A minimal binding domain of the low density lipoprotein receptor family

Molecular chaperone RAP interacts with LRP1 in a dynamic bivalent mode and enhances folding of ligand-binding regions of other LDLR family receptors

The low-density lipoprotein receptor (LDLR) family of receptors are cell-surface receptors that internalize numerous ligands and play crucial role in various processes, such as lipoprotein metabolism, hemostasis, fetal development, etc. Previously, receptor-associated protein (RAP) was described as a molecular chaperone for LDLR-related protein 1 (LRP1), a prominent member of the LDLR family. We aimed to verify this role of RAP for LRP1 and two other LDLR family receptors, LDLR and vLDLR, and to investigate the mechanisms of respective interactions using a cell culture model system, purified system, and in silico modelling. Upon coexpression of RAP with clusters of the ligand-binding complement repeats (CRs) of the receptors in secreted form in insect cells culture, the isolated proteins had increased yield, enhanced folding, and improved binding properties compared with proteins expressed without RAP, as determined by circular dichroism and surface plasmon resonance. Within LRP1 CR-clusters II and IV, we identified multiple sites comprised of adjacent CR doublets, which provide alternative bivalent binding combinations with specific pairs of lysines on RAP. Mutational analysis of these lysines within each of isolated RAP D1/D2 and D3 domains having high affinity to LRP1 and of conserved tryptophans on selected CR-doublets of LRP1, as well as in silico docking of a model LRP1 CR-triplet with RAP, indicated a universal role for these residues in interaction of RAP and LRP1. Consequently, we propose a new model of RAP interaction with LDLR family receptors based on switching of the bivalent contacts between molecules over time in a dynamic mode.

LBD1 of Vitellogenin Receptor Specifically Binds to the Female-Specific Storage Protein SP1 via LBR1 and LBR3

Storage proteins are the major protein synthesized in the fat body, released into hemolymph and re-sequestered into the fat body before pupation in most insect species. Storage proteins are important amino acid and nutrition resources during the non-feeding pupal period and play essential roles for the metamorphosis and oogenesis of insects. The sequestration of storage protein is a selective, specific receptor-mediated process. However, to date, the potential receptor mediating the sequestration of storage protein has not been determined in Bombyx mori. In this study, we expressed and purified the first ligand binding domain of Bombyx mori vitellogenin receptor (BmVgR), LBD1, and found LBD1 could bind with an unknown protein from the hemolymph of the ultimate silkworm larval instar via pull-down assay. This unknown protein was subsequently identified to be the female-specific storage protein SP1 by mass spectrometry. Furthermore, far western blotting assay, immunoprecipitation and isothermal titration calorimetry analysis demonstrated LBD1 specifically bound with the female-specific SP1, rather than another unisex storage protein SP2. The specific binding of LBD1 with SP1 was dependent on the presence of Ca2+ as it was essential for the proper conformation of LBD1. Deletion mutagenesis and ITC analysis revealed the first and third ligand binding repeats LBR1 and LBR3 were indispensable for the binding of LBD1 with SP1, and LBR2 and LBR4 also had a certain contribution to the specific binding. Our results implied BmVgR may mediate the sequestration of SP1 from hemolymph into the fat body during the larval-pupal transformation of Bombyx mori.

Receptor-mediated mechanisms in ovarian follicle and oocyte development

The normal development of the chicken oocyte within the ovarian follicle depends on the coordinated expression and function of several members of the low density lipoprotein receptor gene family. The human low density lipoprotein receptor (LDLR) is the prototype of the gene family; since its discovery and the elucidation of the medical significance of mutations in the ldlr gene, many additional family members have been discovered and characterized, and some important advances have resulted from studies in the chicken. I describe the analogies as well as the differences that exist between the molecular genetics of the mammalian and avian members of this important gene family, with emphasis on receptor-mediated oocyte growth. Recent progress in the molecular characterization of the chicken genes whose products mediate oocyte growth, follicle development, and accessory pathways is described in detail, and emerging information of preliminary nature is included. As the availability of chicken genome sequence data has enhanced the rate of progress in the field, our understanding of the physiological roles of members of this receptor family in general has already gained from studies in the avian model system.

Minimization of the third domain of the LDL receptor-associated protein (RAP)

The third domain of the low-density lipoprotein receptor-associated protein (RAP d3) binds with high-affinity to pairs of complement-type repeats (CR) within the LDLR family of receptors. Structural analyses have defined the contact surface between RAP d3 and a CR pair from the low-density lipoprotein receptor (LDLR). Much of the sequence of RAP d3 has been proposed to stabilize the receptor-binding region without participating directly in formation of the contact surface. We have developed a truncated version of RAP d3 in which these scaffolding regions are excised and replaced with a single, intramolecular disulfide bond. This substitution allows for deletion of as much as a third of the RAP d3 sequence with substantial retention of receptor-binding ability.

Low density lipoprotein receptor relatives in chicken ovarian follicle and oocyte development

The normal development of the chicken oocyte within the ovarian follicle depends on the coordinated expression and function of several members of the low density lipoprotein receptor gene family. The human low density lipoprotein receptor is the prototype of the gene family; since its discovery and the elucidation of the medical significance of mutations in the LDLR gene, many additional family members have been discovered and characterized, and some important advances have resulted from studies in the chicken. I describe the analogies as well as the differences that exist between the molecular genetics of the mammalian and avian members of this important gene family, with emphasis on receptor-mediated oocyte growth. Recent progress in the molecular characterization of the chicken genes whose products mediate oocyte growth, follicle development, and accessory pathways is described in detail, and emerging information of preliminary nature is included. As the availability of chicken genome sequence data has enhanced the rate of progress in the field, our understanding of the physiological roles of members of this receptor family in general has already gained from studies in the avian model system.

