Virtual Screening and Binding Analysis of Potential CD58 Inhibitors in Colorectal Cancer (CRC)

Human cell surface receptor CD58, also known as lymphocyte function-associated antigen 3 (LFA-3), plays a critical role in the early stages of immune response through interacting with CD2. Recent research identified CD58 as a surface marker of colorectal cancer (CRC), which can upregulate the Wnt pathway and promote self-renewal of colorectal tumor-initiating cells (CT-ICs) by degradation of Dickkopf 3. In addition, it was also shown that knockdown of CD58 significantly impaired tumor growth. In this study, we developed a structure-based virtual screening pipeline using Autodock Vina and binding analysis and identified a group of small molecular compounds having the potential to bind with CD58. Five of them significantly inhibited the growth of the SW620 cell line in the following in vitro studies. Their proposed binding models were further verified by molecular dynamics (MD) simulations, and some pharmaceutically relevant chemical and physical properties were predicted. The hits described in this work may be considered interesting leads or structures for the development of new and more efficient CD58 inhibitors.

Modulation of co-stimulatory signal from CD2-CD58 proteins by a grafted peptide

Peptides were designed to inhibit the protein-protein interaction of CD2 and CD58 to modulate the immune response. This work involved the design and synthesis of eight different peptides by replacing each amino acid residue in peptide 6 with alanine as well as grafting the peptide to the sunflower trypsin-inhibitor framework. From the alanine scanning studies, mutation at position 2 of the peptide was shown to result in increased potency to inhibit cell adhesion interactions. The most potent peptide from the alanine scanning was further studied for its detailed three-dimensional structure and binding to CD58 protein using surface plasmon resonance and flow cytometry. This peptide was used to graft to the sunflower trypsin inhibitor to improve the stability of the peptide. The grafted peptide, SFTI-a1, was further studied for its potency as well as its thermal, chemical, and enzymatic stability. The grafted peptide exhibited improved activity compared to our previously grafted peptide and was stable against thermal and enzymatic degradation.

Molecular characterization and expression of CD2 in Nile tilapia (Oreochromis niloticus) in response to Streptococcus agalactiae stimulus

The cluster of differentiation 2 (CD2), functioning as a cell adhesion and costimulatory molecule, plays a crucial role in T-cell activation. In this paper, the CD2 gene of Nile tilapia, Oreochromis niloticus (designated as On-CD2) was cloned and its expression pattern under the stimulation of Streptococcus agalactiae was investigated. Sequence analysis showed On-CD2 protein consists of two extracellular Ig-like domains, a transmembrane region, and a long proline-rich cytoplasmic tail, which is a hallmark of CD2, and several important structural characteristics required for T-cell activation were detected in the deduced amino acid sequence of On-CD2. In healthy tilapia, the On-CD2 transcripts were mainly detected in the head kidney, spleen, blood and thymus. Moreover, there was a clear time-dependent expression pattern of On-CD2 after immunized by formalin-inactivated S. agalactiae and the expression reached the highest level at 12 h in the brain and head kidney, 48 h in the spleen, and 72 h in the thymus, respectively. This is the first report on the expression of CD2 induced by bacteria vaccination in teleosts. These findings indicated that On-CD2 may play an important role in the immune response to intracellular bacteria in Nile tilapia.

Comparative evaluation of T11 target structure and its deglycosylated derivative nullifies the importance of glycan moieties in immunotherapeutic efficacy

Sheep red blood cell (SRBC), a non-specific biological response modifier that has long been used as a classical antigen, has been shown to exert an immunomodulatory and anti-tumor activities in experimental animals. The active component of SRBC, which is responsible for such effects, was found to be a cell surface acidic glycoprotein molecule, known as T11 target structure (T11TS). In the present study, T11TS was isolated and purified to homogeneity using a five-step protocol involving isolation of sheep erythrocyte membrane from packed cell volume, 20% ammonium sulfate cut of the crude membrane proteins mixture, immunoaffinity purification using mouse anti-sheep CD58 mAb (L180/1) tagged matrix, preparative gel electrophoresis, and gel electroelution process. Finally, the purity and identity of the proteins were confirmed by the matrix-assisted laser desorption/ionization (MALDI) mass spectrometric analysis. The in silico glycosylation site analysis showed that the extracellular domain contained three N-glycosylation sites (N-12, N-62, and N-111) and one O-glycosylation site (T-107). However, the experimental analysis negated the presence of O-linked glycan moieties on T11TS. To investigate the role of glycan moieties in the current immunotherapeutic regime, T11TS and its deglycosylated form (dT11TS) were administered intraperitoneally (i.p.) in N-ethyl-N-nitrosourea-induced immune-compromised mice at 0.4 mg/kg body weight. It was observed that both the forms of T11TS could activate the compromised immune status of mice by augmenting immune receptor expression (CD2, CD25, CD8, and CD11b), T-helper 1 shift of cytokine network, enhanced cytotoxicity, and phagocytosis activity. Therefore, the results nullify the active involvement of the N-linked glycan moieties in immunotherapeutic efficacy of T11TS.

N-glycosylation of enhanced aromatic sequons to increase glycoprotein stability

N-glycosylation can increase the rate of protein folding, enhance thermodynamic stability, and slow protein unfolding; however, the molecular basis for these effects is incompletely understood. Without clear engineering guidelines, attempts to use N-glycosylation as an approach for stabilizing proteins have resulted in unpredictable energetic consequences. Here, we review the recent development of three "enhanced aromatic sequons," which appear to facilitate stabilizing native-state interactions between Phe, Asn-GlcNAc and Thr when placed in an appropriate reverse turn context. It has proven to be straightforward to engineer a stabilizing enhanced aromatic sequon into glycosylation-naïve proteins that have not evolved to optimize specific protein-carbohydrate interactions. Incorporating these enhanced aromatic sequons into appropriate reverse turn types within proteins should enhance the well-known pharmacokinetic benefits of N-glycosylation-based stabilization by lowering the population of protease-susceptible unfolded and aggregation-prone misfolded states, thereby making such proteins more useful in research and pharmaceutical applications.

Protein native-state stabilization by placing aromatic side chains in N-glycosylated reverse turns

N-glycosylation of eukaryotic proteins helps them fold and traverse the cellular secretory pathway and can increase their stability, although the molecular basis for stabilization is poorly understood. Glycosylation of proteins at naïve sites (ones that normally are not glycosylated) could be useful for therapeutic and research applications but currently results in unpredictable changes to protein stability. We show that placing a phenylalanine residue two or three positions before a glycosylated asparagine in distinct reverse turns facilitates stabilizing interactions between the aromatic side chain and the first N-acetylglucosamine of the glycan. Glycosylating this portable structural module, an enhanced aromatic sequon, in three different proteins stabilizes their native states by -0.7 to -2.0 kilocalories per mole and increases cellular glycosylation efficiency.

