The structure of receptor-associated protein (RAP)

Receptor-associated protein impairs ligand binding to megalin and megalin-dependent endocytic flux in proximal tubule cells

Proximal tubule (PT) cells retrieve albumin and a broad array of other ligands from the glomerular ultrafiltrate. Efficient uptake of albumin requires PT expression of both megalin and cubilin receptors. Although most proteins engage cubilin selectively, megalin is required to maintain robust flux through the apical endocytic pathway. Receptor-associated protein (RAP) is a chaperone that directs megalin to the cell surface, and recombinant RAP dramatically inhibits the uptake of numerous megalin and cubilin ligands. The mechanism by which this occurs has been suggested to involve competitive inhibition of ligand binding and/or conformational changes in megalin that prevent interaction with ligands and/or with cubilin. To discriminate between these possibilities, we determined the effect of RAP on endocytosis of albumin, which binds to cubilin and megalin receptors with high and low affinity, respectively. Uptake was quantified in opossum kidney (OK) cells and in megalin or cubilin (Cubn) knockout (KO) clones. Surprisingly, RAP inhibited fluid-phase uptake in addition to receptor-mediated uptake in OK cells and Cubn KO cells but had no effect on endocytosis when megalin was absent. The apparent Ki for RAP inhibition of albumin uptake was 10-fold higher in Cubn KO cells compared with parental OK cells. We conclude that in addition to its predicted high-affinity competition for ligand binding to megalin, the primary effect of RAP on PT cell endocytosis is to globally dampen megalin-dependent endocytic flux. Our data explain the complex effects of RAP on binding and uptake of filtered proteins and reveal a novel role in modulating endocytosis in PT cells.NEW & NOTEWORTHY Receptor-associated protein inhibits binding and uptake of all known endogenous ligands by megalin and cubilin receptors via unknown mechanism(s). Here, we took advantage of recently generated knockout cell lines to dissect the effect of this protein on megalin- and cubilin-mediated endocytosis. Our study reveals a novel role for receptor-associated protein in blocking megalin-stimulated endocytic uptake of fluid-phase markers and receptor-bound ligands in proximal tubule cells in addition to its direct effect on ligand binding to megalin receptors.

Proteolytic regulation of a galectin-3/Lrp1 axis controls osteoclast-mediated bone resorption

Bone-resorbing osteoclasts mobilize proteolytic enzymes belonging to the matrix metalloproteinase (MMP) family to directly degrade type I collagen, the dominant extracellular matrix component of skeletal tissues. While searching for additional MMP substrates critical to bone resorption, Mmp9/Mmp14 double-knockout (DKO) osteoclasts-as well as MMP-inhibited human osteoclasts-unexpectedly display major changes in transcriptional programs in tandem with compromised RhoA activation, sealing zone formation and bone resorption. Further study revealed that osteoclast function is dependent on the ability of Mmp9 and Mmp14 to cooperatively proteolyze the β-galactoside-binding lectin, galectin-3, on the cell surface. Mass spectrometry identified the galectin-3 receptor as low-density lipoprotein-related protein-1 (Lrp1), whose targeting in DKO osteoclasts fully rescues RhoA activation, sealing zone formation and bone resorption. Together, these findings identify a previously unrecognized galectin-3/Lrp1 axis whose proteolytic regulation controls both the transcriptional programs and the intracellular signaling cascades critical to mouse as well as human osteoclast function.

Revealing Angiopep-2/LRP1 Molecular Interaction for Optimal Delivery to Glioblastoma (GBM)

