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PubMed1996

Hormone-binding and solubility properties of fusion proteins containing the ligand-binding domain of the human androgen receptor.

The ligand-binding properties of the human androgen receptor (AR) reside in the carboxy terminus of the receptor and are retained in fusion proteins encoding this domain. Despite a wealth of information pertaining to the importance of specific amino acid residues in the binding of ligands, the precise amino terminal boundary of this domain has not been determined for the androgen receptor. The current studies focused on the expression of a series of AR fusion proteins in bacteria. These experiments demonstrate that specific androgen binding is detected in a fusion protein encoding amino acid residues 675-917, although this binding is of lower affinity than that observed for the native AR. Inclusion of an additional segment (residues 622-675) restores the capacity of the hormone-binding domain to bind mibolerone with high affinity. Detailed studies of the ligand-binding properties and solubility of one AR fusion protein (625-917) indicate that although the levels of ligand binding and soluble receptor fusion protein usually vary in parallel, this is not true at all time points and the proportion of soluble AR fusion that is able to bind hormone is highest when the levels of soluble AR fusion protein are lowest. This observation, coupled with the changes in the quantity of soluble and insoluble fusion protein occurring at different times following induction at 12 degrees C, 22 degrees C, and 37 degrees C, suggests that the saturation of one or more steps is required for folding of the AR hormone-binding domain to a conformation that is both soluble and competent to bind hormone under conditions of high-level expression. Analysis of the solubility of six different fusion proteins containing different portions of the AR hormone-binding domain suggests that a discrete segment of this domain is not responsible for its synthesis as predominantly insoluble aggregates in bacteria when expressed at high levels.

Base Sequence↗
PubMed1997

Human low density lipoprotein receptor fragment. Successful refolding of a functionally active ligand-binding domain produced in Escherichia coli.

The low density lipoprotein (LDL) receptor plays a key role in cholesterol homeostasis, mediating cellular uptake of lipoprotein particles by high affinity binding to its ligands, apolipoprotein (apo) B-100 and apoE. The ligand-binding domain of the LDL receptor contains 7 cysteine-rich repeats of approximately 40 amino acids; each repeat contains 6 cysteines, which form 3 intra-repeat disulfide bonds. As a first step toward determining the structure of the LDL receptor, both free and bound to its ligands, we produced in Escherichia coli a soluble fragment containing the ligand-binding domain (residues 1-292) as a thrombin-cleavable, heat-stable thioredoxin fusion. Modest amounts (5 mg/liter) of partially purified but inactive fragment were obtained after cell lysis, heat treatment, thrombin cleavage, and gel filtration under denaturing conditions. We were able to refold the receptor fragment to an active conformation with approximately 10% efficiency. The active fragment was isolated and purified with an LDL affinity column. The refolded receptor fragment was homogeneous, as determined by sodium dodecyl sulfate or non-denaturing polyacrylamide gel electrophoresis and isoelectric focusing. The purified fragment did not react with fluorescein-5-maleimide, indicating that all 42 cysteines were disulfide linked. In addition, the refolded fragment exhibited properties identical to those of the intact native receptor: Ca2+-dependent binding and isoform-dependent apoE binding (apoE2 binding <5% of apoE3). Furthermore, antibodies to the fragment recognized native receptors and inhibited the binding of 125I-LDL to fibroblast LDL receptors. We conclude that we have produced a properly folded and fully active receptor fragment that can be used for further structural studies.

Apolipoprotein E2↗
PubMed2005

Protein stability and ligand binding: new paradigms from in-silico experiments.

Computer simulations are used to investigate two features of proteins: ligand binding and ligand entry/exit. Both reveal surprising new insights into the physics of such complex systems and suggest at possible interpretations that depart from the usual paradigms. A ligand binding study using normal mode analysis suggests that, contrary to the perceived notion that ligand binding induces a tightening of the protein (as would be evidenced by a blue shift in its vibrational spectrum), there seem to be cases where ligand binding causes an increase in the entropy through a red-shift in the vibrational spectrum of the protein; this occurs in the part of the spectrum that is associated with large-scale low-frequency delocalized motions of proteins. Moreover, this increase seems to be dependent on the ability of the ligand to form hydrogen bonds within the polar cavity of the protein. This suggests an additional driving force for stabilizing complex formation. In parallel, pathways of ligand access to cavities in two proteins are mapped and it is found that, in agreement with recent interpretations of experimental data emerging from NMR studies, these pathways are characterized by a ruggedness of the energy landscape, which leads to a picture that has a physically more appealing basis than the traditional two-state paradigm normally invoked for ligand binding.