Capacitive Detection in Ultrathin Chemosensors Prepared by Molecularly Imprinted Grafting Photopolymerization

The usual applications of capacitive detection in chemo- and biosensors are based on changes in effective thickness of insulating layers due to adsorption of analyte onto receptors. Ultrathin chemosensors based on molecularly imprinted polymerization enable a realization of another capacitive approach that exploits changes in electrical capacitance due to modification of the dielectric constant of the polymer. Such chemosensors were prepared by photografted molecularly imprinted polymerization on the surface of gold electrodes. An adsorbed layer of hydrophobic photoinitiator (benzophenone) provided grafted polymerization on the surface of the alkanethiol-modified gold electrode. The chemosensors were characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopy. Binding of analyte was detected by measurements of electrical capacitance. The results indicate a decrease of the dielectric constant of the polymer layer due to analyte binding up to 20%.

Extracellular matrices of the avian ovarian follicle. Molecular characterization of chicken perlecan

In egg-laying species, such as the chicken, the mode of transport of lipoprotein particles from the capillary plasma to endocytic receptors on the oocyte surface is largely unknown. Here we show by molecular characterization that the large prominent heparan sulfate proteoglycan of extracellular matrices, termed perlecan or HSPG2 (the product of the hspg2 gene), is a component of ovarian follicles that may participate in this process. However, although normally a major HSPG of basement membranes or basal laminae, in chicken follicles, perlecan is absent from the membranous structure between the theca interna and granulosa cell layers, which to date has been considered a bona fide basement membrane. Rather, the protein is localized in the extracellular matrix of theca externa cells, which produce this HSPG. Furthermore, in chicken testes, perlecan is localized in the peritubular spaces but in less organized fashion than the classical basement membrane components, agrin and laminin. All five domains and structural hallmarks of chicken perlecan (4071 residues) have been conserved in its mammalian counterparts. We have produced the recombinant domain II (containing low density lipoprotein (LDL) receptor-like binding repeats) of chicken perlecan and demonstrate its capacity to bind LDL and very low density lipoprotein (VLDL), apolipoprotein B-containing lipoproteins ultimately destined for uptake into oocytes via members of the low density lipoprotein receptor family. Binding to perlecan heparan sulfate side chains may facilitate the interaction of lipoproteins with domain II. Based on the current results and on domain-domain interactions revealed by recent ultrastructural investigations of the LDL receptor, nidogen, and laminin (Rudenko, G., Henry, L., Henderson, K., Ichtchenko, K., Brown, M. S., Goldstein, J. L., and Deisenhofer, J. (2002) Science 298, 2353-2358 and Takagi, J., Yang, Y., Liu, J. H., Wang, J. H., and Springer, T. A. (2003) Nature 424, 969-974), we propose a novel role of perlecan in mediating plasma-to-oocyte surface transport of VLDL particles.

A model for modulation of leptin activity by association with clusterin

Transport, biological action, and clearance of leptin are subject to modulation by plasma components responsible for the formation of the so-called "bound" fraction of serum leptin. Candidates for modulators have been identified previously, but mechanisms for their action, and thus their physiological roles, have remained unclear. Here we have obtained evidence for a role of serum-borne clusterin in leptin biology and have delineated a possible mechanism for its action. We demonstrate complex formation between clusterin and leptin by several approaches and show that the binary complex retains the ability to transduce the leptin signal via binding to the leptin receptor and activation of the Janus kinase/signal transducer and activator of transcription pathway. The interaction of leptin with clusterin does not require additional serum components. Furthermore, and importantly for modulation of the bioactivity of leptin, uptake of leptin present in the complex can be mediated by members of the low density lipoprotein (LDL) receptor family, i.e., apolipoprotein receptor type-2 and the very LDL receptor, which here are shown to efficiently endocytose both free and leptin-associated clusterin. Thus, bioavailability of leptin at a given tissue site may be determined by the levels of clusterin and/or by the relative distribution of certain relatives of the LDL receptor vis-à-vis active leptin receptors.

Receptor-ligand interaction between vitellogenin receptor (VtgR) and vitellogenin (Vtg), implications on low density lipoprotein receptor and apolipoprotein B/E. The first three ligand-binding repeats of VtgR interact with the amino-terminal region of Vtg

The vitellogenin receptor (VtgR) belongs to the low density lipoprotein receptor (LDLR) gene family. It mediates the uptake of vitellogenin (Vtg) in oocyte development of oviparous animals. In this study, we cloned and characterized two forms of Oreochromis aureus VtgR. Northern analysis showed that VtgR was specifically expressed in ovarian tissues. However, reverse transcription-PCR indicates that either there are trace levels of expression of VtgR or a homolog of LDLR exists in nonovarian tissues. The VtgR is highly homologous to the very low density lipoprotein receptor. To better understand the mechanism by which similar structural modules in the ligand-binding domain bind different ligands, we used the yeast two-hybrid system to screen for the minimal interaction motifs in Vtg and VtgR. The amino-terminal region of the lipovitellin I domain of Vtg interacts with the ligand-binding domain of VtgR. The first three ligand-binding repeats of the receptor were found to be essential for ligand binding. Computational analysis of the binding sequence indicates that Vtg has a similar receptor-binding region to apolipoprotein (apo) E and apoB. Site-directed mutagenesis of this region indicates electrostatic interaction between Vtg and its receptor. Sequence analysis suggests the coevolution of receptor-ligand pairs for the LDLR/apo superfamily and suggests that the mode of binding of LDLR/very low density lipoprotein receptor to apoB and apoE is inherited from the electrostatic attraction of VtgR and Vtg.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

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.