Background: The family of synthetic peptide angiopeps, and particularly angiopep-2 (ANG-2) demonstrated the ability preclinically and clinically to shuttle active molecules across the blood–brain barrier (BBB) and selectively toward brain tumor cells. The literature has also proved that the transport occurs through a specific receptor-mediated transcytosis of the peptide by LRP-1 receptors present both on BBB and tumor cell membranes. However, contradictory results about exploiting this promising mechanism to engineer complex delivery systems, such as nanoparticles, are being obtained. Methodology: For this reason, we applied a molecular docking (MD)-based strategy to investigate the molecular interaction of ANG-2 and the LRP-1 ligand-binding moieties (CR56 and CR17), clarifying the impact of peptide conjugation on its transport mechanism. Results: MD results proved that ANG-2/LRP-1 binding involves the majority of ANG-2 residues, is characterized by high binding energies, and that it is site-specific for CR56 where the binding to 929ASP recalls a transcytosis mechanism, resembling the binding of the receptor to the receptor-associated protein. On the other hand, ANG-2 binding to CR17 is less site-specific but, as proved for apolipoprotein internalization in physiological conditions, it involves the ANG-2 lysin residue. Conclusions: Overall, our results proved that ANG-2 energetic interaction with the LRP-1 receptor is not hindered if specific residues of the peptide are chemically crosslinked to simple or complex engineered delivery systems.

TAFA4 relieves injury-induced mechanical hypersensitivity through LDL receptors and modulation of spinal A-type K+ current

Pain, whether acute or persistent, is a serious medical problem worldwide. However, its management remains unsatisfactory, and new analgesic molecules are required. We show here that TAFA4 reverses inflammatory, postoperative, and spared nerve injury (SNI)-induced mechanical hypersensitivity in male and female mice. TAFA4 requires functional low-density lipoprotein receptor-related proteins (LRPs) because their inhibition by RAP (receptor-associated protein) dose-dependently abolishes its antihypersensitive actions. SNI selectively decreases A-type K⁺ current (I_A) in spinal lamina II outer excitatory interneurons (L-IIo ExINs) and induces a concomitant increase in I_A and decrease in hyperpolarization-activated current (I_h) in lamina II inner inhibitory interneurons (L-IIi InhINs). Remarkably, SNI-induced ion current alterations in both IN subtypes were rescued by TAFA4 in an LRP-dependent manner. We provide insights into the mechanism by which TAFA4 reverses injury-induced mechanical hypersensitivity by restoring normal spinal neuron activity and highlight the considerable potential of TAFA4 as a treatment for injury-induced mechanical pain.

Calreticulin exploits TGF-β for extracellular matrix induction engineering a tissue regenerative process

Topical application of extracellular calreticulin (eCRT), an ER chaperone protein, in animal models enhances wound healing and induces tissue regeneration evidenced by epidermal appendage neogenesis and lack of scarring. In addition to chemoattraction of cells critical to the wound healing process, eCRT induces abundant neo-dermal extracellular matrix (ECM) formation by 3 days post-wounding. The purpose of this study was to determine the mechanisms involved in eCRT induction of ECM. In vitro, eCRT strongly induces collagen I, fibronectin, elastin, α-smooth muscle actin in human adult dermal (HDFs) and neonatal fibroblasts (HFFs) mainly via TGF-β canonical signaling and Smad2/3 activation; RAP, an inhibitor of LRP1 blocked eCRT ECM induction. Conversely, eCRT induction of α5 and β1 integrins was not mediated by TGF-β signaling nor inhibited by RAP. Whereas eCRT strongly induces ECM and integrin α5 proteins in K41 wild-type mouse embryo fibroblasts (MEFs), CRT null MEFs were unresponsive. The data show that eCRT induces the synthesis and release of TGF-β3 first via LRP1 or other receptor signaling and later induces ECM proteins via LRP1 signaling subsequently initiating TGF-β receptor signaling for intracellular CRT (iCRT)-dependent induction of TGF-β1 and ECM proteins. In addition, TGF-β1 induces 2-3-fold higher level of ECM proteins than eCRT. Whereas eCRT and iCRT converge for ECM induction, we propose that eCRT attenuates TGF-β-mediated fibrosis/scarring to achieve tissue regeneration.

