rDUF1943) and VWD domain (rVWD) was able to bind lipopolysaccharide (LPS), mannose (MAN), peptidoglycan (PGN) and poly (I:C), as well as Gram-positive bacteria (Staphylococcus aureus and Micrococcus luteus), Gram-negative bacteria (Escherichia coli and V. splendidus) and fungi (Pichia pastoris). rDUF1943 exhibited stronger agglutination activity towards S. aureus, M. luteus, E. coli, V. splendidus and P. pastoris, while agglutination was only observed in the rVWD group towards P. pastoris. The rVWD inhibited the growth of E. coli, S. aureus and V. splendidus, while no antibacterial activity was detected in rDUF1943 group. Collectively, CgVg not only functioned as a pattern recognition receptor (PRR) to bind various microorganisms and PAMPs, but also as an immune effector participating in the clearance of invaders, in which DUF1943 and VWD domain were mainly responsible for agglutinating and inhibiting microorganism respectively.

Identification and structural characterization of the factors involved in vitellogenesis and its regulation in the African Osteoglossiforme of aquacultural interest Heterotis niloticus (Cuvier, 1829)

The African bonytongue (Heterotis niloticus) is an excellent candidate for fish farming because it has outstanding biological characteristics and zootechnical performances. However, the absence of sexual dimorphism does not favor its reproduction in captivity or the understanding of its reproductive behavior. Moreover, no molecular data related to its reproduction is yet available. This study therefore focuses on the structural identification of the different molecular actors of vitellogenesis expressed in the pituitary gland, the liver and the ovary of H. niloticus. A transcriptomic approach based on de novo RNA sequencing of the pituitary gland, ovary and liver of females in vitellogenesis led to the creation of three transcriptomes. In silico analysis of these transcriptomes identified the sequences of pituitary hormones such as prolactin (PRL), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) and their ovarian receptors (PRLR, FSHR, LHR). In the liver and ovary, estrogen receptors (ER) beta and gamma, liver vitellogenins (VtgB and VtgC) and their ovarian receptors (VLDLR) were identified. Finally, the partial transcript of an ovarian Vtg weakly expressed compared to hepatic Vtg was identified based on structural criteria. Moreover, a proteomic approach carried out from mucus revealed the presence of one Vtg exclusively in females in vitellogenesis. In this teleost fish that does not exhibit sexual dimorphism, mucus Vtg could be used as a sexing biomarker based on a non-invasive technique compatible with the implementation of experimental protocols in vivo.

Identification and functional characterization of a novel member of low-density lipoprotein receptor-related protein (LRP)-like family in amphioxus

Low-density lipoprotein receptor-related protein (LRP) is a group of important endocytic receptors contributing to binding ligands and maintaining internal environment. In this study, we identified a soluble LRP-like molecule in the amphioxus B. japonicum, BjLRP, with an uncharacterized domain structure combination of LY-EGF-CRD-EGF-CRD. It was mainly expressed in the gill, muscle, notochord and testis, and was significantly up-regulated following the challenge with bacteria. Recombinant BjLRP was capable of interacting with both Gram-negative and positive bacteria as well as PAMPs including lipopolysaccharide (LPS), lipoteichoic acid (LTA) and peptidoglycan (PGN). Interestingly, recombinant LY peptide was also able to bind to the Gram-negative and positive bacteria as well as the PAMPs LPS, LTA and PGN. By contrast, none of recombinant EGF1, EGF2, CRD1 and CRD2 had affinity to the bacteria and the PAMPs. In addition, BjLRPΔLY had no affinity to the PAMPs, although BjLRPΔLY showed slight affinity to the bacteria. These suggest that the interaction of BjLRP with the bacteria and PAMPs was primarily attributable to the LY domain. It is clear that BjLRP is a novel pattern recognition protein capable of identifying and interacting with invading bacteria in amphioxus.

