Solution structure of the sixth LDL-A module of the LDL receptor

A conserved LDL-receptor motif regulates corin and CD320 membrane targeting in polarized renal epithelial cells

Selective protein distribution on distinct plasma membranes is important for epithelial cell function. To date, how proteins are directed to specific epithelial cell surface is not fully understood. Here we report a conserved DSSDE motif in LDL-receptor (LDLR) modules of corin (a transmembrane serine protease) and CD320 (a receptor for vitamin B12 uptake), which regulates apical membrane targeting in renal epithelial cells. Altering this motif prevents specific apical corin and CD320 expression in polarized Madin-Darby canine kidney (MDCK) cells. Mechanistic studies indicate that this DSSDE motif participates in a Rab11a-dependent mechanism that specifies apical sorting. In MDCK cells, inhibition of Rab11a, but not Rab11b, expression leads to corin and CD320 expression on both apical and basolateral membranes. Together, our results reveal a novel molecular recognition mechanism that regulates LDLR module-containing proteins in their specific apical expression in polarized renal epithelial cells.

Structure determination protocol for transmembrane domain oligomers

The transmembrane (TM) anchors of cell surface proteins have been one of the 'blind spots' in structural biology because they are generally very hydrophobic, sometimes dynamic, and thus difficult targets for structural characterization. A plethora of examples show these membrane anchors are not merely anchors but can multimerize specifically to activate signaling receptors on the cell surface or to stabilize envelope proteins in viruses. Through a series of studies of the TM domains (TMDs) of immune receptors and viral membrane proteins, we have established a robust protocol for determining atomic-resolution structures of TM oligomers by NMR in bicelles that closely mimic a lipid bilayer. Our protocol overcomes hurdles typically encountered by structural biology techniques such as X-ray crystallography and cryo-electron microscopy (cryo-EM) when studying small TMDs. Here, we provide the details of the protocol, covering five major technical aspects: (i) a general method for producing isotopically labeled TM or membrane-proximal (MP) protein fragments that involves expression of the protein (which is fused to TrpLE) into inclusion bodies and releasing the target protein by cyanogen bromide (CNBr) cleavage; (ii) determination of the oligomeric state of TMDs in bicelles; (iii) detection of intermolecular contacts using nuclear Overhauser effect (NOE) experiments; (iv) structure determination; and (v) paramagnetic probe titration (PPT) to characterize the membrane partition of the TM oligomers. This protocol is broadly applicable for filling structural gaps of many type I/II membrane proteins. The procedures may take 3-6 months to complete, depending on the complexity and stability of the protein sample.

Genetic Markers for Coronary Artery Disease

Coronary artery disease (CAD) and myocardial infarction (MI) are recognized as leading causes of mortality in developed countries. Although typically associated with behavioral risk factors, such as smoking, sedentary lifestyle, and poor dietary habits, such vascular phenotypes have also long been recognized as being related to genetic background. We review the currently available data concerning genetic markers for CAD in English and non-English articles with English abstracts published between 2003 and 2018. As genetic testing is increasingly available, it may be possible to identify adequate genetic markers representing the risk profile and to use them in a clinical setting.

Low-Density Lipoprotein Receptor-Related Proteins in Skeletal Development and Disease

The identification of the low-density lipoprotein receptor (LDLR) provided a foundation for subsequent studies in lipoprotein metabolism, receptor-mediated endocytosis, and many other fundamental biological functions. The importance of the LDLR led to numerous studies that identified homologous molecules and ultimately resulted in the description of the LDL-receptor superfamily, a group of proteins that contain domains also found in the LDLR. Subsequent studies have revealed that members of the LDLR-related protein family play roles in regulating many aspects of signal transduction. This review is focused on the roles of selected members of this protein family in skeletal development and disease. We present background on the identification of this subgroup of receptors, discuss the phenotypes associated with alterations in their function in human patients and mouse models, and describe the current efforts to therapeutically target these proteins to treat human skeletal disease.

Structural basis for ligand capture and release by the endocytic receptor ApoER2

Apolipoprotein E receptor 2 (ApoER2) is a close homologue of low-density lipoprotein receptor (LDLR) that mediates the endocytosis of ligands, including LDL particles. LDLR family members have been presumed to explore a large conformational space to capture ligands in the extended conformation at the cell surface. Ligands are subsequently released through a pH-titrated structural transition to a self-docked, contracted-closed conformation. In addition to lipoprotein uptake, ApoER2 is implicated in signal transduction during brain development through capture of the extracellular protein reelin. From crystallographic analysis, we determine that the full-length ApoER2 ectodomain adopts an intermediate contracted-open conformation when complexed with the signaling-competent reelin fragment, and we identify a previously unappreciated auxiliary low-affinity binding interface. Based on mutational analyses, we propose that the pH shift during endocytosis weakens the affinity of the auxiliary interface and destabilizes the ligand-receptor complex. Furthermore, this study elucidates that the contracted-open conformation of ligand-bound ApoER2 at neutral pH resembles the contracted-closed conformation of ligand-unbound LDLR at acidic pH in a manner suggestive of being primed for ligand release even prior to internalization.

The closed conformation of the LDL receptor is destabilized by the low Ca(++) concentration but favored by the high Mg(++) concentration in the endosome

The LDL receptor (LDLR) internalizes LDL and VLDL particles. In the endosomes, it adopts a closed conformation important for recycling, by interaction of two modules of the ligand binding domain (LR4-5) and a β-propeller motif. Here, we investigate by SPR the interactions between those two modules and the β-propeller. Our results indicate that the two modules cooperate to bind the β-propeller. The binding is favored by low pH and by high [Ca(++)]. Our data show that Mg(++), at high concentration in the endosome, favors the formation of the closed conformation by replacing the structuring effect of Ca(++) in LR5. We propose a sequential model of LDL release where formation of the close conformation follows LDL release.