Integrated Multiomics Approach to Identify Genetic Underpinnings of Heart Failure and Its Echocardiographic Precursors

Background:

Heart failure (HF) may arise from alterations in metabolic, structural, and signaling pathways, but its genetic architecture is incompletely understood. To elucidate potential genetic contributors to cardiac remodeling and HF, we integrated genome-wide single-nucleotide polymorphisms, gene expression, and DNA methylation using a transomics analytical approach.

Methods:

We used robust rank aggregation (where the position of a certain gene in a rank order list [based on statistical significance level] is tested against a randomly shuffled rank order list) to derive an integrative transomic score for each annotated gene associated with a HF trait.

Results:

We evaluated ≤8372 FHS (Framingham Heart Study) participants (54% women; mean age, 55±17 years). Of these, 62 (0.7%) and 35 (0.4%) had prevalent HF with reduced ejection fraction and HF with preserved left ventricular ejection fraction, respectively. During a mean follow-up of 8.5 years (minimum–maximum, 0.005–18.6 years), 223 (2.7%) and 234 (2.8%) individuals developed incident HF with reduced ejection fraction and HF with reduced ejection fraction, respectively. Top genes included MMP20 and MTSS1 (promotes actin assembly at intercellular junctions) for left ventricular systolic function; ITGA9 (receptor for VCAM1 [vascular cell protein 1]) and C5 for left ventricular remodeling; NUP210 (expressed during myogenic differentiation) and ANK1 (cytoskeletal protein) for diastolic function; TSPAN16 and RAB11FIP3 (involved in regulation of actin cytoskeleton) for prevalent HF with reduced ejection fraction; ANKRD13D and TRIM69 for incident HF with reduced ejection fraction; HPCAL1 and PTTG1IP for prevalent HF with reduced ejection fraction; and ZNF146 (close to the COX7A1 enzyme) and ZFP3 (close to SLC52A1 —the riboflavin transporter) for incident HF with reduced ejection fraction. We tested the HF-related top single-nucleotide polymorphisms in the UK biobank, where rs77059055 in TPM1 (minor allele frequency, 0.023; odds ratio, 0.83; P =0.002) remained statistically significant upon Bonferroni correction.

Conclusions:

Our integrative transomics approach offers insights into potential molecular and genetic contributors to HF and its precursors. Although several of our candidate genes have been implicated in HF in animal models, independent replication is warranted.

PhoX: An IMAC-Enrichable Cross-Linking Reagent

Chemical cross-linking mass spectrometry is rapidly emerging as a prominent technique to study protein structures. Structural information is obtained by covalently connecting peptides in close proximity by small reagents and identifying the resulting peptide pairs by mass spectrometry. However, substoichiometric reaction efficiencies render routine detection of cross-linked peptides problematic. Here, we present a new trifunctional cross-linking reagent, termed PhoX, which is decorated with a stable phosphonic acid handle. This makes the cross-linked peptides amenable to the well-established immobilized metal affinity chromatography (IMAC) enrichment. The handle allows for 300× enrichment efficiency and 97% specificity. We exemplify the approach on various model proteins and protein complexes, e.g., resulting in a structural model of the LRP1/RAP complex. Almost completely removing linear peptides allows PhoX, although noncleavable, to be applied to complex lysates. Focusing the database search to the 1400 most abundant proteins, we were able to identify 1156 cross-links in a single 3 h measurement.

Identification of Receptor Ligands in Apo B100 Reveals Potential Functional Domains

LDL, VLDL and other members of the low-density lipoparticles (LLPs) enter cells through a large family of receptors. The actual receptor ligand(s) in apolipoprotein B100, one of the main proteins of LLP, remain(s) unknown. The objective of this study was to identify true receptor ligand(s) in apo B100, a molecule of 4563 residues. Apo B100 contains 33 analogues of Cardin-Weintraub arginine/lysine-based receptor ligand motifs and shares key lysine motifs and sequence similarity with the LDL receptor-associated protein, MESD, and heat shock proteins. Eleven FITC-labeled synthetic peptides of 21-42 residues, with at least one ligand, were tested for binding and internalization using HeLa cells. All peptides bind but display different binding capacities and patterns. Peptides B0013, B0582, B2366, and B2932 mediate endocytosis and appear in distinct sites in the cytoplasm. B0708 and B3181 bind and remain on the cell surface as aggregates/clusters. Peptides B3119 (Site A) and B3347 (Site B), the putative ligands, showed low binding and no cell entry capacity. Apo B100 regions in this study share similarities with related proteins of known function including chaperone proteins and Apo BEC stimulating protein, and not directly related proteins, e.g., the DNA-binding domain of interferon regulatory factors, MSX2-interacting protein, and snake venom Zinc metalloproteinase-disintegrin-like proteins.