Animals↗
PubMedDate not supplied

Contribution of asymmetric ligand binding to the apparent permanent dipole moment of DNA.

Despite its antiparallel symmetry, DNA often appears to possess a permanent electric dipole moment in transient electro-optical experiments. We propose that this may be due to the asymmetric binding of charged ligands to the DNA. We have used the fluctuating dipole theory of Kirkwood and Shumaker to calculate the contribution of asymmetric ligand binding to the electro-optic orientation function, and Monte Carlo computer simulation to calculate the reversing pulse behavior, as a function of ligand binding density. The results indicate that the effect should be observable even against the background of the sizable induced dipole moment produced by polarization of the counterion atmosphere.

Chemical Phenomena↗
PubMed2005

The extracellular domains of ErbB3 retain high ligand binding affinity at endosome pH and in the locked conformation.

The extracellular, ligand binding regions of ErbB receptors consist of four domains that can assume at least two alternative conformations, extended and locked. The locked conformation, observed in several crystal structures, is held together by a noncovalent intramolecular tether and is incompatible with current models for receptor dimerization and ligand activation. Based on structures of ligand-receptor complexes in the extended conformation, the high affinity ligand binding pocket between domains I and III is disrupted in the locked conformation. Therefore the biological role of the locked conformation is not clear. To address the impact of the locked conformation on ligand binding, we compared extracellular domains of wild-type ErbB3, mutant domains in a constitutively locked or extended conformation and partial extracellular domain constructs. We found that the constitutively locked receptor domains and truncated constructs carrying only domains I-II or III-IV strongly bind ligand, albeit with reduced affinity compared to wild-type receptor. This suggests that the locked conformation cannot be discounted for ligand binding. The significant binding by both partial interfaces in domains I and III also suggests that "partial bivalency" may be the reason for the low nanomolar and high picomolar binding observed for ErbB3 in the respective "low" and high affinity states. In contrast to EGFR (ErbB1), ErbB3 retains high ligand binding affinity at an endosome-comparable pH in both the extended and locked conformations. Ligand affinity for the locked conformation even improves at low pH. For ErbB3, the contribution of domain I to ligand binding is strong and increases at low pH while its contribution is thought to be minimal for EGFR, regardless of pH. This shift in domain contribution and pH dependency provides a mechanistic explanation for some of the divergent properties of EGFR and ErbB3.

Amino Acid Substitution↗
PubMed2003

Crystal structures of nucleosome core particles in complex with minor groove DNA-binding ligands.

We determined the crystal structures of three nucleosome core particles in complex with site-specific DNA-binding ligands, the pyrrole-imidazole polyamides. While the structure of the histone octamer and its interaction with the DNA remain unaffected by ligand binding, nucleosomal DNA undergoes significant structural changes at the ligand-binding sites and in adjacent regions to accommodate the ligands. Our findings suggest that twist diffusion occurs over long distances through tightly bound nucleosomal DNA. This may be relevant to the mechanism of ATP-dependent and spontaneous nucleosome translocation, and to the effect of bound factors on nucleosome dynamics.

Animals↗
PubMed2001

A yeast sensor of ligand binding.

We describe a biosensor that reports the binding of small-molecule ligands to proteins as changes in growth of temperature-sensitive yeast. The yeast strains lack dihydrofolate reductase (DHFR) and are complemented by mouse DHFR containing a ligand-binding domain inserted in a flexible loop. Yeast strains expressing two ligand-binding domain fusions, FKBP12-DHFR and estrogen receptor-alpha (ERalpha)-DHFR, show increased growth in the presence of their corresponding ligands. We used this sensor to identify mutations in residues of ERalpha important for ligand binding, as well as mutations generally affecting protein activity or expression. We also tested the sensor against a chemical array to identify ligands that bind to FKBP12 or ERalpha. The ERalpha sensor was able to discriminate among estrogen analogs, showing different degrees of growth for the analogs that correlated with their relative binding affinities (RBAs). This growth assay provides a simple and inexpensive method to select novel ligands and ligand-binding domains.