Intradomain Confinement of Disulfides in the Folding of Two Consecutive Modules of the LDL Receptor

The LDL receptor internalizes circulating LDL and VLDL particles for degradation. Its extracellular binding domain contains ten (seven LA and three EGF) cysteine-rich modules, each bearing three disulfide bonds. Despite the enormous number of disulfide combinations possible, LDLR oxidative folding leads to a single native species with 30 unique intradomain disulfides. Previous folding studies of the LDLR have shown that non native disulfides are initially formed that lead to compact species. Accordingly, the folding of the LDLR has been described as a "coordinated nonvectorial” reaction, and it has been proposed that early compaction funnels the reaction toward the native structure. Here we analyze the oxidative folding of LA4 and LA5, the modules critical for ApoE binding, isolated and in the LA45 tandem. Compared to LA5, LA4 folding is slow and inefficient, resembling that of LA5 disease-linked mutants. Without Ca⁺⁺, it leads to a mixture of many two-disulfide scrambled species and, with Ca⁺⁺, to the native form plus two three-disulfide intermediates. The folding of the LA45 tandem seems to recapitulate that of the individual repeats. Importantly, although the folding of the LA45 tandem takes place through formation of scrambled isomers, no interdomain disulfides are detected, i.e. the two adjacent modules fold independently without the assistance of interdomain covalent interactions. Reduction of incredibly large disulfide combinatorial spaces, such as that in the LDLR, by intradomain confinement of disulfide bond formation might be also essential for the efficient folding of other homologous disulfide-rich receptors.

Cluster III of low-density lipoprotein receptor-related protein 1 binds activated blood coagulation factor VIII

Low-density lipoprotein receptor-related protein 1 (LRP) mediates clearance of blood coagulation factor VIII (FVIII). In LRP, FVIII binds the complement-type repeats (CRs) of clusters II and IV, which also bind a majority of other LRP ligands. No ligand is known for LRP cluster I, and only three ligands, including the LRP chaperone alpha-2 macroglobulin receptor-associated protein (RAP), bind cluster III. Using surface plasmon resonance, we found that in addition to clusters II and IV, activated FVIII (FVIIIa) binds cluster III. The specificity of this interaction was confirmed using an anti-FVIII antibody fragment, which inhibited the binding. Recombinant fragments of cluster III and its site-directed mutagenesis were used to localize the cluster's site for binding FVIIIa to CR.14-19. The interactive site of FVIIIa was localized within its A1/A3'-C1-C2 heterodimer (HDa), which is a major physiological remnant of FVIIIa. In mice, the clearance of HDa was faster than that of FVIII and prolonged in the presence of RAP, which is known to inhibit interactions of LRP with its ligands. In accordance with this, the cluster III site for RAP (CR.15-19) was found to overlap that for FVIIIa. Altogether, our findings support the involvement of LRP in FVIIIa catabolism and suggest a greater significance of the biological role of cluster III compared to that previously known.

Orchestration of secretory protein folding by ER chaperones

The endoplasmic reticulum is a major compartment of protein biogenesis in the cell, dedicated to production of secretory, membrane and organelle proteins. The secretome has distinct structural and post-translational characteristics, since folding in the ER occurs in an environment that is distinct in terms of its ionic composition, dynamics and requirements for quality control. The folding machinery in the ER therefore includes chaperones and folding enzymes that introduce, monitor and react to disulfide bonds, glycans, and fluctuations of luminal calcium. We describe the major chaperone networks in the lumen and discuss how they have distinct modes of operation that enable cells to accomplish highly efficient production of the secretome. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Mapping the binding region on the low density lipoprotein receptor for blood coagulation factor VIII

Low density lipoprotein receptor (LDLR) was shown to mediate clearance of blood coagulation factor VIII (FVIII) from the circulation. To elucidate the mechanism of interaction of LDLR and FVIII, our objective was to identify the region of the receptor necessary for binding FVIII. Using surface plasmon resonance, we found that LDLR exodomain and its cluster of complement-type repeats (CRs) bind FVIII in the same mode. This indicated that the LDLR site for FVIII is located within the LDLR cluster. Similar results were obtained for another ligand of LDLR, α-2-macroglobulin receptor-associated protein (RAP), a common ligand of receptors from the LDLR family. We further generated a set of recombinant fragments of the LDLR cluster and assessed their structural integrity by binding to RAP and by circular dichroism. A number of fragments overlapping CR.2-5 of the cluster were positive for binding RAP and FVIII. The specificity of these interactions was tested by site-directed mutagenesis of conserved tryptophans within the LDLR fragments. For FVIII, the specificity was also tested using a single-chain variable antibody fragment directed against the FVIII light chain as a competitor. Both cases resulted in decreased binding, thus confirming its specificity. The mutagenic study also showed an importance of the conserved tryptophans in LDLR for both ligands, and the competitive binding results showed an involvement of the light chain of FVIII in its interaction with LDLR. In conclusion, the region of CR.2-5 of LDLR was defined as the binding site for FVIII and RAP.

Gentamicin binds to the megalin receptor as a competitive inhibitor using the common ligand binding motif of complement type repeats: insight from the nmr structure of the 10th complement type repeat domain alone and in complex with gentamicin

Gentamicin is an aminoglycoside widely used in treatments of, in particular, enterococcal, mycobacterial, and severe Gram-negative bacterial infections. Large doses of gentamicin cause nephrotoxicity and ototoxicity, entering the cell via the receptor megalin. Until now, no structural information has been available to describe the interaction with gentamicin in atomic detail, and neither have any three-dimensional structures of domains from the human megalin receptor been solved. To address this gap in our knowledge, we have solved the NMR structure of the 10th complement type repeat of human megalin and investigated its interaction with gentamicin. Using NMR titration data in HADDOCK, we have generated a three-dimensional model describing the complex between megalin and gentamicin. Gentamicin binds to megalin with low affinity and exploits the common ligand binding motif previously described (Jensen, G. A., Andersen, O. M., Bonvin, A. M., Bjerrum-Bohr, I., Etzerodt, M., Thogersen, H. C., O'Shea, C., Poulsen, F. M., and Kragelund, B. B. (2006) J. Mol. Biol. 362, 700-716) utilizing the indole side chain of Trp-1126 and the negatively charged residues Asp-1129, Asp-1131, and Asp-1133. Binding to megalin is highly similar to gentamicin binding to calreticulin. We discuss the impact of this novel insight for the future structure-based design of gentamicin antagonists.