Voltage-gated calcium channel α 2δ subunits: an assessment of proposed novel roles

Voltage-gated calcium (Ca_V) channels are associated with β and α₂δ auxiliary subunits. This review will concentrate on the function of the α₂δ protein family, which has four members. The canonical role for α₂δ subunits is to convey a variety of properties on the Ca_V1 and Ca_V2 channels, increasing the density of these channels in the plasma membrane and also enhancing their function. More recently, a diverse spectrum of non-canonical interactions for α₂δ proteins has been proposed, some of which involve competition with calcium channels for α₂δ or increase α₂δ trafficking and others which mediate roles completely unrelated to their calcium channel function. The novel roles for α₂δ proteins which will be discussed here include association with low-density lipoprotein receptor-related protein 1 (LRP1), thrombospondins, α-neurexins, prion proteins, large conductance (big) potassium (BK) channels, and N-methyl-d-aspartate (NMDA) receptors.

Recombinant Expression of the Full-length Ectodomain of LDL Receptor-related Protein 1 (LRP1) Unravels pH-dependent Conformational Changes and the Stoichiometry of Binding with Receptor-associated Protein (RAP)

LDL receptor-related protein 1 (LRP1) is a highly modular protein and the largest known mammalian endocytic receptor. LRP1 binds and internalizes many plasma components, playing multiple crucial roles as a scavenger and signaling molecule. One major challenge to studying LRP1 has been that it is difficult to express such a large, highly glycosylated, and cysteine-rich protein, limiting structural studies to LRP1 fragments. Here, we report the first recombinant expression of the complete 61 domains of the full-length LRP1 ectodomain. This advance was achieved with a multistep cloning approach and by using DNA dilutions to improve protein yields. We investigated the binding properties of LRP1 using receptor-associated protein (RAP) as a model ligand due to its tight binding interaction. The LRP1 conformation was studied in its bound and unbound state using mass spectrometry, small-angle X-ray scattering, and negative-stain electron microscopy at neutral and acidic pH. Our findings revealed a pH-dependent release of the ligand associated with a conformational change of the receptor. In summary, this investigation of the complete LRP1 ectodomain significantly advances our understanding of this important receptor and provides the basis for further elucidating the mechanism of action of LRP1 in a whole and integrated system.

Evidence That Factor VIII Forms a Bivalent Complex with the Low Density Lipoprotein (LDL) Receptor-related Protein 1 (LRP1): IDENTIFICATION OF CLUSTER IV ON LRP1 AS THE MAJOR BINDING SITE

Hemophilia A is a bleeding disorder caused by a deficiency in coagulation factor VIII (fVIII) that affects 1 in 5,000 males. Current prophylactic replacement therapy, although effective, is difficult to maintain due to the cost and frequency of injections. Hepatic clearance of fVIII is mediated by the LDL receptor-related protein 1 (LRP1), a member of the LDL receptor family. Although it is well established that fVIII binds LRP1, the molecular details of this interaction are unclear as most of the studies have been performed using fragments of fVIII and LRP1. In the current investigation, we examine the binding of intact fVIII to full-length LRP1 to gain insight into the molecular interaction. Chemical modification studies confirm the requirement for lysine residues in the interaction of fVIII with LRP1. Examination of the ionic strength dependence of the interaction of fVIII with LRP1 resulted in a Debye-Hückel plot with a slope of 1.8 ± 0.5, suggesting the involvement of two critical charged residues in the interaction of fVIII with LRP1. Kinetic studies utilizing surface plasmon resonance techniques reveal that the high affinity of fVIII for LRP1 results from avidity effects mediated by the interactions of two sites in fVIII with complementary sites on LRP1 to form a bivalent fVIII·LRP1 complex. Furthermore, although fVIII bound avidly to soluble forms of clusters II and IV from LRP1, only soluble cluster IV competed with the binding of fVIII to full-length LRP1, revealing that cluster IV represents the major fVIII binding site in LRP1.