Rational design of a novel calcium-binding site adjacent to the ligand-binding site on CD2 increases its CD48 affinity

Electrostatic interactions are important for molecular recognition processes including Ca2+-binding and cell adhesion. To understand these processes, we have successfully introduced a novel Ca2+-binding site into the non-Ca2+-dependent cell adhesion protein CD2 using our criteria that are specifically tailored to the structural and functional properties of the protein environment and charged adhesion surface. This designed site with ligand residues exclusively from the beta-sheets selectively binds to Ca2+ and Ln3+ over other mono- and divalent cations. While Ca2+ and Ln3+ binding specifically alters the local environment of the designed Ca2+-binding site, the designed protein undergoes a significantly smaller conformation change compared with those observed in naturally occurring Ca2+-binding sites that are composed of at least part of the flexible loop and helical regions. In addition, the CD2-CD48-binding affinity increased approximately threefold after protein engineering, suggesting that the cell adhesion of CD2 can be modulated by altering the local electrostatic environment. The study provides site-specific information for regulating cell adhesion within CD2 and gives insight into the structural factors required for Ca2+-modulated biological processes.

Structure-function studies of peptides for cell adhesion inhibition: identification of key residues by alanine mutation and peptide-truncation approach

Blockage of the interaction of CD2 with its ligand CD58 is expected to bring out potential therapeutic value for autoimmune diseases and organ transplantation. Three series of peptides (cVL, cIL and cAQ series) were designed from ratCD2 and humanCD2 to modulate CD2-CD58 interaction. To determine the specific segments in parent peptides responsible for inhibitory activity as lead sequence, we generated shorter fragments of the parent peptides and evaluated their biological activity with cell adhesion assay. The structure-activity relationship studies indicated that small cyclic peptides derived from CD2 ligand binding epitopes could mimic native beta-turn structure, and thus modulate CD2-CD58 interaction.

Structure-based rational design of beta-hairpin peptides from discontinuous epitopes of cluster of differentiation 2 (CD2) protein to modulate cell adhesion interaction

Modulation or inhibition of interaction of cluster of differentiation (CD) adhesion molecules CD2-CD58 has been shown to be therapeutically useful. The analysis of the crystal structure of CD2 complexed with CD58 was carried out to define the epitopes that are important for the interaction of the two proteins. The crystal structure of CD2 indicated that the interaction surface of CD2 with CD58 has two beta-strand structures (F and C strands) with charged residues. On the basis of the crystal structure of the complex CD2-CD58, we have designed beta-hairpin peptides from the beta-strand region of CD2 by conjugating the discontinuous sequences in the protein. The peptides were modeled by molecular dynamics simulation, and their inhibitory activities were evaluated in vitro using two heterotypic cell adhesion assays, E-rosetting and lymphocyte-epithelial cell adhesion assays. Results indicated that 12- and 14-residue conjugate cyclic peptides cKS12 and cDD14 exhibited 60% and 50% inhibition activity, respectively, at 90 microM.

The contribution of conformational adjustments and long-range electrostatic forces to the CD2/CD58 interaction

CD2 is a T cell surface molecule that enhances T and natural killer cell function by binding its ligands CD58 (humans) and CD48 (rodents) on antigen-presenting or target cells. Here we show that the CD2/CD58 interaction is enthalpically driven and accompanied by unfavorable entropic changes. Taken together with structural studies, this indicates that binding is accompanied by energetically significant conformational adjustments. Despite having a highly charged binding interface, neither the affinity nor the rate constants of the CD2/CD58 interaction were affected by changes in ionic strength, indicating that long-range electrostatic forces make no net contribution to binding.

Impact of salt bridges on the equilibrium binding and adhesion of human CD2 and CD58

This study describes quantitative investigations of the impact of single charge mutations on equilibrium binding, kinetics, and the adhesion strength of the CD2-CD58 interaction. Previously steered molecular dynamics simulations guided the selection of the charge mutants investigated, which include the CD2 mutants D31A, K41A, K51A, and K91A. This set includes mutations in which the previous cell aggregation and binding data either agreed or disagreed with the steered molecular dynamics predictions. Surface plasmon resonance measurements quantified the solution binding properties. Adhesion was quantified with the surface force apparatus, which was used previously to study the closely related CD2-CD48 interaction. The results reveal roles that these salt bridges play in equilibrium binding and adhesion. We discuss both the molecular basis of this behavior and its implications for cell adhesion.

NTB-A receptor crystal structure: insights into homophilic interactions in the signaling lymphocytic activation molecule receptor family

The signaling lymphocytic activation molecule (SLAM) family includes homophilic and heterophilic receptors that regulate both innate and adaptive immunity. The ectodomains of most SLAM family members are composed of an N-terminal IgV domain and a C-terminal IgC2 domain. NK-T-B-antigen (NTB-A) is a homophilic receptor that stimulates cytotoxicity in natural killer (NK) cells, regulates bactericidal activities in neutrophils, and potentiates T helper 2 (Th2) responses. The 3.0 A crystal structure of the complete NTB-A ectodomain revealed a rod-like monomer that self-associates to form a highly kinked dimer spanning an end-to-end distance of approximately 100 A. The NTB-A homophilic and CD2-CD58 heterophilic dimers show overall structural similarities but differ in detailed organization and physicochemical properties of their respective interfaces. The NTB-A structure suggests a mechanism responsible for binding specificity within the SLAM family and imposes physical constraints relevant to the colocalization of SLAM-family proteins with other signaling molecules in the immunological synapse.

Electrostatic contribution to the binding stability of protein-protein complexes

To investigate roles of electrostatic interactions in protein binding stability, electrostatic calculations were carried out on a set of 64 mutations over six protein-protein complexes. These mutations alter polar interactions across the interface and were selected for putative dominance of electrostatic contributions to the binding stability. Three protocols of implementing the Poisson-Boltzmann model were tested. In vdW4 the dielectric boundary between the protein low dielectric and the solvent high dielectric is defined as the protein van der Waals surface and the protein dielectric constant is set to 4. In SE4 and SE20, the dielectric boundary is defined as the surface of the protein interior inaccessible to a 1.4-A solvent probe, and the protein dielectric constant is set to 4 and 20, respectively. In line with earlier studies on the barnase-barstar complex, the vdW4 results on the large set of mutations showed the closest agreement with experimental data. The agreement between vdW4 and experiment supports the contention of dominant electrostatic contributions for the mutations, but their differences also suggest van der Waals and hydrophobic contributions. The results presented here will serve as a guide for future refinement in electrostatic calculation and inclusion of nonelectrostatic effects.