The structure, dynamics, and binding of the LA45 module pair of the low-density lipoprotein receptor suggest an important role for LA4 in ligand release

The low-density lipoprotein receptor (LDLR), the primary receptor for cholesterol uptake, binds ligands through its seven LDL-A modules (LAs). We present nuclear magnetic resonance (NMR) and ligand binding measurements on the fourth and fifth modules of the LDLR (LA45), the modules critical for ApoE binding, at physiological pH. Unlike LA5 and all other modules in LDLR, LA4 has a very weak calcium affinity, which probably plays a critical role in endosomal ligand release. The NMR solution structure of each module in the LA45 pair only showed minor differences compared to the analogous domains in previously determined crystal structures. The 12-residue linker connecting the modules, though slightly structured through an interaction with LA4, is highly flexible. Although no intermodule nuclear Overhauser effects were detected, chemical shift perturbations and backbone dynamics suggest cross talk between the two modules. The ligand affinity of both modules is enhanced when the two are linked. LA4 is more flexible than LA5 and remains so even in the module pair, which likely is related to its weaker calcium binding affinity.

Functional characterization of splicing and ligand-binding domain variants in the LDL receptor

Familial hypercholesterolemia (FH) is an autosomal dominant disorder mostly caused by mutations in the LDLR gene. Although the detection of functional mutations in the LDLR gene provides an unequivocal diagnosis of the FH condition, there are many variants whose pathogenicity is still unknown. The aims of this study were to set up a rapid method to determine the effect of LDLR mutations, thereby providing an accurate diagnosis of FH, and to functionally characterize six LDLR mutations detected at high frequency by the LIPOchip(®) platform (Progenika Biopharma, Spain) in the Spanish population. LDLR expression and activity were analyzed by one-single-step flow cytometry assay and confocal microscopy. Splicing effects were determined by sequencing reverse transcription polymerase chain reaction products. The analysis of three heterozygous variants with a single point mutation within the low-density lipoprotein binding domain allowed us to classify the c.806G>A variant as nonpathogenic, and c.862G>A and c.895G>A variants as causative of FH. The results obtained for three variants affecting donor splice sites of the LDLR mRNA, c.313+2dupT, c.1186+5G>A, and c.1845+1G>C, demonstrated that these mutations are pathogenic. These results expand our knowledge of mutations responsible for FH, providing an accurate diagnosis and leading to early treatment to reduce the risk of premature cardiovascular events.

A novel dimeric inhibitor targeting Beta2GPI in Beta2GPI/antibody complexes implicated in antiphospholipid syndrome

BACKGROUND

β2GPI is a major antigen for autoantibodies associated with antiphospholipid syndrome (APS), an autoimmune disease characterized by thrombosis and recurrent pregnancy loss. Only the dimeric form of β2GPI generated by anti-β2GPI antibodies is pathologically important, in contrast to monomeric β2GPI which is abundant in plasma.

PRINCIPAL FINDINGS

We created a dimeric inhibitor, A1-A1, to selectively target β2GPI in β2GPI/antibody complexes. To make this inhibitor, we isolated the first ligand-binding module from ApoER2 (A1) and connected two A1 modules with a flexible linker. A1-A1 interferes with two pathologically important interactions in APS, the binding of β2GPI/antibody complexes with anionic phospholipids and ApoER2. We compared the efficiency of A1-A1 to monomeric A1 for inhibition of the binding of β2GPI/antibody complexes to anionic phospholipids. We tested the inhibition of β2GPI present in human serum, β2GPI purified from human plasma and the individual domain V of β2GPI. We demonstrated that when β2GPI/antibody complexes are formed, A1-A1 is much more effective than A1 in inhibition of the binding of β2GPI to cardiolipin, regardless of the source of β2GPI. Similarly, A1-A1 strongly inhibits the binding of dimerized domain V of β2GPI to cardiolipin compared to the monomeric A1 inhibitor. In the absence of anti-β2GPI antibodies, both A1-A1 and A1 only weakly inhibit the binding of pathologically inactive monomeric β2GPI to cardiolipin.

CONCLUSIONS

Our results suggest that the approach of using a dimeric inhibitor to block β2GPI in the pathological multivalent β2GPI/antibody complexes holds significant promise. The novel inhibitor A1-A1 may be a starting point in the development of an effective therapeutic for antiphospholipid syndrome.

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.

Structural insights into recognition of beta2-glycoprotein I by the lipoprotein receptors

The interactions of beta2 glycoprotein I (B2GPI) with the receptors of the low-density lipoprotein receptor (LDLR) family are implicated in the clearance of negatively charged phospholipids and apoptotic cells and, in the presence of autoimmune anti-B2GPI antibodies, in cell activation, which might play a role in the pathology of antiphospholipid syndrome (APS). The ligand-binding domains of the lipoprotein receptors consist of multiple homologous LA modules connected by flexible linkers. In this study, we investigated at the atomic level the features of the LA modules required for binding to B2GPI. To compare the binding interface in B2GPI/LA complex to that observed in the high-resolution co-crystal structure of the receptor associated protein (RAP) with a pair of LA modules 3 and 4 from the LDLR, we used LA4 in our studies. Using solution NMR spectroscopy, we found that LA4 interacts with B2GPI and the binding site for B2GPI on the (15)N-labeled LA4 is formed by the calcium coordinating residues of the LA module. We built a model for the complex between domain V of B2GPI (B2GPI-DV) and LA4 without introducing any experimentally derived constraints into the docking procedure. Our model, which is in the agreement with the NMR data, suggests that the binding interface of B2GPI for the lipoprotein receptors is centered at three lysine residues of B2GPI-DV, Lys 308, Lys 282, and Lys317.