Amino Acid Sequence↗
PubMed2003

Curariform antagonists bind in different orientations to the nicotinic receptor ligand binding domain.

Curariform alkaloids competitively inhibit muscle acetylcholine receptors (AChR) by bridging the alpha and non-alpha subunits that form the ligand-binding site. Here we delineate bound orientations of d-tubocurarine (d-TC) and its methylated derivative metocurine using mutagenesis, ligand binding measurements, and computational methods. When tested against a series of lysine mutations in the epsilon subunit, the two antagonists show marked differences in the consequences of the mutations on binding affinity. The mutations epsilon L117K, epsilon Y111K, and epsilon L109K decrease affinity of metocurine by up to 3 orders of magnitude but only slightly alter affinity of d-TC. At the alpha subunit face of the binding site, the mutation alpha Y198T decreases affinity of both antagonists, but alpha Y198F preferentially enhances affinity of d-TC. Computation of antagonist docking orientations, based on our structural model of the alpha-epsilon site of the human AChR, indicates distinct orientations of each antagonist; the flatter metocurine fits into a pocket formed principally by the epsilon subunit, whereas the more compact d-TC spans the narrower crevasse between alpha and epsilon subunits. The side chains of epsilon Tyr-111 and epsilon Thr-117 juxtapose one of two quaternary nitrogens in metocurine but are remote from the equivalent quaternary nitrogen in d-TC, which instead closely approaches alpha Tyr-198. The different docked orientations arise through tilt of the curariform scaffold by approximately 60 degrees normal to the nitrogen-nitrogen axis, together with a 20 degrees rotation about the axis. The overall mutagenesis and computational results show that despite their similar structures, d-TC and metocurine bind in distinctly different orientations to the adult human AChR.

Alkaloids↗
PubMed2004

Ligand-binding modes in cationic biogenic amine receptors.

The binding site in G protein-coupled cationic biogenic amine receptors is formed in the cleft of the seven transmembrane segments. Upon binding the ligand, the receptors are activated or inactivated through the conformational changes of the transmembrane segments. G protein-coupled receptors bind four functionally distinct ligands; inverse agonists, antagonists, partial agonists, and full agonists. Hence, putative structural models for biogenic amine receptors corresponding to the ligand function (inverse agonist-, antagonist-, partial agonist-, and full agonist-bound receptor models) were built by using photointermediate models in the rhodopsin photocascade (M. Ishiguro et al. ChemBioChem. 2004, 5, 298-310). The ligand-receptor recognition of each was examined by modeling receptor-ligand complexes with functional ligands. The complex models suggested that each functional ligand binds the corresponding receptor structure and that ligand-specific interactions contribute to stabilization of the corresponding receptor structure.

Amino Acids↗
PubMed2005

Glucocorticoid receptor point mutation V571M facilitates coactivator and ligand binding by structural rearrangement and stabilization.

Two-point mutations in the human glucocorticoid receptor have been studied by computer simulations to rationalize experimental data, where mutants comprising the V571M substitution improve both transcriptional activity and affinity for aldosterone despite large distances between the mutated residue and the coactivator-binding surface and ligand-binding pocket, respectively. Our molecular dynamics simulations show that the V571M mutation modifies the coactivator-binding site defined by helices 3, 4, and 12, and that specific structural rearrangement of the coactivator-binding site correlates well with transactivation data. A similar reorganization of the coactivator-binding cleft is observed in crystallographic structures of the estrogen receptor in the presence of coactivator peptide, compared with structures without peptide, indicating that induced fit for coactivator binding is a general phenomenon for nuclear receptors. These results suggest that the V571M substitution facilitates recruitment of coactivator protein by promotion of a conformational substate reducing the energetic penalty for the induced fit of the receptor-coactivator complex. Furthermore, our simulations of V571M mutants showed reduced fluctuations of residues lining the ligand-binding pocket. This indicates that a reduction of the entropic cost for ligand binding may explain the increased affinity of V571M mutants for certain ligands.

Allosteric Site↗
PubMed1997

Altered ligand binding properties and enhanced stability of a constitutively active estrogen receptor: evidence that an open pocket conformation is required for ligand interaction.