Crystal structure and binding properties of the CD2 and CD244 (2B4)-binding protein, CD48

The structural analysis of surface proteins belonging to the CD2 subset of the immunoglobulin superfamily has yielded important insights into transient cellular interactions. In mice and rats, CD2 and CD244 (2B4), which are expressed predominantly on T cells and natural killer cells, respectively, bind the same, broadly expressed ligand, CD48. Structures of CD2 and CD244 have been solved previously, and we now present the structure of the receptor-binding domain of rat CD48. The receptor-binding surface of CD48 is unusually flat, as in the case of rat CD2, and shares a high degree of electrostatic complementarity with the equivalent surface of CD2. The relatively simple arrangement of charged residues and this flat topology explain why CD48 cross-reacts with CD2 and CD244 and, in rats, with the CD244-related protein, 2B4R. Comparisons of modeled complexes of CD2 and CD48 with the complex of human CD2 and CD58 are suggestive of there being substantial plasticity in the topology of ligand binding by CD2. Thermodynamic analysis of the native CD48-CD2 interaction indicates that binding is driven by equivalent, weak enthalpic and entropic effects, in contrast to the human CD2-CD58 interaction, for which there is a large entropic barrier. Overall, the structural and biophysical comparisons of the CD2 homologues suggest that the evolutionary diversification of interacting cell surface proteins is rapid and constrained only by the requirement that binding remains weak and specific.

Amyloid Fibril Formation by a Domain of Rat Cell Adhesion Molecule

Amyloid fibrils are protein aggregates implicated in several amyloidotic diseases. Cellular membranes with local decrease in pH and dielectric constant are associated with the amyloid formation. In this study, domain 1 of cell adhesion molecule CD2 (CD2-1) is used for studying amyloid fibril formation in a water/trifluoroethanol (TFE) mixture. CD2-1 is an all beta-sheet protein similar in topology to the amyloidogenic light chain variable domain, which deposits as amyloid in light chain amyloidosis at acidic pH. When incubated at pH 2.0 in the presence of 18% TFE, CD2-1 initiates the process to assemble into amyloid fibrils. It has been shown that TFE induces CD2-1 conformational change with a chemical transition (C(m)) of 18% (v/v). ANS (1-anilinonapthalene-8-sulfonic acid) binding was used to show that the hydrophobic surface becomes exposed under these solvent conditions. Our studies indicate that partial formation of a non-native conformation and the exposure of the hydrophobic interior could be the origins of oligomerization and fibril formation of CD2-1.

Low Energy Pathways and Non-native Interactions

Four versions of a beta-sheet protein (CD2.d1) have been made, each with a single artificial disulfide bond inserted into hairpin structures. Folding kinetics of reduced and oxidized forms shows bridge position strongly influences its effect on the folding reaction. Bridging residues 58 and 62 does not affect the rapidly formed intermediate (I) or rate-limiting transition (t) state, whereas bridging 33 and 38, or 31 and 41, lowers the t-state energy, with the latter having the stronger influence. Bridging residues 79 and 90 stabilizes both I- and t-states. To assess additivity in the energetic effects of these bridges, four double-bridge variants have also been made. All show precise additivity of overall stability, with two showing additivity when ground states and the rate-limiting t-state are assessed, i.e. no measurable change in the folding mechanism occurs. However, combining 31-41 and 79-90 bridges produces a molecule that folds through a different pathway, with a much more stable intermediate than expected and a much higher t-state barrier. This is explained by the artificial introduction of stabilizing, non-native contacts in the I-state. More surprisingly, for another double-bridge version (58-62 and 79-90) both I- and t-states are less stable than expected, showing that conformational constraints introduced by the two bridges prevent formation of non-native contacts that would otherwise stabilize the I- and t-states, thereby lowering the energy of the folding landscape in the wild-type (unbridged) molecule. We conclude that the lowest energy path for folding has I- and t-state structures that are stabilized by non-native interactions.

Structure-activity studies of peptides from the "hot-spot" region of human CD2 protein: development of peptides for immunomodulation

CD2 is a cell surface protein belonging to the immunoglobulin superfamily (IgSF) that plays a key role in mediating adhesion between human T-lymphocytes and target cells. The interaction between cell-adhesion molecules CD2 and CD58 is critical for immune response. Modulation or inhibition of these interactions has been shown to be therapeutically useful. Synthetic 12-mer linear and cyclic peptides and cyclic hexapeptides from the beta-turn and beta-strand region (hot spot) of human CD2 protein were designed to modulate CD2-CD58 interaction. The 12-amino acid synthetic cyclic peptides effectively blocked the interaction between CD2 and CD58 proteins as demonstrated by E-rosetting and heterotypic adhesion assays. NMR and molecular modeling studies indicated that these cyclic peptides exhibit beta-turn structure in solution and closely mimic the beta-turn structure of the surface epitopes of CD2 protein. The designed cyclic peptides with beta-turn structure have the ability to modulate CD2-CD58 interaction.

The effects of Ca2+ binding on the dynamic properties of a designed Ca2+-binding protein

The effects of Ca(2+) binding on the dynamic properties of Ca(2+)-binding proteins are important in Ca(2+) signaling. To understand the role of Ca(2+) binding, we have successfully designed a Ca(2+)-binding site in the domain 1 of rat CD2 (denoted as Ca.CD2) with the desired structure and retained function. In this study, the backbone dynamic properties of Ca.CD2 have been investigated using (15)N spin relaxation NMR spectroscopy to reveal the effect of Ca(2+) binding on the global and local dynamic properties without the complications of multiple interactive Ca(2+) binding and global conformational change. Like rat CD2 (rCD2) and human CD2 (hCD2), residues involved in the recognition of the target molecule CD48 exhibit high flexibility. Mutations N15D and N17D that introduce the Ca(2+) ligands increase the flexibility of the neighboring residues. Ca(2+)-induced local dynamic changes occur mainly at the residues proximate to the Ca(2+)-binding pocket or the residues in loop regions. The beta-strand B of Ca.CD2 that provides two Asp for the Ca(2+) undergoes an S(2) decrease upon the Ca(2+) binding, while the DE-loop that provides one Asn and one Asp undergoes an S(2) increase. Our study suggests that Ca(2+) binding has a differential effect on the rigidity of the residues depending on their flexibility and location within the secondary structure.