Calcium as a crucial cofactor for low density lipoprotein receptor folding in the endoplasmic reticulum

The family of low density lipoprotein (LDL) receptors mediate uptake of a plethora of ligands from the circulation and couple this to signaling, thereby performing a crucial role in physiological processes including embryonic development, cancer development, homeostasis of lipoproteins, viral infection, and neuronal plasticity. Structural integrity of individual ectodomain modules in these receptors depends on calcium, and we showed before that the LDL receptor folds its modules late after synthesis via intermediates with abundant non-native disulfide bonds and structure. Using a radioactive pulse-chase approach, we here show that for proper LDL receptor folding, calcium had to be present from the very early start of folding, which suggests at least some native, essential coordination of calcium ions at the still largely non-native folding phase. As long as the protein was in the endoplasmic reticulum (ER), its folding was reversible, which changed only upon both proper incorporation of calcium and exit from the ER. Coevolution of protein folding with the high calcium concentration in the ER may be the basis for the need for this cation throughout the folding process even though calcium is only stably integrated in native repeats at a later stage.

The role of calcium in lipoprotein release by the low-density lipoprotein receptor

The LDL receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the uptake process, the LDLR releases lipoproteins in endosomes. Released lipoproteins are subsequently trafficked to lysosomes for degradation, while the LDLR recycles back to the cell surface for further rounds of uptake. Endosomes have at least two features that can promote lipoprotein release: an acidic pH and low concentrations of free calcium. The relative contributions of acidic pH and low free calcium to lipoprotein release are not known. Here, we generated fibroblasts that express either normal LDLR or an LDLR variant that is unable to employ the acid-dependent release mechanism to determine the relative contributions of acidic pH and low free calcium on lipoprotein release. We show that endosomal concentrations of free calcium can drive lipoprotein release at rates that are similar to those of acid-dependent release and that the calcium-dependent and acid-dependent mechanisms can cooperate during lipoprotein release. Assessment of lipoprotein uptake by these two cell lines showed that LDL uptake requires the acid-dependent mechanism, while uptake of the VLDL remnant, beta-VLDL, does not. We propose that endosomes use both the acid-dependent and calcium-dependent release mechanisms to drive lipoprotein release and that the acid-dependent process is only required for LDL release.

Insect vitellogenin/lipophorin receptors: molecular structures, role in oogenesis, and regulatory mechanisms

Insect vitellogenin and lipophorin receptors (VgRs/LpRs) belong to the low-density lipoprotein receptor (LDLR) gene superfamily and play a critical role in oocyte development by mediating endocytosis of the major yolk protein precursors Vg and Lp, respectively. Precursor Vg and Lp are synthesized, in the majority of insects, extraovarially in the fat body and are internalized by competent oocytes through membrane-bound receptors (i.e., VgRs and LpRs, respectively). Structural analysis reveals that insect VgRs/LpRs and all other LDLR family receptors share a group of five structural domains: clusters of cysteine-rich repeats constituting the ligand-binding domain (LBD), epidermal growth factor (EGF)-precursor homology domain that mediates the acid-dependent dissociation of ligands, an O-linked sugar domain of unknown function, a transmembrane domain anchoring the receptor in the plasma membrane, and a cytoplasmic domain that mediates the clustering of the receptor into the coated pits. The sequence analysis indicates that insect VgRs harbor two LBDs with five repeats in the first and eight repeats in the second domain as compared to LpRs which have a single 8-repeat LBD. Moreover, the cytoplasmic domain of all insect VgRs contains a LI internalization signal instead of the NPXY motif found in LpRs and in the majority of other LDLR family receptors. The exception is that of Solenopsis invicta VgR, which also contains an NPXY motif in addition to LI signal. Cockroach VgRs still harbor another motif, NPTF, which is also believed to be a functional internalization signal. The expression studies clearly demonstrate that insect VgRs are ovary-bound receptors of the LDLR family as compared to LpRs, which are transcribed in a wide range of tissues including ovary, fat body, midgut, brain, testis, Malpighian tubules, and muscles. VgR/LpR mRNA and the protein were detected in the germarium, suggesting that the genes involved in receptor-endocytotic machinery are specifically expressed long before they are functionally required.

Molecular modeling of D151Y and M391T mutations in the LDL receptor

The low-density lipoprotein receptor (LDLR) is a key regulator of cholesterol homeostasis, and defects in the function of LDLR result in familial hypercholesterolemia (FH). In the present study, we performed structural analyses of two novel LDLR mutations, D151Y and M391T. Both mutations occurred in conserved residues of LDLR. The D151Y mutation, in the ligand binding domain, caused an elimination of a hydrogen bond in the calcium binding site, higher solvent accessibility and a loss of negative charge in the Y151 residue. On the other hand, the M391T mutation, in the beta-propeller of the epidermal growth factor (EGF) precursor homology domain, caused an additional hydrogen bond to form, higher solvent accessibility and a distortion of the beta-strand. These data suggest that the irregular structures of the mutated LDLRs are likely to cause the functional defect that contributes to the pathology of FH.

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.

The epidermal growth factor homology domain of the LDL receptor drives lipoprotein release through an allosteric mechanism involving H190, H562, and H586

The low density lipoprotein (LDL) receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the endocytic process, the LDLR releases lipoproteins in endosomes. The release process correlates with an acid-dependent conformational change in the receptor from an extended, "open" state to a compact, "closed" state. The closed state has an intramolecular contact involving H190, H562, and H586. The current model for lipoprotein release holds that protonation of these histidines drives the conformational change that is associated with release. We tested the roles of H190, H562, and H586 on LDLR conformation and on lipoprotein binding, uptake, and release using variants in which the three histidines were replaced with alanine (AAA variant) or in which the histidines were replaced with charged residues that can form ionic contacts at neutral pH (DRK variant). Contrary to expectation, both the AAA and the DRK variants exhibited normal acid-dependent transitions from open to closed conformations. Despite this similarity, both the AAA and DRK mutations modulated lipoprotein release, indicating that H190, H562, and H586 act subsequent to the conformational transition. These observations also suggest that the intramolecular contact does not drive release through a competitive mechanism. In support of this possibility, mutagenesis experiments showed that beta-VLDL binding was inhibited by mutations at D203 and E208, which are exposed in the closed conformation of the LDLR. We propose that H190, H562, and H586 are part of an allosteric mechanism that drives lipoprotein release.