Alterations of tissue metallothionein and vitellogenin concentrations in tropical cup oysters (Saccostrea sp.) following short-term (96h) exposure to cadmium

Metallothioneins and vitellogenins are low molecular weight proteins that have been used widely in environmental monitoring as biomarkers of exposure and damage to metals and estrogenic compounds, respectively. In the present study, the responses of metallothionein and vitellogenin tissue concentrations were measured following acute (96h) aqueous exposures to cadmium in Saccostrea sp., a tropical cup oyster native to the Western Pacific Ocean that has recently established itself in the Caribbean Sea. Adult oysters (1.5-5.0cm shell length) collected from the municipal marina of Santa Marta, Colombia (Caribbean Sea) and acclimated for 5days in the laboratory, were exposed to Cd at five concentrations (0, 1, 10, 100 and 1000μg/L) and their tissues (gills, digestive gland and adductor muscle) were analyzed in pools of 5 individuals (3 replicates per concentration). Metallothioneins in digestive glands of oysters exposed to Cd concentrations≥100μg/L showed a significant increase, from 8.0 to 14.8μg MT/mg total protein, whereas metallothionein concentrations in gills increased to lesser extent, and no differences were observed in adductor muscle. Metallothionein concentrations in digestive gland and gills correlated directly with whole soft tissue Cd concentrations (ranging from 2 to 297μg/g dw Cd). Vitellogenin in homogenates of oyster gonad tissue, after 96h of exposure to 1000μg/L Cd, were significantly lower (0.04mg P/g gonad) compared to control oysters (0.68mg P/g gonad), suggestive of an anti-estrogenic effect of Cd at high concentrations, whereas no significant changes in vitellogenin concentrations were observed at intermediate Cd exposure concentrations. This study confirms acute responses of metallothionein and vitellogenin concentrations in tissues of Saccostrea sp. exposed to high concentrations of cadmium (Cd≥100μg/L, 96h). The present results are first step towards validating the use of these two proteins as biomarkers of metal exposure in this species.

Molecular Characterization of Vitellogenin and Vitellogenin Receptor of Bemisia tabaci

Vitellogenin (Vg) plays vital role in oocytes and embryo development in insects. Vg is synthesized in the fat body, moves through haemolymph and accumulates in oocytes. Vitellogenin receptors (VgR) present on the surface of oocytes, are responsible for Vg transportation from haemolymph to oocytes. Here, we cloned and characterized these genes from Bemisia tabaci Asia1 (BtA1) species. The cloned BtA1Vg and BtA1VgR genes consisted of 6,330 and 5,430 bp long open reading frames, which encoded 2,109 and 1,809 amino acid (AA) residues long protein. The BtA1Vg protein comprised LPD_N, DUF1943 and VWFD domains, typical R/KXXR/K, DGXR and GL/ICG motifs, and polyserine tracts. BtA1VgR protein contained 12 LDLa, 10 LDLb and 7 EGF domains, and a trans-membrane and cytoplasmic region at C-terminus. Phylogenetic analyses indicated evolutionary association of BtA1Vg and BtA1VgR with the homologous proteins from various insect species. Silencing of BtA1VgR by siRNA did not affect the transcript level of BtA1Vg. However, BtA1Vg protein accumulation in oocytes was directly influenced with the expression level of BtA1VgR. Further, BtA1VgR silencing caused significant mortality and reduced fecundity in adult whiteflies. The results established the role of BtA1VgR in transportation of BtA1Vg in oocytes. Further, these proteins are essential for fecundity, and therefore these can be potential RNAi targets for insect control in crop plants.

Lipoprotein assembly and function in an evolutionary perspective

Circulatory fat transport in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). ApoB and apoLp-II/I, constituting the structural (non-exchangeable) basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride-transfer protein, another LLTP family member, and bind them by means of amphipathic α-helical and β-sheet structural motifs. Comparative research reveals that LLTPs evolved from the earliest animals and highlights the structural adaptations in these lipid-binding proteins. Thus, in contrast to apoB, apoLp-II/I is cleaved post-translationally by a furin, resulting in the appearance of two non-exchangeable apolipoproteins in the single circulatory lipoprotein in insects, high-density lipophorin (HDLp). The remarkable structural similarities between mammalian and insect lipoproteins notwithstanding important functional differences relate to the mechanism of lipid delivery. Whereas in mammals, partial delipidation of apoB-containing lipoproteins eventually results in endocytic uptake of their remnants, mediated by members of the low-density lipoprotein receptor (LDLR) family, and degradation in lysosomes, insect HDLp functions as a reusable lipid shuttle capable of alternate unloading and reloading of lipid. Also, during muscular efforts (flight activity), an HDLp-based lipoprotein shuttle provides for the transport of lipid for energy generation. Although a lipophorin receptor - a homolog of LDLR - was identified that mediates endocytic uptake of HDLp during specific developmental periods, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. These data highlight that the functional adaptations in the lipoprotein lipid carriers in mammals and insects also emerge with regard to the functioning of their cognate receptors.