Single-molecule microscopy reveals plasma membrane microdomains created by protein-protein networks that exclude or trap signaling molecules in T cells

Membrane subdomains have been implicated in T cell signaling, although their properties and mechanisms of formation remain controversial. Here, we have used single-molecule and scanning confocal imaging to characterize the behavior of GFP-tagged signaling proteins in Jurkat T cells. We show that the coreceptor CD2, the adaptor protein LAT, and tyrosine kinase Lck cocluster in discrete microdomains in the plasma membrane of signaling T cells. These microdomains require protein-protein interactions mediated through phosphorylation of LAT and are not maintained by interactions with actin or lipid rafts. Using a two color imaging approach that allows tracking of single molecules relative to the CD2/LAT/Lck clusters, we demonstrate that these microdomains exclude and limit the free diffusion of molecules in the membrane but also can trap and immobilize specific proteins. Our data suggest that diffusional trapping through protein-protein interactions creates microdomains that concentrate or exclude cell surface proteins to facilitate T cell signaling.

Design of a calcium-binding protein with desired structure in a cell adhesion molecule

Ca2+, "a signal of life and death", controls numerous cellular processes through interactions with proteins. An effective approach to understanding the role of Ca2+ is the design of a Ca2+-binding protein with predicted structural and functional properties. To design de novo Ca2+-binding sites in proteins is challenging due to the high coordination numbers and the incorporation of charged ligand residues, in addition to Ca2+-induced conformational change. Here, we demonstrate the successful design of a Ca2+-binding site in the non-Ca2+-binding cell adhesion protein CD2. This designed protein, Ca.CD2, exhibits selectivity for Ca2+ versus other di- and monovalent cations. In addition, La3+ (Kd 5.0 microM) and Tb3+ (Kd 6.6 microM) bind to the designed protein somewhat more tightly than does Ca2+ (Kd 1.4 mM). More interestingly, Ca.CD2 retains the native ability to associate with the natural target molecule. The solution structure reveals that Ca.CD2 binds Ca2+ at the intended site with the designed arrangement, which validates our general strategy for designing de novo Ca2+-binding proteins. The structural information also provides a close view of structural determinants that are necessary for a functional protein to accommodate the metal-binding site. This first success in designing Ca2+-binding proteins with desired structural and functional properties opens a new avenue in unveiling key determinants to Ca2+ binding, the mechanism of Ca2+ signaling, and Ca2+-dependent cell adhesion, while avoiding the complexities of the global conformational changes and cooperativity in natural Ca2+-binding proteins. It also represents a major achievement toward designing functional proteins controlled by Ca2+ binding.

Alternative Binding Modes of Proline-Rich Peptides Binding to the GYF Domain

Recognition of proline-rich sequences plays an important role for the assembly of multiprotein complexes during the course of eukaryotic signal transduction and is mediated by a set of protein folds that share characteristic features. The GYF (glycine-tyrosine-phenylalanine) domain is known as a member of the superfamily of recognition domains for proline-rich sequences. Recent studies on the complexation of the CD2BP2-GYF domain with CD2 peptides showed that the peptide adopts an extended conformation and forms a polyproline type-II helix involving residues Pro4-Pro7 [Freund et al. (2002) EMBO J. 21, 5985-5995]. R/K/GxxPPGxR/K is the key signature for the peptides that bind to the GYF domain [Kofler et al. (2004) J. Biol. Chem. 279, 28292-28297]. In our combined theoretical and experimental study, we show that the peptides adopt a polyproline II helical conformation in the unbound form as well as in the complex. From molecular dynamics simulations, we identify a novel binding mode for the G8W mutant and the wild-type peptide (shifted by one proline in register). In contrast, the conformation of the peptide mutant H9M remains close to the experimentally derived wild-type GYF-peptide complex. Possible functional implications of this altered conformation of the bound ligand are discussed in the light of our experimental and theoretical results.

A Model for CD2/CD58-Mediated Adhesion Strengthening

Stable cell adhesion is vital for structural integrity and functional efficacy. Yet how low affinity adhesion molecules such as CD2 and CD58 can produce stable cell adhesion is still not completely understood. In this paper, we present a theoretical model that simulates the accumulation of CD2 and CD58 in the contact area of a Jurkat T lymphoblast and a CD58-containing substrate. The cell is assumed to have a spherical shape initially and it is allowed to spread gradually on a circular substrate. Mobile CD2 and CD58 can diffuse freely on both the cell and substrate. Their binding in the contact area is controlled by first-order kinetics. The contact area grows linearly with the total number of CD2/CD58 bonds. Cellular deformation and cytoskeleton involvement were not considered. This time-dependent moving-boundary problem was solved with the Crank-Nicolson finite difference scheme and the variable space grid method. Our simulated results are in reasonable agreement with the experimental observations. The role of diffusion becomes more and more prominent during the contact area increase, which is not sensitive to the kinetic rate constants tested in this study. However, it is very sensitive to the dissociation equilibrium constant and the concentrations of CD2 and CD58.

Probing Site-Specific Calmodulin Calcium and Lanthanide Affinity by Grafting

Ca2+ binding is essential for the biological functions of calmodulin (CaM) as a trigger/sensor protein to regulate many biological processes in the Ca2+ -signaling cascade. A challenge in understanding the mechanism of Ca2+ signaling is to obtain site-specific information about the Ca2+ binding properties of individual Ca2+ -binding sites of EF-hand proteins, especially for CaM. In this paper, we report the first estimation of the intrinsic Ca2+ affinities of the four EF-hand loops of calmoduin (I-IV) by individually grafting into the domain 1 of CD2. Taking advantage of the Trp residues in the host protein, we first determined metal-binding affinities for Tb3+, Ca2+, and La3+ for all four grafted EF-loops using Tb3+ aromatic resonance energy transfer. EF-loop I exhibits the strongest binding affinity for Ca2+, La3+, and Tb3+, while EF-loop IV has the weakest metal-binding affinity. EF-loops I-IV of CaM have dissociation constants for Ca2+ of 34, 245, 185, and 814 microM, respectively, with the order I > III approximately equal to II > IV. These findings support a charge-ligand-balanced model in which both the number of negatively charged ligand residues and the balanced electrostatic dentate-dentate repulsion by the adjacent charged residues are two major determinants for the relative Ca2+ -binding affinities of EF-loops in CaM. Our grafting method provides a new strategy to obtain site-specific Ca2+ binding properties and a better estimation of the cooperativity and conformational change contributions of coupled EF-hand proteins.