Scrambled isomers as key intermediates in the oxidative folding of ligand binding module 5 of the low density lipoprotein receptor

The ligand binding module five (LA5) of the low density lipoprotein receptor is a small, single-domain protein of 40 residues and three disulfide bonds with a calcium binding motif that is essential for its structure and function. Several mutations in LA5 have been reported to cause familial hypercholesterolemia by impairing a proper folding of the module. The current study reports the oxidative folding and reductive unfolding pathways of wild type and mutant LA5 modules through kinetic and structural analysis of the trapped intermediates. Wild type LA5 folding involves an initial phase of nonspecific packing where the sequential oxidation of its cysteines gives rise to complex equilibrated populations of intermediates. In the presence of calcium, the attainment of a coordination-competent conformation becomes the rate-limiting step of folding while binding of the ion funnels both thermodynamically and kinetically the folding reaction toward the native state. In the absence of calcium, a scrambled isomer (termed Xa) constitutes the global free energy minimum of the folding process. Xa and the native form are stable, inter-convertible species whose relative populations at equilibrium appear displaced in disease-linked mutants toward the scrambled form. Because stable scrambled isomers such as Xa avoid the exposition of reactive cysteines in misfolded modules, they might constitute a strategy to prevent wrong interactions with other domains during folding of the receptor. Comparison of the folding pathways of wild type and mutant LA5 provides the molecular basis to understand how LA modules fold into a functional conformation or upon mutation misfold and lead to disease.

Solution structure of the twelfth cysteine-rich ligand-binding repeat in rat megalin

Megalin, an approx. 600 kDa transmembrane glycoprotein that acts as multi-ligand transporter, is a member of the low density lipoprotein receptor gene family. Several cysteine-rich repeats, each consisting of about 40 residues, are responsible for the multispecific binding of ligands. The solution structure of the twelfth cysteine-rich ligand-binding repeat with class A motif found in megalin features two short beta-strands and two helical turns, yielding the typical fold with a I-III, II-V and IV-VI disulfide bridge connectivity pattern and a calcium coordination site at the C-terminal end. The resulting differences in electrostatic surface potential compared to other ligand-binding modules of this gene family, however, may be responsible for the functional divergence.

The NMR Solution Structure of the Relaxin (RXFP1) Receptor Lipoprotein Receptor Class A Module and Identification of Key Residues in the N-terminal Region of the Module That Mediate Receptor Activation

The receptors for the peptide hormones relaxin and insulin-like peptide 3 (INSL3) are the leucine-rich repeat-containing G-protein-coupled receptors LGR7 and LGR8 recently renamed as the relaxin family peptide (RXFP) receptors, RXFP1 and RXFP2, respectively. These receptors differ from other LGRs by the addition of an N-terminal low density lipoprotein receptor class A (LDLa) module and are the only human G-protein-coupled receptors to contain such a domain. Recently it was shown that the LDLa module of the RXFP1 and RXFP2 receptors is essential for ligand-stimulated cAMP signaling. The mechanism by which the LDLa module modulates receptor signaling is unknown; however, it represents a unique paradigm in understanding G-protein-coupled receptor signaling. Here we present the structure of the RXFP1 receptor LDLa module determined by solution NMR spectroscopy. The structure is similar to other LDLa modules but shows small differences in side chain orientations and inter-residue packing. Interchange of the module with the second ligand binding domain of the LDL receptor, LB2, results in a receptor that binds relaxin with full affinity but is unable to signal. Furthermore, we demonstrate via structural studies on mutated LDLa modules and functional studies on mutated full-length receptors that a hydrophobic surface within the N-terminal region of the module is essential for activation of RXFP1 receptor signal in response to relaxin stimulation. This study has highlighted the necessity to understand the structural effects of single amino acid mutations on the LDLa module to fully interpret the effects of these mutations on receptor activity.

Computational diagnosis of protein conformational diseases: Short molecular dynamics simulations reveal a fast unfolding of r-LDL mutants that cause familial hypercholesterolemia

The molecular basis of conformational diseases frequently resides in mutant proteins constituting a subset of the vast mutational space. While the subtleties of protein structure point to molecular dynamics (MD) techniques as promising tools for an efficient exploration of such a space, the average size of proteins and the time scale of unfolding events make this goal difficult with present computational capabilities. We show here, nevertheless, that an efficient approach is already feasible for modular proteins. Familial hypercholesterolemia (FH) is a conformational disease linked to mutations in the gene encoding the low density lipoprotein receptor. A high percentage of these mutations has been found in the seven small modular binding repeats of the receptor. Taking advantage of its small size, we have performed an in depth MD study of the fifth binding repeat. Fast unfolding dynamics have been observed in the absence of a structural bound calcium ion, which agrees with its reported essential role in the stability of the module. In addition, several mutations detected in FH patients have been analyzed, starting from the native conformation. Our results indicate that in contrast with the wild type protein and an innocuous control mutant, disease-related mutants experience, in short simulation times (2-8 ns), gross departures from the native state that lead to unfolded conformations and, in some cases, to binding site desorganization deriving in calcium release. Computational diagnosis of mutations leading to conformational diseases seems thus feasible, at least for small or modular pathogenic proteins.