Analogs of LDL Receptor Ligand Motifs in Dengue Envelope and Capsid Proteins as Potential Codes for Cell Entry

It is established that cell entry of low density lipoprotein particles (LLPs) containing Apo B100 and Apo E is mediated by receptors and GAGs. Receptor ligand motifs, XBBBXXBX, XBBXBX, and ΨBΨXB, and mono- and bipartite NLS sequences are abundant in Apo E and Apo B100 as well as in envelope and capsid proteins of Dengue viruses 1-4 (DENV1-4). Synthetic, fluorescence-labeled peptides of sequences in DENV2 envelope protein, and DENV3 capsid that include these motifs were used to conduct a qualitative assessment of cell binding and entry capacity using HeLa cells. DENV2 envelope peptide, Dsp2EP, ⁰⁵⁶⁴Gly-Gly⁰⁵⁹⁵, was shown to bind and remain at the cell surface. In contrast, DENV3 capsid protein peptide, Dsp3CP, ⁰⁰⁰²Asn-Gln⁰⁰²⁸, readily enters HeLa cells and accumulates at discrete loci in the nucleus. FITC-labeled dengue synthetic peptides colocalize with Low Density Lipoprotein-CM-DiI and Apo E-CM-DiI to a degree that suggests that Dengue viruses may utilize cell entry pathways used by LLPs.

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.

A low-density lipoprotein receptor-related protein (LRP)-like molecule identified from Chlamys farreri participated in immune response against bacterial infection

Low-density lipoprotein receptor-related protein (LRP) is a group of important endocytic receptors contributing to binding ligands and maintaining internal environment. In the present study, an LRP-like molecule was identified from Zhikong scallop Chlamys farreri (CfLPR), and its mRNA expression profiles, tissue location, and immunology activities were analyzed to explore its possible function in the innate immune system. The ORF of CfLRP was of 1971 bp encoding a polypeptide of 656 amino acids with ten low-density lipoprotein-receptor YWTD (LY) domains and one scavenger receptor cysteine-rich (SRCR) domain. It shared similar structure with out-membrane domains of LRP family members in mammalian. The mRNA transcripts of CfLRP were dominantly expressed in hepatopancreas and mantle (P < 0.01), and its mRNA level in hemocytes was up-regulated (P < 0.01) significantly after the stimulations of lipopolysaccharides (LPS), peptidoglycan (PGN) and β-glucan. Western blotting assay using polyclonal antibody specific for CfLRP revealed that CfLRP was localized in the plasma. The recombinant protein of CfLRP (rCfLRP) could bind acetylated low density lipoprotein (Ac-LDL), metalloprotease SPF1 of Vibrio splendidus and mannan, but could not bind other typical PAMPs such as LPS, PGN, β-glucan and zymosan. Meanwhile, rCfLRP also exhibited strong bacteriostatic activity to Gram-negative bacteria Vibrio anguillarum and V. splendidus. These results indicated that CfLRP could serve as a receptor to recognize and eliminate the invading pathogens, which provided a new implication in the function of LRP-like molecules in invertebrate immunity.