Association of extensive polymorphisms in the SLAM/CD2 gene cluster with murine lupus

Susceptibility to autoimmunity in B6.Sle1b mice is associated with extensive polymorphisms between two divergent haplotypes of the SLAM/CD2 family of genes. The B6.Sle1b-derived SLAM/CD2 family haplotype is found in many other laboratory mouse strains but only causes autoimmunity in the context of the C57Bl/6 (B6) genome. Phenotypic analyses have revealed variations in the structure and expression of several members of the SLAM/CD2 family in T and B lymphocytes from B6.Sle1b mice. T lymphocytes from B6.Sle1b mice have modified signaling responses to stimulation at 4-6 weeks of age. While autoimmunity may be mediated by a combination of genes in the SLAM/CD2 family cluster, the strongest candidate is Ly108, a specific isoform of which is constitutively upregulated in B6.Sle1b lymphocytes.

Analysis of human and primate CD2 molecules by protein sequence and epitope mapping with anti-human CD2 antibodies

A panel of anti-human CD2 monoclonal antibodies (mAb) and soluble human CD58 (LFA-3) were tested for binding to human peripheral blood mononuclear cells (PBMCs), recombinant human CD2 and mononuclear cells from Cynomolgus, Rhesus and African green monkey, Stump-tail, Pig-tail and Assamese macaque, Chimpanzee and Baboon. This analysis revealed that whilst some antibodies recognized all species, there were differential binding profiles with others. Three antibodies, MEDI-507, 6F10.3 and 4B2, recognized CD2 from human and Chimpanzee but not that from the other primates. We have cloned eight of the previously unknown primate CD2 molecules and report here their sequences for the first time. This analysis revealed that 12 amino acids formed a common set of residues in the extra cellular domain of human and Chimpanzee CD2. Using a "knock-in" mutagenesis approach starting with Baboon CD2, which does not bind MEDI-507, 6F10.3 and 4B2, we have identified three residues in the adhesion domain of human CD2 which are critical for its binding to these mAbs. These residues, N18, K55 and T59 define a region located outside of the previously described binding regions on CD2. Affinity measurements of the mutants revealed a variety of degrees of binding restoration for MEDI-507, 6F10.3 and 4B2, indicating that there are fine differences within a given epitope. Furthermore, the analysis of the competition of several of the anti-human CD2 antibodies with each other and CD58 demonstrated the existence of a continuum of overlapping epitopes on human CD2, which is in contrast to the commonly held belief that epitopes on human CD2 are clearly segregated.

Design, structure and biological activity of β-turn peptides of CD2 protein for inhibition of T-cell adhesion

The interaction between cell-adhesion molecules CD2 and CD58 is critical for an immune response. Modulation or inhibition of these interactions has been shown to be therapeutically useful. Synthetic 12-mer linear and cyclic peptides, and cyclic hexapeptides based on rat CD2 protein, were designed to modulate CD2-CD58 interaction. The synthetic peptides effectively blocked the interaction between CD2-CD58 proteins as demonstrated by antibody binding, E-rosetting and heterotypic adhesion assays. NMR and molecular modeling studies indicated that the synthetic cyclic peptides exhibit beta-turn structure in solution and closely mimic the beta-turn structure of the surface epitopes of the CD2 protein. Docking studies of CD2 peptides and CD58 protein revealed the possible binding sites of the cyclic peptides on CD58 protein. The designed cyclic peptides with beta-turn structure have the ability to modulate the CD2-CD58 interaction.

Structural biology of the cell adhesion protein CD2: from molecular recognition to protein folding and design

CD2 (cluster of differentiation 2) is a cell adhesion molecule expressed on T cells and is recognized as a target for CD48 (rats) and CD58 (humans). Tremendous progress has been achieved in understanding the function of CD2, the mechanism of molecular recognition and protein folding, thus, leading towards the use of this protein as a scaffold for protein design. CD2 has been shown to set quantitative thresholds in T cell activation both in vivo and in vitro. Further, intracellular CD2 signaling pathways and networks are being discovered by the identification of several cytosolic tail binding proteins. In addition, a new method for directly measuring heterophilic adhesion has been developed. The functional "hot spot" for the adhesion surface of CD2 and CD58 has been dissected. Detailed NMR studies reveal that rat CD2 weakly self-associates to form a homodimeric structure in solution. Dynamic interaction of CD2 with the GYF and SH3 domains has been investigated. CD2 has been shown to form fibrils in the presence of 2,2,2-trifluoroethanol (TFE) and at low pH. Furthermore, kinetic studies have been completed to monitor the effect of surface hydrophobic residues and intramolecular bridges on the folding pathways of CD2. Our lab has de novo designed single calcium-binding sites into domain 1 of rat CD2 (CD2-D1) with strong metal selectivity. In addition, the EF-hand motifs have been grafted into CD2 to understand the site-specific calcium-binding affinity of calmodulin and calcium-dependent cell adhesion.

The CD2v protein of African swine fever virus interacts with the actin-binding adaptor protein SH3P7

The predicted extracellular domain of the CD2v protein of African swine fever virus (ASFV) shares significant similarity to that of the CD2 protein in T cells but has a unique cytoplasmic domain of unknown function. Here we have shown that CD2v is expressed as a glycoprotein of approximately 105 kDa in ASFV-infected cells. In the absence of an extracellular ligand, the majority of CD2v appears to localize to perinuclear membrane compartments. Furthermore, we have shown using the yeast two-hybrid system and by direct binding studies that the cytoplasmic tail of CD2v binds to the cytoplasmic adaptor protein SH3P7 (mAbp1, HIP55), which has been reported to be involved in diverse cellular functions such as vesicle transport and signal transduction. A cDNA clone encoding a variant form of SH3P7 could also be identified and was found to be expressed in a wide range of porcine tissues. Deletion mutagenesis identified proline-rich repeats of sequence PPPKPC in the ASFV CD2v protein to be necessary and sufficient for binding to the SH3 domain of SH3P7. In ASFV-infected cells, CD2v and SH3P7 co-localized in areas surrounding the perinuclear virus factories. These areas also stained with an antibody that recognizes a Golgi network protein, indicating that they contained membranes derived from the Golgi network. Our data provide a first molecular basis for the understanding of the immunomodulatory functions of CD2v in ASFV-infected animals.