High Affinity Binding of the Receptor-associated Protein D1D2 Domains with the Low Density Lipoprotein Receptor-related Protein (LRP1) Involves Bivalent Complex Formation: CRITICAL ROLES OF LYSINES 60 AND 191

The LDL receptor-related protein 1 (LRP1) is a large endocytic receptor that binds and mediates the endocytosis of numerous structurally diverse ligands. Currently, the basis for ligand recognition by LRP1 is not well understood. LRP1 requires a molecular chaperone, termed the receptor-associated protein (RAP), to escort the newly synthesized receptor from the endoplasmic reticulum to the Golgi. RAP is a three-domain protein that contains the following two high affinity binding sites for LRP1: one is located within domains 1 and 2, and one is located in its third domain. Studies on the interaction of the RAP third domain with LRP1 reveal critical contributions by lysine 256 and lysine 270 for this interaction. From these studies, a model for ligand recognition by this class of receptors has been proposed. Here, we employed surface plasmon resonance to investigate the binding of RAP D1D2 to LRP1. Our results reveal that the high affinity of D1D2 for LRP1 results from avidity effects mediated by the simultaneous interactions of lysine 60 in D1 and lysine 191 in D2 with sites on LRP1 to form a bivalent D1D2-LRP1 complex. When lysine 60 and 191 are both mutated to alanine, the binding of D1D2 to LRP1 is ablated. Our data also reveal that D1D2 is able to bind to a second distinct site on LRP1 to form a monovalent complex. The studies confirm the canonical model for ligand recognition by this class of receptors, which is initiated by pairs of lysine residues that dock into acidic pockets on the receptor.

A novel chemical footprinting approach identifies critical lysine residues involved in the binding of receptor-associated protein to cluster II of LDL receptor-related protein

Tandem mass tags (TMTs) were utilized in a novel chemical footprinting approach to identify lysine residues that mediate the interaction of receptor-associated protein (RAP) with cluster II of LDL (low-density lipoprotein) receptor (LDLR)-related protein (LRP). The isolated RAP D3 domain was modified with TMT-126 and the D3 domain-cluster II complex with TMT-127. Nano-LC-MS analysis revealed reduced modification with TMT-127 of peptides including Lys(256), Lys(270) and Lys(305)-Lys(306) suggesting that these residues contribute to cluster II binding. This agrees with previous findings that Lys(256) and Lys(270) are critical for binding cluster II sub-domains [Fisher, Beglova and Blacklow (2006) Mol. Cell 22, 277-283]. Cluster II-binding studies utilizing D3 domain variants K(256)A, K(305)A and K(306)A now showed that Lys(306) contributes to cluster II binding as well. For full-length RAP, we observed that peptides including Lys(60), Lys(191), Lys(256), Lys(270) and Lys(305)-Lys(306) exhibited reduced modification with TMT in the RAP-cluster II complex. Notably, Lys(60) has previously been implicated to mediate D1 domain interaction with cluster II. Our results suggest that also Lys(191) of the D2 domain contributes to cluster II binding. Binding studies employing the RAP variants K(191)A, K(256)A, K(305)A and K(306)A, however, revealed a modest reduction in cluster II binding for the K(256)A variant only. This suggests that the other lysine residues can compensate for the absence of a single lysine residue for effective complex assembly. Collectively, novel insight has been obtained into the contribution of lysine residues of RAP to cluster II binding. In addition, we propose that TMTs can be utilized to identify lysine residues critical for protein complex formation.