New horizons for lipoprotein receptors: communication by β-propellers

The lipoprotein receptor (LR) family constitutes a large group of structurally closely related receptors with broad ligand-binding specificity. Traditionally, ligand binding to LRs has been anticipated to involve merely the complement type repeat (CR)-domains omnipresent in the family. Recently, this dogma has transformed with the observation that β-propellers of some LRs actively engage in complex formation too. Based on an in-depth decomposition of current structures and sequences, we suggest that exploitation of the β-propellers as binding targets depends on receptor subgroups. In particular, we highlight the shutter mechanism of β-propellers as a general recognition motif for NxI-containing ligands, and we present indications that the generalized β-propeller-induced ligand release mechanism is not applicable for the larger LRs. For the giant LR members, we present evidence that their β-propellers may also actively engage in ligand binding. We therefore advocate for an increased focus on solving the structure-function relationship of this group of important biological receptors.

cDNA isolation, expression, and hormonal regulation of yolk protein genes in the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

Yolk protein (YP) or vitellogenin (Vg), the main component of yolk, is the key nutrient for embryonic development. YPs, encoded from uncleaved genes existing mainly in cyclorraphan flies, are different from VGs that are present in most non-cyclorraphan dipterans and other insects. In this study, cDNAs of two YPs, namely Bdyp1 and Bdyp2 (GenBank accession Nos. AF368053 and AF368054), were isolated in the oriental fruit fly, Bactrocera dorsalis (Hendel). RT-PCR analysis revealed that Bdyp1 and 2 are expressed in the fat body and ovary during egg development. However, the expression profiles of Bdyp1 and 2 in the fat body are different, indicating that divergent mechanisms might exist in the regulation of these two genes. Twenty-hydroxyecdysone (20E) plays a major role in promoting Bdyp1 expression, yet the expression of Bdyp2 exhibits a greater response to juvenile hormone (JH) in fat body in vitro. Unexpectedly, 20E-induced expression of both Bdyp1 and 2 is suppressed by JH prior to 20E treatment of in vitro fat body; conversely, it is enhanced by the addition of JH following 20E treatment.

Low-density lipoprotein receptor (LDLR) family orchestrates cholesterol homeostasis

The LDLR family of proteins is involved in lipoproteins trafficking. While the role of LDLR in cardiovascular disease has been widely studied, only recently the role of other members of the LDLR proteins in lipoprotein homeostasis and atherosclerosis has emerged. LDLR, VLDLR, and LRPs bind and internalize apoE- and apoB-containing lipoprotein, including LDL and VLDL, and regulate their cellular uptake. LRP6 is a unique member of this family for its function as a co-receptor for Wnt signal transduction. The work in our laboratory has shown that LRP6 also plays a key role in lipoprotein and TG clearance, glucose homoeostasis, and atherosclerosis. The role of these receptor proteins in pathogenesis of diverse metabolic risk factors is emerging, rendering them targets of novel therapeutics for metabolic syndrome and atherosclerosis. This manuscript reviews the physiological role of the LDLR family of proteins and describes its involvement in pathogenesis of hyperlipidemia and atherosclerosis.

Circulatory lipid transport: lipoprotein assembly and function from an evolutionary perspective

Circulatory transport of neutral lipids (fat) in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). Latter proteins, which constitute the structural basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride transfer protein (MTP)--another LLTP family member--and bind them by means of amphipathic structures. Comparative research reveals that LLTPs have evolved from the earliest animals and additionally highlights the structural and functional adaptations in these lipid carriers. For instance, in contrast to mammalian apoB, the insect apoB homologue, apoLp-II/I, is post-translationally cleaved by a furin, resulting in their appearance of two non-exchangeable apolipoproteins in the insect low-density lipoprotein (LDL) homologue, high-density lipophorin (HDLp). An important difference between mammalian and insect lipoproteins relates to the mechanism of lipid delivery. Whereas in mammals, endocytic uptake of lipoprotein particles, mediated via members of the LDL receptor (LDLR) family, results in their degradation in lysosomes, the insect HDLp was shown to act as a reusable lipid shuttle which is capable of reloading lipid. Although the recent identification of a lipophorin receptor (LpR), a homologue of LDLR, reveals that endocytic uptake of HDLp may constitute an additional mechanism of lipid delivery, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. Binding studies indicate that the HDLp-LpR complex, in contrast to the LDL-LDLR complex, is resistant to dissociation at endosomal pH as well as by treatment with EDTA mimicking the drop in Ca(2+) concentration in the endosome. This remarkable stability of the ligand-receptor complex may provide a crucial key to the recycling mechanism. Based on the binding and dissociation capacities of mutant and hybrid receptors, the specific binding interaction of the ligand-binding domain of the receptor with HDLp was characterized. These structural similarities and functional adaptations of the lipid transport systems operative in mammals and insects are discussed from an evolutionary perspective.