Molecular interactions mediating T cell antigen recognition

Over the past decade, key protein interactions contributing to T cell antigen recognition have been characterized in molecular detail. These have included interactions involving the T cell antigen receptor (TCR) itself, its coreceptors CD4 and CD8, the accessory molecule CD2, and the costimulatory receptors CD28 and CTLA-4. A clear view is emerging of how these molecules interact with their ligands at the cell-cell interface. Structural and binding studies have confirmed that the proteins span small but comparable distances and that, overall, they interact very weakly. However, there have been important surprises as well: that TCR interactions with peptide-MHC are topologically constrained and characterized by considerable conformational flexibility at the binding interface; that coreceptors engage peptide-MHC with extraordinarily fast kinetics and at angles apparently precluding direct interactions with the TCR bound to the same peptide-MHC; that the structural mechanisms allowing recognition by costimulatory and accessory molecules to be weak and yet specific are very heterogeneous; and that because of differences in both binding affinity and stoichiometry, there is enormous variation in the stability of the various costimulatory receptor/ligand complexes. These studies provide the necessary framework for exploring how these molecular interactions initiate T cell activation.

Linking the T cell surface protein CD2 to the actin-capping protein CAPZ via CMS and CIN85

Recruitment of CD2 to the immunological synapse in response to antigen is dependent on its proline-rich cytoplasmic tail. A peptide from this region (CD2:322-339) isolated CMS (human CD2AP); a related protein, CIN85; and the actin capping protein, CAPZ from a T cell line. In BIAcore analyses, the N-terminal SH3 domains of CMS and CIN85 bound CD2:322-339 with similar dissociation constants (KD = approximately 100 microm). CAPZ bound the C-terminal half of CMS and CIN85. Direct binding between CMS/CIN85 and CAPZ provides a link with the actin cytoskeleton. Overexpression of a fragment from the C-terminal half or the N-terminal SH3 domain of CD2AP in a mouse T cell hybridoma resulted in enhanced interleukin-2 production and reduced T cell receptor down-modulation in response to antigen. These adaptor proteins are important in T cell signaling consistent with a role for CD2 in regulating pathways initiated by CMS/CIN85 and CAPZ.

A grafting approach to obtain site-specific metal-binding properties of EF-hand proteins

The EF-hand calcium-binding loop III from calmodulin was inserted with glycine linkers into the scaffold protein CD2.D1 at three locations to study site-specific calcium binding properties of EF-hand motifs. After insertion, the host protein retains its native structure and forms a 1:1 metal-protein complex for calcium and its analog, lanthanum. Tyrosine-sensitized Tb3+ energy transfer exhibits metal binding and La3+ and Ca2+ compete for the metal binding site. The grafted EF-loop III in different environments has similar La3+ binding affinities, suggesting that it is largely solvated and functions independently from the host protein.

Rational design of a calcium-binding protein

Calcium ions play key roles as structural components in biomineralization and as a second messenger in signaling pathways. We have introduced a de novo designed calcium-binding site into the framework of a non-calcium-binding protein, domain 1 of CD2. The resulting protein selectively binds calcium over magnesium with calcium-binding affinity comparable to that of natural extracellular calcium-binding proteins (K(d) of 50 microM). This experiment is the first successful metalloprotein design that has a high coordination number (seven) metal-binding site constructed into a beta-sheet protein. Our results demonstrate the feasibility of designing a single calcium-binding site into a host protein, taking into account only local properties of a calcium-binding site obtained by a survey of natural calcium-binding proteins and chelators. The resulting site exhibits strong metal selectivity, suggesting that it should now be feasible to understand and manipulate signaling processes by designing novel calcium-modulated proteins with specifically desired functions and to affect their stability.

Forced detachment of the CD2-CD58 complex

The force-induced detachment of the adhesion protein complex CD2-CD58 was studied by steered molecular dynamics simulations. The forced detachment of CD2 and CD58 shows that the system can respond to an external force by two mechanisms, which depend on the loading rate. At the rapid loading rates of 70 and 35 pN/ps (pulling speeds of 1 and 0.5 A/ps) the two proteins unfold before they separate, whereas at slower loading rates of 7 and 3.5 pN/ps (pulling speeds of 0.1 and 0.05 A/ps), the proteins separate before the domains can unfold. When subjected to a constant force of 400 pN, the two proteins separated without significant structural distortion. These findings suggest that protein unfolding is not coupled to the adhesive function of CD2 and CD58. The simulations further confirm that salt bridges primarily determine the tensile strength of the protein-to-protein bond, and that the order of salt bridge rupture depends mainly on the position of the bond, relative to the line of action of the applied force. Salt bridges close to this line break first. The importance of each of the salt bridges for adhesion, determined from the simulations, correlates closely with their role in cell-to-cell adhesion and equilibrium binding determined by site-directed mutagenesis experiments.

CD45 ectodomain controls interaction with GEMs and Lck activity for optimal TCR signaling

The transmembrane phosphatase CD45 regulates both Lck activity and T cell receptor (TCR) signaling. Here we have tested whether the large ectodomain of CD45 has a role in this regulation. A CD45 chimera containing the large ectodomain of CD43 efficiently rescues TCR signaling in CD45-null T cells, whereas CD45 chimeras containing small ectodomains from other phosphatases do not. Both basal Lck activity in unstimulated cells and the TCR-induced increase in tyrosine phosphorylation of the TCR zeta-chain and in Lck activity depend on the expression of CD45 with a large ectodomain. Unlike CD45 chimeras containing small ectodomains, both the CD45 chimera with a large ectodomain and wild-type CD45 itself are partially localized to glycosphingolipid-enriched membranes (GEMs). Taken together, these data show that the large CD45 ectodomain is required for optimal TCR signaling.