The Single Ligand-binding Repeat of Tva, a Low Density Lipoprotein Receptor-related Protein, Contains Two Ligand-binding Surfaces

The receptor for avian sarcoma/leukosis virus subtype A (ASLV-A), Tva, is the simplest member of the low density lipoprotein receptor family containing a single ligand-binding repeat (LBR). Most LBRs contain a central Trp (Trp33 in Tva) that is important for ligand binding and, for the low density lipoprotein receptor, is associated with familial hypercholesterolemia. The Tva ligand-binding module contains a second Trp (Trp48) that is part of a DEW motif present in a subset of LBRs. Trp48 is important for ASLV-A infectivity. A soluble Tva (sTva) ligand-binding module is sufficient for ASLV-A infectivity. Tva interacts with the viral glycoprotein, and a soluble receptor-binding domain (SUA) binds sTva with picomolar affinity. We investigated whether Tva, a retroviral receptor, could behave as a classic LBR by assessing sTva interactions with the universal receptor-associated protein (RAP) and comparing these interactions with those between sTva and its viral ligand (SUA). To address the role of the two Trp residues in Tva function, we prepared sTva harboring mutations of Trp33, Trp48, or both and determined the binding kinetics with RAP and SUA. We found that sTva behaved as a "normal" receptor toward RAP, requiring both calcium and Trp33 for binding. However, sTva binding to SUA required neither calcium nor Trp33. Furthermore, sTva could bind both RAP and SUA simultaneously. These results show that the single LBR of Tva has two ligand-binding sites, raising the possibility that other LBRs may also.

Binding site structure of one LRP-RAP complex: implications for a common ligand-receptor binding motif

The low-density lipoprotein receptor-related protein (LRP) interacts with more than 30 ligands of different sizes and structures that can all be replaced by the receptor-associated protein (RAP). The double module of complement type repeats, CR56, of LRP binds many ligands including all three domains of RAP and alpha2-macroglobulin, which promotes the catabolism of the Abeta-peptide implicated in Alzheimer's disease. To understand the receptor-ligand cross-talk, the NMR structure of CR56 has been solved and ligand binding experiments with RAP domain 1 (RAPd1) have been performed. From chemical shift perturbations of both binding partners upon complex formation, a HADDOCK model of the complex between CR56 and RAPd1 has been obtained. The binding residues are similar to a common binding motif suggested from alpha2-macroglobulin binding studies and provide evidence for an understanding of their mutual cross-competition pattern. The present structural results convey a simultaneous description of both binding partners of an LRP-ligand complex and open a route to a broader understanding of the binding specificity of the LRP receptor, which may involve a general four-residue receptor-ligand recognition motif common to all LRP ligands. The present result may be beneficial in the design of antagonists of ligand binding to the LDL receptor family, and especially of drugs for treatment of Alzheimer's disease.

Linker chains of the gigantic hemoglobin of the earthworm Lumbricus terrestris: primary structures of linkers L2, L3, and L4 and analysis of the connectivity of the disulfide bonds in linker L1

The extracellular hemoglobin (Hb) of the earthworm, Lumbricus terrestris, has four major kinds of globin chains: a, b, c, and d, present in equimolar proportions, and additional non-heme, non-globin scaffolding chains called linkers that are required for the calcium-dependent assembly of the full-sized molecule. The amino acid sequences of all four of the globin chains and one of the linkers (L1) have previously been determined. The amino acid sequences via cDNA of each of the three remaining linkers, L2, L3, and L4, have been determined so that the sequences of all constituent polypeptides of the hemoglobin are now known. Each linker has a highly conserved cysteine-rich segment of approximately 40 residues that is homologous with the seven ligand-binding repeats of the human low-density lipoprotein receptor (LDLR). Analysis of linker L1 shows that the connectivity of the three disulfide bonds is exactly the same as in the LDLR ligand-binding repeats. The presence of a calcium-binding site comprising one glutamyl and three aspartyl residues in both the LDLR repeats and in the linkers supports the suggestion that calcium is required for the folding and disulfide connectivity of the linkers as in the LDLR repeats. Linker L2 is markedly heterogeneous and contains unusual glycine-rich sequences near the NH2-terminus and a polar zipper-like sequence with imperfect repeats of Asp-Asp-His at the carboxyl terminus. Similar Asp-Asp-His repeats have been found in a protein homologous to superoxide dismutase in the hemolymph of certain mussels. These repeats may function as metal-binding sites.

Identification of two residues within the LDL-A module of Tva that dictate the altered receptor specificity of mutant subgroup A avian sarcoma and leukosis viruses

Avian sarcoma and leukosis virus subgroup A (ASLV-A) entry is mediated by interactions between the viral glycoprotein EnvA and its cognate receptor Tva. Previously, some interesting mutants of ASLV-A have been selected by others which can use chicken Tva, but not quail Tva, for efficient entry. The mutant phenotypes are caused by two point mutations within the surface subunit of EnvA (S. L. Holmen, D. C. Melder, and M. J. Federspiel, J. Virol. 75:726-737, 2001). In this study, we have shown that the altered receptor specificity maps to the LDL-A module of Tva. Further, we have identified two residues in the chicken LDL-A module that allow more efficient viral entry by the mutant viruses. These results demonstrate that the altered receptor specificity of the mutant viruses is determined by specific interactions with residues in the LDL-A module of Tva.

The Human LGR7 Low-Density Lipoprotein Class A Module Requires Calcium for Structure

The relaxin and INSL3 receptors, LGR7 and LGR8, are the only human G-protein-coupled receptors to contain a low-density lipoprotein class-A (LDL-A) module. LDL-A modules are well characterized in a variety of diverse biological functions that involve ligand binding to elicit a response. Common features of the LDL-A modules characterized to date are the conservation of three disulfide bonds and the structural arrangement around a bound calcium ion. In this study we recombinantly produce the human LGR7 LDL-A module for NMR studies and demonstrate that calicum is required for the module to form a stable and correctly folded structure.

Structure and physiologic function of the low-density lipoprotein receptor

The low-density lipoprotein receptor (LDLR) is responsible for uptake of cholesterol-carrying lipoprotein particles into cells. The receptor binds lipoprotein particles at the cell surface and releases them in the low-pH environment of the endosome. The focus of the current review is on biochemical and structural studies of the LDLR and its ligands, emphasizing how structural features of the receptor dictate the binding of low-density lipoprotein (LDL) and beta-migrating forms of very low-density lipoprotein (beta-VLDL) particles, how the receptor releases bound ligands at low pH, and how the cytoplasmic tail of the LDLR interfaces with the endocytic machinery.