NMR and small-angle scattering-based structural analysis of protein complexes in solution

Structural analysis of multi-domain protein complexes is a key challenge in current biology and a prerequisite for understanding the molecular basis of essential cellular processes. The use of solution techniques is important for characterizing the quaternary arrangements and dynamics of domains and subunits of these complexes. In this respect solution NMR is the only technique that allows atomic- or residue-resolution structure determination and investigation of dynamic properties of multi-domain proteins and their complexes. As experimental NMR data for large protein complexes are sparse, it is advantageous to combine these data with additional information from other solution techniques. Here, the utility and computational approaches of combining solution state NMR with small-angle X-ray and Neutron scattering (SAXS/SANS) experiments for structural analysis of large protein complexes is reviewed. Recent progress in experimental and computational approaches of combining NMR and SAS are discussed and illustrated with recent examples from the literature. The complementary aspects of combining NMR and SAS data for studying multi-domain proteins, i.e. where weakly interacting domains are connected by flexible linkers, are illustrated with the structural analysis of the tandem RNA recognition motif (RRM) domains (RRM1-RRM2) of the human splicing factor U2AF65 bound to a nine-uridine (U9) RNA oligonucleotide.

Structure of the minimal interface between ApoE and LRP

Clusters of complement-type ligand-binding repeats (CRs) in the low-density lipoprotein receptor (LDLR) family are thought to mediate the interactions with their various ligands. Apolipoprotein E (ApoE), a key ligand for cholesterol homeostasis, has been shown to interact with LDLR-related protein 1 (LRP) through these clusters. The segment comprising the receptor-binding portion of ApoE (residues 130-149) has been found to have a weak affinity for isolated CRs. We have fused this region of ApoE to a high-affinity CR from LRP (CR17) for structural elucidation of the complex. The interface reveals a motif that has previously been observed in CR domains with other binding partners, but with several novel features. Comparison to free CR17 reveals that very few structural changes result from this binding event, but significant changes in intrinsic dynamics are observed upon binding. NMR perturbation experiments suggest that this interface may be similar to several other ligand interactions with LDLRs.

Receptor-associated protein interacts with amyloid-beta peptide and promotes its cellular uptake

Brain amyloid-beta (Abeta) peptide accumulation and aggregation are critical events in the pathogenesis of Alzheimer disease. Increasing evidence has demonstrated that LRP1 is involved in Alzheimer disease pathogenesis. The physiological ligands of LRP1, including apoE, play significant roles in the cellular clearance of Abeta. The receptor-associated protein (RAP) is a specialized chaperone for members of the low density lipoprotein receptor family. RAP shares structural and receptor-binding properties with apoE. Here, we show that RAP binds to both Abeta40 and Abeta42 in a concentration-dependent manner and forms complexes with them. Fluorescence-activated cell sorter analysis showed that RAP significantly enhances the cellular internalization of Abeta in different cell types, including brain vascular smooth muscle, neuroblastoma, glioblastoma, and Chinese hamster ovary cells. This effect of RAP was confirmed by fluorescence microscopy and enzyme-linked immunosorbent assay. RAP binds to both LRP1 and heparin; however, the ability of RAP to enhance Abeta cellular uptake was blocked by heparin and heparinase treatment but not by LRP1 deficiency. Furthermore, the effects of RAP were significantly decreased in heparan sulfate proteoglycan-deficient Chinese hamster ovary cells. Our findings reveal that RAP is a novel Abeta-binding protein that promotes cellular internalization of Abeta.