The pH dependence of CD2 domain 1 self-association and 15N chemical exchange broadening is correlated with the anomalous pKa of Glu41

We have previously shown using (15)N nuclear relaxation measurements that the concentration-dependent rotational correlation time and chemical exchange broadening for selected resonances of rat CD2 domain 1 (CD2d1) are consistent with a model of low-affinity self-association of the protein molecules. The exchange broadening data, which at high protein concentrations highlight selected nuclei in the major C'-C-F-G beta-sheet face of the immunoglobulin fold, implicate a surface reminiscent of the major lattice contact within crystals of the intact CD2 ectodomain. In a separate study, we have also demonstrated that the beta-strand C' surface-exposed residue Glu41 possesses an anomalously elevated acidity constant (pK(a) = 6.7 at a protein concentration of 1.2 mM). Mutagenesis studies showed that the close contact of residue Glu41 with Glu29 (beta-strand C) is the primary cause of the high pK(a). However, the measured pK(a) of Glu41 also shows a weak dependence on protein concentration, implicating Glu41 in the mechanism of CD2d1 self-association. In the study presented here, we demonstrate a correlation of the pH dependence of the chemical shift and (15)N nuclear relaxation parameters measured for wild-type and mutant forms of CD2d1 with pH and the protonation state of Glu41. Self-association of CD2d1 molecules is promoted whenever the side chain charge of residue 41 is neutralized. These observations are consistent with a model for CD2d1 self-association that corresponds to the crystal structure lattice contact where the interatomic distances are consistent with Glu41 being in the protonated state. This study reinforces the conclusion that residue-specific chemical exchange broadening of protein resonances can arise from weak self-association phenomena. In addition, the electrostatic profile of rat CD2 interfacial residues parallels that of the homologous human CD2 in a manner that suggests a rationalization of similar exchange broadening observations.

Dynamic interaction of CD2 with the GYF and the SH3 domain of compartmentalized effector molecules

Intracellular protein interaction domains are essential for eukaryotic signaling. In T cells, the CD2BP2 adaptor binds two membrane-proximal proline-rich motifs in the CD2 cytoplasmic tail via its GYF domain, thereby regulating interleukin-2 production. Here we present the structure of the GYF domain in complex with a CD2 tail peptide. Unlike SH3 domains, which use two surface pockets to accommodate proline residues of ligands, the GYF domain employs phylogenetically conserved hydrophobic residues to create a single interaction surface. NMR analysis shows that the Fyn but not the Lck tyrosine kinase SH3 domain competes with CD2BP2 GYF-domain binding to the same CD2 proline-rich sequence in vitro. To test the in vivo significance of this competition, we used co-immunoprecipitation experiments and found that CD2BP2 is the ligand of the membrane-proximal proline-rich tandem repeat of CD2 in detergent-soluble membrane compartments, but is replaced by Fyn SH3 after CD2 is translocated into lipid rafts upon CD2 ectodomain clustering. This unveils the mechanism of a switch of CD2 function due to an extracellular mitogenic signal.

The influence of intramolecular bridges on the dynamics of a protein folding reaction

Thirteen versions of a beta-sheet protein have been constructed, each with a single, surface-exposed disulfide bridge. A comparison of folding kinetics, in oxidizing and reducing conditions, is used to elucidate the order in which beta-strands become associated during the folding process and, hence, the relationship between topology and folding dynamics. In common with the wild-type molecule, all the proteins fold through a two-step (three state) mechanism with a rapidly formed intermediate which slowly converts to the native state. In a majority of cases, the bridge is seen to stabilize the folded state, and for five of the modified proteins, the additional stability is greater than 3 kcal/mol. Surprisingly, cross-links which connect beta-strands which are distant in sequence predominantly stabilize the rapidly formed intermediate state, suggesting that these strand-strand interactions occur in the initial stages of folding. Cross-links which stabilize local hairpins have their major influence on the second, rate-determining step leading to significant enhancements in the folding rate. We find that enhancement of the folding rate in the second, rate-limiting step is correlated with a reduction in contact order in the same way as in naturally occurring proteins of different folds. The large increases in native-state stability resulting from the insertion of disulfide bridges on the surface of beta-sheet structures have implications for enhancing the robustness of proteins by molecular engineering.

Temperature-induced formation of a non-native intermediate state of the all beta-sheet protein CD2

Domain 1 of the cell adhesion protein CD2 (CD2-1) has an all beta-structure typical of proteins belonging to the immunoglobin superfamily. It has a remarkable ability to fold as a native monomer or a metastable intertwined dimer. To understand the origin of structural rearrangements of CD2-1, we have studied equilibrium unfolding of the protein using various biophysical spectroscopic techniques. At temperatures above approx 68 degrees C, a partially folded state of CD2-1 (H state) with a distinct secondary structure, involving largely exposed aromatic and hydrophobic residues and a substantially perturbed tertiary structure, is observed. In contrast, an unfolded state (D state) of CD2-1 with random-coil-like secondary and tertiary structures is observed in 6 M GuHCl. This partially folded high-temperature state has increased negative molar ellipticity at 222 nm in far-ultraviolet CD spectra, implying formation of a non-native helical conformation. The existence of this non-native high-temperature intermediate is consistent with relatively high intrinsic helical propensities in the primary sequence of CD2-1. This conformational flexibility may be important in the observed domain swapping of CD2-1.

CD2 physically associates with CD5 in rat T lymphocytes with the involvement of both extracellular and intracellular domains

T lymphocytes can be activated and induced to proliferate through stimulation of the CD2 glycoprotein with functional combinations of CD2 antibodies. However, this mechanism of signal transduction via CD2 is still not fully understood. We have investigated which molecules on the T cell surface preferentially associate in Cis with CD2 and may regulate its signaling properties. Though a quantification method we found that CD5 represents the antigen capable of co-precipitating a larger proportion of CD2. Using co-capping assays and immunoprecipitations from cell lysates, we show that an association between CD2 and CD5 can be found in rat thymocytes, T lymphocytes and in a thymoma cell line. Possibly, this interaction is a direct one, since CD2 and CD5 transiently expressed in Cos7 cells co-precipitate each other. Furthermore, using CD2 chimeric proteins containing different domains of CD2, expressed in Cos7 cells as well as in stably transfected Jurkat cells, we show that the interaction between CD2 and CD5 is held at both the intra- and extracellular levels, but does not involve the transmembrane domain. The fact that both the extracellular and the cytoplasmic domains of CD2 interact with CD5 suggests a specific and tight association between the two molecules, possibly relevant for the fine-tuning of signal transduction in T lymphocytes.

A feline CD2 homologue interacts with human red blood cells

A cDNA encoding a feline homologue of CD2 (fCD2) was identified. Several amino acids (aa) important for ligand interaction, molecular folding or signal transduction, found in other mammalian CD2, were found to be highly conserved in the predicted fCD2 aa sequence. fCD2-expressing cells were able to form rosettes with human red blood cells (probably via human CD58), and the rosette formation was inhibited by an anti-fCD2 monoclonal antibody. These results are indicative of the similarity of feline and human CD2 structures. fCD2 was found to be expressed in feline peripheral blood T lymphocytes, monocytes and cultured lymphoid cells.