LRP1B functions as a receptor for Pseudomonas exotoxin

Pseudomonas aeruginosa is an opportunistic pathogen that produces several virulence factors, among them Pseudomonas Exotoxin A (PE). Previously, low-density lipoprotein receptor-related protein 1 (LRP 1) was shown to be the primary receptor for PE. In this report, we show that a close family member, LRP 1B, can also function as a receptor.

The LDL receptor: how acid pulls the trigger

The low-density lipoprotein receptor normally carries lipoprotein particles into cells, and releases them upon delivery to the low pH milieu of the endosome. Recent structural and functional studies of the receptor, combined with the plethora of prior knowledge about normal receptor function and the effects of disease-associated mutations that cause familial hypercholesterolemia, reveal a detailed molecular model for how the acidic environment of the endosome triggers the release of bound lipoprotein particles. Remarkably, the receptor dynamically interconverts between open (ligand-active) and closed (ligand-inactive) conformations in response to pH, relying on a specific arrangement of fixed and flexible interdomain connections to facilitate efficient binding and release of its lipoprotein ligands.

Global defects in the expression and function of the low density lipoprotein receptor (LDLR) associated with two familial hypercholesterolemia mutations resulting in misfolding of the LDLR epidermal growth factor-AB pair

The low density lipoprotein (LDL) receptor is a modular protein involved in the endocytosis of cholesterol-rich lipoproteins from the circulation. Mutations to the receptor result in familial hypercholesterolemia, and over 60 of these occur in the calcium-binding epidermal growth factor-like domain pair. Two selected mutations in this region (G322S and R329P) were introduced into the domain pair and analyzed by in vitro refolding. Both exhibited differing levels of protein misfolding with R329P being the most pronounced. Solution NMR studies of the mutant domain pairs after purification established that a fraction of protein maintains a native-like fold and that this fraction contains two intact calcium-binding sites. An in vivo analysis of intact receptors containing these binding sites showed significantly reduced cell-surface expression compared with the native LDL receptor levels, again with R329P showing the most severe decrease. The sum of these results suggests that either local changes in structure or domain misfolding may be associated with the mutations. There is also the possibility that the misfolding of the calcium-binding epidermal growth factor-like pair region is propagated to other regions of the intact receptor, resulting in more global defects. Surprisingly, for both mutants, those full-length receptors that fold and reach the cell surface retain the ability to bind LDL and release the ligand upon exposure to low pH. This analysis provides significant insight into the protein defect resulting from each of the two mutations and allows their classification to be 2B (partially transport-defective). The results also highlight a range of misfolding defects that may be associated with familial hypercholesterolemia and may enable the prediction of the consequences of homologous disease-causing mutations to other proteins.

The spacing between cysteines two and three of the LDL-A module of Tva is important for subgroup A avian sarcoma and leukosis virus entry

Rong et al. have demonstrated previously that with a few substitutions, the fourth repeat of human low-density lipoprotein (hLDL-A4) receptor can functionally replace the LDL-A module of Tva, the cellular receptor for subgroup A avian sarcoma and leukosis virus (ASLV-A), in viral entry (L. Rong, K. Gendron, and P. Bates, Proc. Natl. Acad. Sci. USA 95:8467-8472, 1998). Here we have shown that swapping the amino terminus of hLDL repeat 5 (hLDL-A5) with that of Tva, in addition to the corresponding substitutions made in human LDL-A4, was required to convert hLDL-A5 into an efficient ASLV-A receptor. These results substantiated our previous findings regarding the role of the specific residues in the viral interaction domain of Tva and demonstrated the critical role of the amino terminus of the Tva LDL-A module in ASLV-A infection. Furthermore, we have shown that the residues between cysteines 2 and 3 of the Tva LDL-A module in a Tva/LDL-A5 chimeric protein can be functionally replaced by the corresponding region of another LDL-A module, human LDL receptor-related protein repeat 22 (LDL-A22), to mediate efficient ASLV-A entry. Since the only conserved feature between the C2-C3 region of LDL-A22 and the Tva LDL-A module is that both contain nine amino acids of which none are conserved, we conclude that the spacing between C2 and C3 of the LDL-A module of Tva is an important determinant for ASLV-A entry. Thus, the present study provides strong evidence to support our hypothesis that one role of the N terminus of the LDL-A module of Tva is to allow proper folding and conformation of the protein for optimal interaction with the viral glycoprotein EnvA in ASLV-A entry.

The low-density lipoprotein receptor: ligands, debates and lore

Like pieces belonging to a large mosaic, the structures of low-density lipoprotein receptor (LDL-R) modules have been elucidated one by one in recent years. LDL-Rs localized on hepatocytes play an important role in removing cholesterol-transporting LDL particles from the plasma by receptor-mediated endocytosis. Key steps in this process involve the LDL-R binding LDL at neutral pH at the cell surface and, after internalization, releasing it again at acidic pH in the endosomes. How the modules of the LDL-R might interact within the intact receptor to carry out ligand binding and release has been revealed by the recent crystal structure of the extracellular domain of the LDL-R.

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.

Structure of the LDL receptor extracellular domain at endosomal pH

The low-density lipoprotein receptor mediates cholesterol homeostasis through endocytosis of lipoproteins. It discharges its ligand in the endosome at pH < 6. In the crystal structure at pH = 5.3, the ligand-binding domain (modules R2 to R7) folds back as an arc over the epidermal growth factor precursor homology domain (the modules A, B, beta propeller, and C). The modules R4 and R5, which are critical for lipoprotein binding, associate with the beta propeller via their calcium-binding loop. We propose a mechanism for lipoprotein release in the endosome whereby the beta propeller functions as an alternate substrate for the ligand-binding domain, binding in a calcium-dependent way and promoting lipoprotein release.