Receptor-associated protein (RAP) has two high-affinity binding sites for the low-density lipoprotein receptor-related protein (LRP): consequences for the chaperone functions of RAP

RAP (receptor-associated protein) is a three domain 38 kDa ER (endoplasmic reticulum)-resident protein that is a chaperone for the LRP (low-density lipoprotein receptor-related protein). Whereas RAP is known to compete for binding of all known LRP ligands, neither the location, the number of binding sites on LRP, nor the domains of RAP involved in binding is known with certainty. We have systematically examined the binding of each of the three RAP domains (D1, D2 and D3) to tandem and triple CRs (complement-like repeats) that span the principal ligand-binding region, cluster II, of LRP. We found that D3 binds with low nanomolar affinity to all (CR)₂ species examined. Addition of a third CR domain increases the affinity for D3 slightly. A pH change from 7.4 to 5.5 gave only a 6-fold increase in K_d for D3 at 37 °C, whereas temperature change from 22 °C to 37 °C has a similar small effect on affinity, raising questions about the recently proposed D3-destabilization mechanism of RAP release from LRP. Surprisingly, and in contrast to literature suggestions, D1 and D2 also bind to most (CR)₂ and (CR)₃ constructs with nanomolar affinity. Although this suggested that there might be three high-affinity binding sites in RAP for LRP, studies with intact RAP showed that only two binding sites are available in the intact chaperone. These findings suggest a new model for RAP to function as a folding chaperone and also for the involvement of YWTD domains in RAP release from LRP in the Golgi.

Activated protein C ligation of ApoER2 (LRP8) causes Dab1-dependent signaling in U937 cells

Binding of activated protein C (APC) to cells triggers multiple beneficial cytoprotective activities that suppress apoptosis, inflammation, and endothelial barrier breakdown. One paradigm for APC's signaling emphasizes its binding to endothelial cell protein C receptor (EPCR) and subsequent protease activated receptor (PAR)-1 activation. Here we used human monocytic-like U937 cells to evaluate apolipoprotein E receptor 2 (ApoER2)-dependent signaling by APC and found that APC initiated rapid phosphorylation of Tyr-220 in the adaptor protein disabled-1 (Dab1) and of Ser-473 in Akt. APC also induced phosphorylation of Ser-9 in glycogen synthase kinase 3beta (GSK3beta), which was blocked by the PI3K inhibitor LY294002. Receptor-associated protein (RAP), a general antagonist for binding of ligands to LDL receptor family members, inhibited APC-induced phosphorylation of Dab1 and GSK3beta, whereas anti-EPCR or anti-PAR1 blocking antibodies did not. Knocking down ApoER2 by using siRNA-ablated APC induced Dab1 phosphorylation, suggesting that RAP-sensitive APC-induced signaling requires ApoER2. In surface plasmon resonance equilibrium binding studies, APC bound with high affinity to soluble (s) ApoER2 (apparent K(d), approximately 30 nM) but not to soluble very low density lipoprotein receptor. RAP blocked APC binding to sApoER2 but not to sEPCR. RAP blocked binding of U937 cells to immobilized APC. RAP also blocked APC's ability to inhibit endotoxin-induced tissue factor pro-coagulant activity of U937 cells. Thus, we propose that ligation of ApoER2 by APC signals via Dab1 phosphorylation and subsequent activation of PI3K and Akt and inactivation of GSK3beta, thereby contributing to APC's beneficial effects on cells.

LDL receptor-related protein 1: unique tissue-specific functions revealed by selective gene knockout studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

Functional mimicry of the LDL receptor-associated protein through multimerization of a minimized third domain

The LDL receptor-associated protein (RAP) is a ligand for the LDL receptor-related protein (LRP1). The first and third domains of RAP can each bind to one of many sequence-related pairs of complement-type repeats (CR) found within the LRP1 ectodomain. Multiple sites of interaction between the multivalent RAP ligand and the multivalent LRP1 receptor yield strong binding avidity for the complex. The third domain of RAP can be significantly truncated, with material retention of monovalent CR pair-binding affinity, provided that the minimized sequence is stabilized with an intramolecular disulfide bond. We demonstrate that the avidity of full-length RAP for LRP1 in vitro can be partially reconstituted by assembly of truncated, disulfide-linked RAP peptides on tetravalent streptavidin or bivalent immunoglobulin scaffolds. The peptide complex with streptavidin shows pronounced hepatotropism in vivo, replicating the biodistribution of full-length RAP.