To elucidate the ligand binding properties of the estrogen receptor (ER) and how ligand access to and release from the ligand binding pocket is affected by the conformational state of the receptor, we have measured the rates of estradiol association and dissociation, the equilibrium binding, and the stability of estradiol binding to denaturants, comparing wild-type human ER and a point mutant (Y537S ER) that shows full constitutive activity, i.e., the same full transcriptional activity in the absence or presence of estrogen. Ligand binding kinetics and affinity were measured with the full-length (1-595) ERs and with truncated forms of both receptors containing domains C through F (including the DNA binding, hinge, and ligand binding domains, amino acids 175-595) or domains E and F (the ligand binding domain; amino acids 304-595). With all ERs, the rates of ligand association and dissociation were considerably slower with the Y537S mutant ER than with wild-type ER (6-fold and 3-4-fold, respectively). These marked differences in ligand on and off rates for the wild-type and Y537S receptors result in a predicted (k-1/k+1) and measured Kd that is 2-fold lower for Y537S ER compared to wild-type ER. The binding of estradiol by wild-type ER was disrupted by high concentrations of urea (above 2 M), whereas the Y537S ER was distinctly more resistant to this disruption. These results are consistent with a model in which wild-type ER in the absence of ligand adopts a transcriptionally inactive collapsed pocket conformation, stabilized by specific interactions of Y537 with nearby regions of ER. When estradiol is bound, the wild-type ER adopts a transcriptionally active, closed pocket (ligand occupied) conformation. By contrast, the Y537S mutant ER favors the transcriptionally active closed pocket conformation, whether occupied by ligand or not, the latter state (closed pocket but unoccupied) accounting for its constitutive activity. Our findings suggest that the entry or exit of ligand from the binding pocket requires that ER adopt an open pocket conformation. The reduced rates of ligand association and dissociation in the constitutively active form of the ER, as well as its greater resistance to disruption of ligand binding by urea, support the supposition that the rate at which this open pocket conformation can be accessed from the unoccupied or ligand-occupied Y537S ER is slower than from the unoccupied or occupied forms of wild-type ER. Thus, the binding and release of ligand by ER require that the receptor access an open pocket state, and the ease with which this state can be accessed is affected by mutations that alter receptor conformation.

Binding Sites↗
PubMed1993

N-linked glycosylation of the ligand-binding domain of the human urokinase receptor contributes to the affinity for its ligand.

Variations in glycosylation exist among urokinase plasminogen activator receptors (u-PARs) from different cell types. We have studied the functional role of N-linked carbohydrate within the ligand-binding domain of u-PAR. Treatment with glycosidases demonstrated that all the N-linked carbohydrates on u-PAR are complex-type oligosaccharides. Substitution of a single Asn (Asn52) to Gln by means of site-directed mutagenesis led to an active receptor mutant with a ligand-binding domain devoid of carbohydrate. The cellular distribution, the glycosyl-phosphatidylinositol anchoring, and the conformational stability after solubilization were unaffected by this single substitution. However, ligand binding analysis demonstrated a 4- 5-fold decrease in affinity as compared with the wild type receptor. Two different strategies were used in order to obtain a u-PAR type completely devoid of N-linked carbohydrates. 1) Tunicamycin treatment of wild type u-PAR-expressing cells. 2) Mutation of all glycosylation sites (Hu-PARN5-mut). In neither case, unglycosylated receptors with ligand binding activity were identified. However, immunofluorescence studies demonstrated that the Hu-PARN5-mut was retained inside the cells in the endoplasmic reticulum. The same result was found for Hu-PARN4-mut, where only the glycosylation sites outside the binding domain were mutated. These results demonstrate that some extent of glycosylation of u-PAR is necessary for cellular transport and for molecular maturation events leading to ligand binding activity. Glycosylation of the binding domain per se affects only the affinity of the receptor. The positive modulation of the Asn52 carbohydrate side chain on ligand affinity suggests that the u-PAR glycosylation variants observed in various cell types may have different functional roles.