Characterization of the LDL-A module mutants of Tva, the subgroup A Rous sarcoma virus receptor, and the implications in protein folding

Tva is the cellular receptor for subgroup A Rous sarcoma virus (RSV-A), and the viral receptor function is solely determined by a 40-residue motif called the LDL-A module of Tva. In this report, an integral approach of molecular, biochemical, and biophysical techniques was used to examine the role of a well-conserved tryptophan of the LDL-A module of Tva in protein folding and ligand binding. We show that substitution of tryptophan by glycine adversely affected the correct folding of the LDL-A module of Tva, with only a portion giving a calcium-binding conformation. Furthermore, we show that the misfolded LDL-A conformations of Tva could not efficiently bind to its ligand. These results indicate that this conserved tryptophan in the LDL-A module of Tva plays an important role in correct protein folding and ligand recognition. Furthermore, these results suggest that the familial hypercholesterolemia (FH) French Canadian-4 mutation is likely caused by protein misfolding of low-density lipoprotein receptor, thus explaining the defect for this class of FH.

NMR structure and dynamics of a receptor-active apolipoprotein E peptide

Apolipoprotein E (apoE) is important in lipid metabolism due to its interaction with members of the low density lipoprotein (LDL) receptor family. ApoE is able to interact with the LDL receptor only when it is bound to lipid particles. To address structural aspects of this phenomenon, a receptor-active apoE peptide, encompassing the receptor-binding region of the protein, was studied by NMR in the presence of the lipid-mimicking agent trifluoroethanol. In 50% trifluoroethanol, apoE-(126-183) forms a continuous amphipathic alpha-helix over residues Thr(130)-Glu(179). Detailed NMR relaxation analysis indicates a high degree of plasticity for the residues surrounding 149-159. This intrinsic flexibility imposes a curvature to the peptide that may be important in terms of interaction of apoE with various sized lipid particles and the LDL receptor. Residues 165-179 of apoE may act as a molecular switch whereby these residues are unstructured in the absence of lipids and prevent interaction with the LDL receptor. In the presence of lipids, these residues become helical resulting in a receptor-active conformation of the protein. Furthermore, the electrostatic characteristics and geometric features of apoE-(126-183) suggest that apoE binds to the LDL receptor by interacting with more than one of the receptor ligand-binding repeats.

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.

Solution structure of the viral receptor domain of Tva and its implications in viral entry

Tva is the cellular receptor for subgroup A avian sarcoma and leukosis virus (ASLV-A). The viral receptor function of Tva is determined by a 40-residue, cysteine-rich motif called the LDL-A module. Here we report the solution structure of the LDL-A module of Tva, determined by nuclear magnetic resonance (NMR) spectroscopy. Although the carboxyl terminus of the Tva LDL-A module has a structure similar to those of other reported LDL-A modules, the amino terminus adopts a different conformation. The LDL-A module of Tva does not contain the signature antiparallel beta-sheet observed in other LDL-A modules, and it is more flexible than other reported LDL-A modules. The LDL-A structure of Tva provides mechanistic insights into how a simple viral receptor functions in retrovirus entry. The side chains of H38 and W48 of Tva, which have been identified as viral contact residues by mutational analysis, are solvent exposed, suggesting that they are directly involved in EnvA binding. However, the side chain of L34, another potential viral contact residue identified previously, is buried inside of the module and forms the hydrophobic core with other residues. Thus L34 likely stabilizes the Tva structure but is not a viral interaction determinant. In addition, we propose that the flexible amino-terminal region of Tva plays an important role in determining specificity in the Tva-EnvA interaction.

The solution structure of the viral binding domain of Tva, the cellular receptor for subgroup A avian leukosis and sarcoma virus

The cellular receptor for subgroup A avian leukosis and sarcoma virus (ALSV-A) is Tva, which contains a motif related to repeats in the low density lipoprotein receptor (LDLR) ligand binding repeat (LBr) and which is necessary for viral entry. As observed with LBr repeats of LDLR, the 47 residue LBr domain of Tva (sTva47) requires calcium during oxidative folding to form the correct disulfide bonds, and calcium enhances the structure of correctly oxidized sTva47, as well as its ability to bind the viral envelope protein (Env). However, solution nuclear magnetic resonance studies indicate that, even in the presence of excess calcium, sTva47 exists in an ensemble of conformations. Nonetheless, as reported here, the structure of the predominant sTva47 solution conformer closely resembles that of other LBr repeats, with identical S-S binding topology and octahedral calcium coordination. The location of W48 and other critical residues on the surface suggests a region of the molecule necessary for Env binding and to mediate post-binding events important for ALSV-A cell entry.

Analysis of a two-domain binding site for the urokinase-type plasminogen activator-plasminogen activator inhibitor-1 complex in low-density-lipoprotein-receptor-related protein

The low-density-lipoprotein-receptor (LDLR)-related protein (LRP) is composed of several classes of domains, including complement-type repeats (CR), which occur in clusters that contain binding sites for a multitude of different ligands. Each approximately 40-residue CR domain contains three conserved disulphide linkages and an octahedral Ca(2+) cage. LRP is a scavenging receptor for ligands from extracellular fluids, e.g. alpha(2)-macroglobulin (alpha(2)M)-proteinase complexes, lipoprotein-containing particles and serine proteinase-inhibitor complexes, like the complex between urokinase-type plasminogen activator (uPA) and the plasminogen activator inhibitor-1 (PAI-1). In the present study we analysed the interaction of the uPA-PAI-1 complex with an ensemble of fragments representing a complete overlapping set of two-domain fragments accounting for the ligand-binding cluster II (CR3-CR10) of LRP. By ligand blotting, solid-state competition analysis and surface-plasmon-resonance analysis, we demonstrate binding to multiple CR domains, but show a preferential interaction between the uPA-PAI-1 complex and a two-domain fragment comprising CR domains 5 and 6 of LRP. We demonstrate that surface-exposed aspartic acid and tryptophan residues at identical positions in the two homologous domains, CR5 and CR6 (Asp(958,CR5), Asp(999,CR6), Trp(953,CR5) and Trp(994,CR6)), are critical for the binding of the complex as well as for the binding of the receptor-associated protein (RAP) - the folding chaperone/escort protein required for transport of LRP to the cell surface. Accordingly, the present work provides (1) an identification of a preferred binding site within LRP CR cluster II; (2) evidence that the uPA-PAI-1 binding site involves residues from two adjacent protein domains; and (3) direct evidence identifying specific residues as important for the binding of uPA-PAI-1 as well as for the binding of RAP.