The evolution of plasma cholesterol: direct utility or a "spandrel" of hepatic lipid metabolism?

Fats provide a concentrated source of energy for multicellular organisms. The efficient transport of fats through aqueous biological environments raises issues concerning effective delivery to target tissues. Furthermore, the utilization of fatty acids presents a high risk of cytotoxicity. Improving the efficiency of fat transport while simultaneously minimizing the cytotoxic risk confers distinct selective advantages. In humans, most of the plasma cholesterol is associated with low-density lipoprotein (LDL), a metabolic by-product of very-low-density lipoprotein (VLDL), which originates in the liver. However, the functions of VLDL are not clear. This paper reviews the evidence that LDL arose as a by-product during the natural selection of VLDL. The latter, in turn, evolved as a means of improving the efficiency of diet-derived fatty acid storage and utilization, as well as neutralizing the potential cytotoxicity of fatty acids while conserving their advantages as a concentrated energy source. The evolutionary biology of lipid transport processes has provided a fascinating insight into how and why these VLDL functions emerged during animal evolution. As causes of historical origin must be separated from current utilities, our spandrel-LDL theory proposes that LDL is a spandrel of VLDL selection, which appeared non-adaptively and may later have become crucial for vertebrate fitness.

Apolipocrustacein, formerly vitellogenin, is the major egg yolk precursor protein in decapod crustaceans and is homologous to insect apolipophorin II/I and vertebrate apolipoprotein B

BACKGROUND

In animals, the biogenesis of some lipoprotein classes requires members of the ancient large lipid transfer protein (LLTP) superfamily, including the cytosolic large subunit of microsomal triglyceride transfer protein (MTP), vertebrate apolipoprotein B (apoB), vitellogenin (Vtg), and insect apolipophorin II/I precursor (apoLp-II/I). In most oviparous species, Vtg, a large glycolipoprotein, is the main egg yolk precursor protein.

RESULTS

This report clarifies the phylogenetic relationships of LLTP superfamily members and classifies them into three families and their related subfamilies. This means that the generic term Vtg is no longer a functional term, but is rather based on phylogenetic/structural criteria. In addition, we determined that the main egg yolk precursor protein of decapod crustaceans show an overall greater sequence similarity with apoLp-II/I than other LLTP, including Vtgs. This close association is supported by the phylogenetic analysis, i.e. neighbor-joining, maximum likelihood and Bayesian inference methods, of conserved sequence motifs and the presence of three common conserved domains: an N-terminal large lipid transfer module marker for LLTP, a DUF1081 domain of unknown function in their central region exclusively shared with apoLp-II/I and apoB, and a von Willebrand-factor type D domain at their C-terminal end. Additionally, they share a conserved functional subtilisin-like endoprotease cleavage site with apoLp-II/I, in a similar location.

CONCLUSION

The structural and phylogenetic data presented indicate that the major egg yolk precursor protein of decapod crustaceans is surprisingly closely related to insect apoLp-II/I and vertebrate apoB and should be known as apolipocrustacein (apoCr) rather than Vtg. These LLTP may arise from an ancient duplication event leading to paralogs of Vtg sequences. The presence of LLTP homologs in one genome may facilitate redundancy, e.g. involvement in lipid metabolism and as egg yolk precursor protein, and neofunctionalization and subfunctionalization, e.g. involvement in clotting cascade and immune response, of extracellular LLTP members. These protein-coding nuclear genes may be used to resolve phylogenetic relationships among the major arthropod groups, especially the Pancrustacea-major splits.