Amino Acid Sequence↗
PubMed2001

Ca2+ binding to alpha-synuclein regulates ligand binding and oligomerization.

alpha-Synuclein is a protein normally involved in presynaptic vesicle homeostasis. It participates in the development of Parkinson's disease, in which the nerve cell lesions, Lewy bodies, accumulate alpha-synuclein filaments. The synaptic neurotransmitter release is primarily dependent on Ca(2+)-regulated processes. A microdialysis technique was applied showing that alpha-synuclein binds Ca(2+) with an IC(50) of about 2-300 microm and in a reaction uninhibited by a 50-fold excess of Mg(2+). The Ca(2+)-binding site consists of a novel C-terminally localized acidic 32-amino acid domain also present in the homologue beta-synuclein, as shown by Ca(2+) binding to truncated recombinant and synthetic alpha-synuclein peptides. Ca(2+) binding affects the functional properties of alpha-synuclein. First, the ligand binding of (125)I-labeled bovine microtubule-associated protein 1A is stimulated by Ca(2+) ions in the 1-500 microm range and is dependent on an intact Ca(2+) binding site in alpha-synuclein. Second, the Ca(2+) binding stimulates the proportion of (125)I-alpha-synuclein-containing oligomers. This suggests that Ca(2+) ions may both participate in normal alpha-synuclein functions in the nerve terminal and exercise pathological effects involved in the formation of Lewy bodies.

Biopolymers↗
PubMed2005

Mesd binds to mature LDL-receptor-related protein-6 and antagonizes ligand binding.

Wnt co-receptors LRP5 and LRP6 are two members of the low-density lipoprotein receptor family. Receptor-associated protein is not only a specialized chaperone but also a universal antagonist for members of the low-density lipoprotein receptor family. Here we test whether Mesd, a newly identified chaperone for members of the low-density lipoprotein receptor family, also binds to mature receptors at the cell surface and antagonizes ligand binding. We found that Mesd binds to cell surface LRP5 and LRP6, but not to other members of the low-density lipoprotein receptor family. Scatchard analysis revealed that Mesd binds cell surface LRP6 with high affinity (K(d) approximately 3.3 nM). Interestingly, the C-terminal region of Mesd, which is absent in sequences from invertebrates, is necessary and sufficient for binding to mature LRP6, and is required for LRP6 folding. We also found that LRP6 is not a constitutively active endocytosis receptor and binding of the receptor-associated protein to LRP6 partially competes for Mesd binding. Finally, we demonstrated that Mesd antagonizes ligand binding to LRP6 at the cell surface. Together our results show that in addition to serving as a folding chaperone, Mesd can function as a receptor antagonist by inhibiting ligand binding to mature LRP6.

Animals↗
PubMed2007

Identification of amino acid residues in the Ah receptor involved in ligand binding.

The Ah receptor (AhR) is a ligand-activated transcription factor. Five amino acids as candidate amino acids necessary for ligand binding within or near the ligand-binding domain were selected based on their evolutional conservation and their aromatic nature that could interact with xenobiotic ligands. These amino acids were changed to Ala, and the mutated AhRs were subjected to a test of their transactivation activity in HeLa cells. Mutation of Phe318 completely lost its activity whereas other mutations only weakly impaired activity. The Leu-substituted mutant, AhR(Phe318Leu), activated the luciferase activity to the level comparable to wild type in the cells treated with 3-methylcholanthrene (MC) but not at all with beta-naphthoflavone (beta-NF). Ligand-binding activity of mutants was examined with [3H]MC in vitro. AhR(Phe318Ala) could not bind to [3H]MC. [3H]MC bound by AhR(Phe318Leu) was competed with unlabeled MC but not with beta-NF. A structural model of the ligand-binding domain was constructed.

Amino Acid Sequence↗
PubMed2005

The thermodynamic landscape of testosterone binding to cytochrome P450 3A4: ligand binding and spin state equilibria.

Human cytochrome P450 (CYP) 3A4 catalyzes the oxygen-dependent metabolism of greater than 60% of known drugs. CYP3A4 binds multiple ligands simultaneously, and this contributes to complex allosteric kinetic behavior. Substrates that bind to this enzyme change the ferric spin state equilibrium of the heme, which can be observed by optical absorbance and electron paramagnetic resonance (EPR) spectroscopy. The ligand-dependent spin state equilibrium has not been quantitatively understood for any ligands that exhibit multiple binding. The CYP3A4 substrate testosterone (TST) has been shown previously by absorbance spectroscopy to induce spin state changes that are characteristic of a low spin to high spin conversion. Here, EPR was used to examine the equilibrium binding of TST to CYP3A4 at [CYP3A4] > K(D), which allows for characterization of the singly occupied state (i.e., CYP3A4.TST). We also have used absorbance spectroscopy to examine equilibrium binding, where [CYP3A4] < K(D), which allows for determination of K(D)'s. The combination of absorbance and EPR spectroscopy at different CYP3A4 concentrations relative to K(D) and curve fitting of the resultant equilibrium binding titration curves to the Adair-Pauling equations, and modifications of it, reveals that the first equivalent of TST binds with higher affinity than the second equivalent of TST and its binding is positively cooperative with respect to ligand-dependent spin state conversion. Careful analysis of the EPR and absorbance spectral results suggests that the binding of the second TST induces a shift to the high spin state and thus that the second TST binding causes displacement of the bound water. A model involving six thermodynamic states is presented and this model is related to the turnover of the enzyme.