BLAST screening of chlamydial genomes to identify signature proteins that are unique for the Chlamydiales, Chlamydiaceae, Chlamydophila and Chlamydia groups of species

Background

Chlamydiae species are of much importance from a clinical viewpoint. Their diversity both in terms of their numbers as well as clinical involvement are presently believed to be significantly underestimated. The obligate intracellular nature of chlamydiae has also limited their genetic and biochemical studies. Thus, it is of importance to develop additional means for their identification and characterization.

Results

We have carried out analyses of available chlamydiae genomes to identify sets of unique proteins that are either specific for all Chlamydiales genomes, or different Chlamydiaceae family members, or members of the Chlamydia and Chlamydophila genera, or those unique to Protochlamydia amoebophila, but which are not found in any other bacteria. In total, 59 Chlamydiales-specific proteins, 79 Chlamydiaceae-specific proteins, 20 proteins each that are specific for both Chlamydia and Chlamydophila and 445 ORFs that are Protochlamydia-specific were identified. Additionally, 33 cases of possible gene loss or lateral gene transfer were also detected.

Conclusion

The identified chlamydiae-lineage specific proteins, many of which are highly conserved, provide novel biomarkers that should prove of much value in the diagnosis of these bacteria and in exploration of their prevalence and diversity. These conserved protein sequences (CPSs) also provide novel therapeutic targets for drugs that are specific for these bacteria. Lastly, functional studies on these chlamydiae or chlamydiae subgroup-specific proteins should lead to important insights into lineage-specific adaptations with regards to development, infectivity and pathogenicity.

How to make an egg: transcriptional regulation in oocytes

The oocyte is a highly differentiated cell. It makes organelles specialized to its unique functions and progresses through a series of developmental stages to acquire a fertilization competent phenotype. This review will integrate the biology of the oocyte with what is known about oocyte-specific gene regulation and transcription factors involved in oocyte development. We propose that oogenesis is reliant on a dynamic gene regulatory network that includes oocyte-specific transcriptional regulators.

Tissue- and stage-specific expression of two lipophorin receptor variants with seven and eight ligand-binding repeats in the adult mosquito

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 in AaLpRfb 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.

Species-specific receptor recognition by a minor-group human rhinovirus (HRV): HRV serotype 1A distinguishes between the murine and the human low-density lipoprotein receptor

Human rhinoviruses (HRV) of the minor receptor group use several members of the low-density lipoprotein receptor superfamily for cell entry. These proteins are evolutionarily highly conserved throughout species and are almost ubiquitously expressed. Their common building blocks, cysteine-rich ligand binding repeats about 40 amino acids in length, exhibit considerable sequence similarity. Various numbers of these repeats are present in the different receptors. We here demonstrate that HRV type 1A (HRV1A) replicates in mouse cells without adaptation. Furthermore, although closely related to HRV2, it fails to bind to the human low-density lipoprotein receptor but recognizes the murine protein, whereas HRV2 binds equally well to both homologues. This difference went unnoticed due to the presence of other receptors, such as the low-density lipoprotein receptor-related protein, which allow species-independent attachment. The species specificity of HRV1A reported here will aid in defining amino acid residues establishing the contact between the viral surface and the receptor.

Yolk protein endocytosis by oocytes in Drosophila melanogaster: immunofluorescent localization of clathrin, adaptin and the yolk protein receptor

The process of yolk protein (YP) uptake by developing oocytes in Drosophila melanogaster has been investigated by immunofluorescent localization of the endocytosis proteins, clathrin, alpha-adaptin and the putative yolk protein receptor (YP receptor). Data suggests that YPs from the follicle cells are trafficked into the oocyte during early stages of vitellogenesis, and that hemolymph YPs are sequestered by nurse cells adjacent to the developing oocyte during late stages of vitellogenesis. Yolk proteins were immunolocalized to both follicle cells and nurse cells during these processes. Diapausing female Drosophila melanogaster undergo a pre-vitellogenic arrest of ovarian development associated with the absence of ovarian alpha-adaptin, clathrin and putative YP receptor. Diapause termination by transfer of whole animals from 11 degrees C to 25 degrees C, or by 20-hydroxyecdysone injection, results in the appearance of immunopositive material in the nurse cells for all three proteins between 12 h and 16 h post upshift and within four days of injection. Immunopositive material was not noted in the follicle cells during diapause termination. In vitro warming of diapausing ovaries, or incubation in the presence of 1 &mgr;M 20-hydroxyecdysone failed to initiate early vitellogenic development suggesting that diapause termination requires factor(s) external to the ovary. Western blotting analysis of extracts of 24 h post-eclosion wild type and ap(56f) females identified putative yolk protein receptor with a molecular weight of 208 kDa and clathrin with a molecular weight of 178 kDa.