Computer Simulation↗
PubMed1993

Definition of a novel ligand binding domain of a nuclear bHLH receptor: co-localization of ligand and hsp90 binding activities within the regulable inactivation domain of the dioxin receptor.

The dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and binds to DNA target sequences as a heterodimer of the approximately 100 kDa ligand binding receptor and the approximately 85 kDa auxiliary factor, Arnt. Both of these factors encompass an N-terminal basic helix-loop-helix (bHLH) motif required for DNA binding and dimerization. In this study we describe the construction of glucocorticoid/dioxin receptor fusion proteins which allow the regulation of glucocorticoid receptor activity by dioxin in transient transfections of CHO and hepatoma cells. Thus, in the absence of dioxin, chimeric receptor constructs which contain large 500-720 amino acid C-terminal dioxin receptor fragments, but lack the N-terminal bHLH motif, confer repression upon the transcriptional activity of a glucocorticoid receptor derivative, tau DBD, containing its N-terminal strong transactivating signal (tau) and its DNA binding domain (DBD). In the presence of dioxin, this repression is reversed. Importantly, these chimeric receptors did not require the bHLH Arnt co-factor for function. A considerably smaller region of the dioxin receptor, located between amino acids 230 and 421, showed specific dioxin binding activity in vitro. Moreover, dioxin binding in vitro correlated with the ability of receptor fragments to form stable complexes in vitro with the molecular chaperone hsp90. These findings support the notion that hsp90 may be important for folding of a dioxin binding configuration of the receptor. Finally, tau DBD activity was constitutively repressed in a dioxin non-responsive manner by dioxin receptor fragments which failed to bind ligand but also failed to bind hsp90 in vitro, indicating that alternative mechanisms in addition to hsp90 binding may contribute to the inactivation function. In summary, the dioxin receptor system provides a novel and complex model of regulation of bHLH factors that may also give important insights into the mechanism of action of ligand-activated nuclear receptors.

Aryl Hydrocarbon Receptor Nuclear Translocator↗
PubMed1996

Substitution of two amino acids confers C3b binding to the C4b binding site of CR1 (CD35). Analysis based on ligand binding by chimpanzee erythrocyte complement receptor.

The chimpanzee (Ch) E complement receptor type 1 (CR177) appears to be an alternatively spliced product of the Ch CR1 gene transcript. Its cDNA-derived amino acid sequence contains complement protein-repeating modules (CP) 1-6, 28, 29, and 30 in tandem and is 98.8% homologous to the corresponding regions of human (Hu) CR1. It differs from the C4b binding site of Hu CR1 only by two amino acids, Tyr for Ser37 in CP 1 and Asp for Gly79 in CP 2. However, in addition to binding C4b, Ch E binds C3b. As homologous substitution of one of these amino acids (Tyr for Ser37) was previously shown to not confer C3b binding, we reasoned that either single substitution of the other amino acid or a combination of the two amino acid changes would be required for C3b binding. To test this, using a truncated form of Hu CR1 that has a binding site only for C4b, we made these additional constructs. Single substitution of either amino acid did not affect the ligand binding or cofactor activity. However, the double substitution induced C3b binding and increased cofactor activity for C3b without changing the C4b-binding property. Of interest, these two amino acids are also found in the homologous positions of CP 9 and 16, which form part of the C3b binding site of Hu CR1. Thus, Ch E CR177, one-third of the size and with only a single ligand binding site, by acquiring key amino acid substitutions, binds C3b and C4b and functions analogous to Hu E CR1.

Amino Acid Sequence↗