Molecular characterization of the VLDL receptor homolog mediating binding of lipophorin in oocyte of the mosquito Aedes aegypti

Lipophorin (Lp) functions as a yolk protein precursor in the mosquito Aedes aegypti and it is internalized via receptor-mediated endocytosis (Insect Biochem. Mol. Biol., 30 (2000) 1161). We cloned and molecularly characterized a putative mosquito ovarian lipophorin receptor (AaLpRov) cDNA. The cDNA has a length of 3468 bp coding for a 1156-residue protein with a predicted molecular mass of 128.9 kDa. The deduced amino acid sequence of the cDNA revealed that it encodes a protein homolog of the LDL receptor superfamily, and that it harbors eight cysteine-rich ligand binding repeats at the N-terminus like vertebrate VLDL receptors. The deduced amino acid sequence of this mosquito ovarian receptor is most similar to that of the locust lipophorin receptor (LmLpR) (64.3%), and is only distantly related to the mosquito vitellogenin receptor (VgR) (18.3%), another ovarian LDLR homolog with a different ligand. The AaLpRov cDNA was expressed in a TnT Coupled Reticulocyte Lysate system, and co-immunoprecipitation experiments confirmed that the receptor protein specifically binds Lp. Developmental expression profiles clearly showed that AaLpRov transcripts are present in the vitellogenic ovary, with peak expression at 24-36 h post blood meal. In situ hybridization indicated that AaLpRov transcripts are present only in female germ line cells. Distance-based phylogenetic analyses suggest that the insect LpR and vertebrate LDL/VLDL receptor lineages separated after divergence from the insect VgR lineage.

The low-density lipoprotein receptor family: genetics, function, and evolution

With ever increasing sophistication in molecular biological approaches, the low-density lipoprotein receptor supergene family continues to grow rapidly. From the well-defined key role of these receptors in lipoprotein metabolism, the new members move the field into many different and diverse physiologic and developmental areas. We observe an expansion of the functional spectrum of the family members, which is due to 1) the binding to their extracellular domains of more and more components lacking homology to apolipoproteins, and 2) the recently uncovered interaction of the receptors' cytoplasmic tails with adaptor proteins that are part of signaling pathways. As this review attempts to describe, the task of delineation of the evolutionary history of the gene family may be aided by concepts that consider events, both divergent and convergent, within and between the intra- and extracellular domains.

Molecular cloning and characterization of LR3, a novel LDL receptor family protein with mitogenic activity

We report molecular cloning and initial functional characterization of a novel member of the low density lipoprotein receptor (LDLR) gene family. The cDNA was isolated from a human osteoblast cDNA library and encoded a 1,615 amino acids protein designated as LR3. It has, in the extracellular region, a cluster of three LDLR ligand binding repeats at a juxtamembrane position and four EGF precursor homology domains separated by YWTD spacer repeats. The entire ectodomain shares the same modular organization with the middle portion of the extracellular regions of two LDLR family members, LDLR-related protein (LRP), and gp330/megalin. LR3 mRNA was expressed in most of the adult and fetal tissues examined. The highest expression level was seen in aorta. In human osteosarcoma cells examined, LR3 mRNA was highly enriched in TE85 cells, moderately expressed in MG63 cells and primary human osteoblasts, and undetectable in SaOS-2 cells. NIH 3T3 cells transfected with either full length LR3 or its ectodomain showed significantly increased proliferation, whereas transfection of intracellular domain had no proliferative effect. We predict that LR3 is a multi-functional protein with potential mitogenic activity.