TreeDock: a tool for protein docking based on minimizing van der Waals energies

Predicting protein-protein and protein-ligand docking remains one of the challenging topics of structural biology. The main problems are (i) to reliably estimate the binding free energies of docked states, (ii) to enumerate possible docking orientations at a high resolution, and (iii) to consider mobility of the docking surfaces and structural rearrangements upon interaction. Here we present a novel algorithm, TreeDock, that addresses the enumeration problem in a rigid-body docking search. By representing molecules as multidimensional binary search trees and by exploring a sufficient number of docking orientations such that two chosen atoms, one from each molecule, are always in contact, TreeDock is able to explore all clash-free orientations at very fine resolution in a reasonable amount of time. Due to the speed of the program, many contact pairs can be examined to search partial or complete surface areas. The deterministic systematic search of TreeDock is in contrast to most other docking programs that use stochastic searches such as Monte Carlo or simulated annealing methods. At this point, we have used the Lennard-Jones potential as the only scoring function and show that this can predict the correct docked conformation for a number of protein-protein and protein-ligand complexes. The program is most powerful if some information is known about the location of binding faces from NMR chemical-shift perturbation studies, orientation information from residual dipolar coupling, or mutational screening. The approach has the potential to include docking-site mobility by performing molecular dynamics or other randomization methods of the docking site and docking families to families of structures. The performance of the algorithm is demonstrated by docking three complexes of immunoglobulin superfamily domains, CD2 to CD58, the V(alpha) domain of a T-cell receptor to its V(beta) domain, and a T-cell receptor to a pMHC complex as well as a small molecule inhibitor to a phosphatase.

A facile regio- and stereoselective synthesis of mannose octasaccharide of the N-glycan in human CD2 and mannose hexasaccharide antigenic factor 13b

A highly concise and effective synthesis of the mannose octasaccharide of the N-linked glycan in the adhesion domain of human CD2 was achieved via TMSOTf-promoted selective 6-glycosylation of a trisaccharide 4,6-diol acceptor with a pentasaccharide donor, followed by deprotection. The pentasaccharide was constructed by selective 3,6-diglycosylation of 1,2-O-ethylidene-beta-D-mannopyranose with 2-O-acetyl-3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->2)-3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl trichloroacetimidate, while the trisaccharide was obtained by selective 3-O-glycosylation of allyl 4,6-O-benzylidene-alpha-D-mannopyranoside with the same disaccharide trichloroacetimidate, followed by debenzylidenation. The mannose hexasaccharide antigenic factor 13b was synthesized by condensation of a trisaccharide donor, 2-O-acetyl-3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->2)-3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->3)-4,6-di-O-acetyl-2-O-benzoyl-alpha-D-mannopyranosyl trichloroacetimidate, with a trisaccharide acceptor, methyl 3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->2)-3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->2)-3,4,6-tri-O-benzoyl-alpha-D-mannopyranoside, followed by deprotection.

Identification and characterization of SF2000 and SF2001, two new members of the immune receptor SLAM/CD2 family

The SLAM family of human genes currently consists of seven related members of the immunoglobulin superfamily, membrane-associated proteins, including CD150 (SLAM), CD244 (2B4), CD84, CD229 ( Ly-9), BLAME, CD48, and 19A. These genes are expressed to varying degrees in subsets of immune cells (T, B, natural killer, and myeloid cells) and may function as ligands or receptors. This set of genes, related to CD2 and CD58 on Chromosome (Chr) 1p98, are found clustered close together in the human genome on Chr 1q22. Four of these family members (CD150, CD244, CD84, CD229) contain conserved tyrosine motifs in their cytoplasmic tails that enable them to bind intracellular signaling molecules SAP and EAT-2. SAP is mutated in human X-linked lymphoproliferative disease (XLP), and studies in XLP patients have shown that improper signaling via molecules that bind SAP contributes to the disease. We have identified two new members of the SLAM family (SF), which we term SF2000 and SF2001, which are expressed in immune cells and map in the SLAM gene cluster. SF2001 does not contain SAP-binding motifs in its short cytoplasmic tail. SF2000, which is co-expressed with SAP in T cells, binds both SAP and EAT-2. The data suggest that signaling through SF2000, together with CD150, CD244, CD84, and CD229, is controlled by SAP and therefore contributes to the pathogenesis of XLP.

A novel immunoglobulin superfamily receptor (19A) related to CD2 is expressed on activated lymphocytes and promotes homotypic B-cell adhesion

A novel lymphocyte-specific immunoglobulin superfamily protein (19A) has been cloned. The predicted 335-amino-acid sequence of 19A represents a Type 1 membrane protein with homology with the CD2 family of receptors. A molecular model of the two predicted extracellular immunoglobulin-like domains of 19A has been generated using the crystal structure of CD2 as a template. In isolated lymphocytes, expression of 19A is induced by various activation stimuli, and enforced expression of the 19A gene promotes homotypic cell adhesion in a B-cell-line model. Collectively these data imply that the 19A protein plays a role in regulation of lymphocyte adhesion.

Metal binding affinity and structural properties of an isolated EF-loop in a scaffold protein

To establish an approach to obtain the site-specific calcium binding affinity of EF-hand proteins, we have successfully designed a series of model proteins, each containing the EF-hand calcium-binding loop 3 of calmodulin, but with increasing numbers of Gly residues linking the loop to domain 1 of CD2. Structural analyses, using different spectroscopic methods, have shown that the host protein is able to retain its native structure after insertion of the 12-residue calcium-binding loop and retains a native thermal stability and thermal unfolding behavior. In addition, calcium binding to the engineered CD2 variants does not result in a significant change from native CD2 conformation. The CD2 variant with two Gly linkers has been shown to have the strongest metal binding affinity to Ca(II) and La(III). These experimental results are consistent with our molecular modeling studies, which suggest that this protein with the engineered EF-loop has a calmodulin-like calcium binding geometry and backbone conformation. The addition of two Gly linkers increases the flexibility of the inserted EF-loop 3 from calmodulin, which is essential for the proper binding of metal ions.

Molecular dissection of the CD2-CD58 counter-receptor interface identifies CD2 Tyr86 and CD58 Lys34 residues as the functional "hot spot"

The heterophilic CD2-CD58 adhesion interface contains interdigitating residues that impart high specificity and rapid binding kinetics. To define the hot spot of this counter-receptor interaction, we characterized CD2 adhesion domain variants harboring a single mutation of the central Tyr86 or of each amino acid residue forming a salt link/hydrogen bond. Alanine mutations at D31, D32 and K34 on the C strand and K43 and R48 on the C' strand reduce affinity for CD58 by 47-127-fold as measured by isothermal titration calorimetry. The Y86A mutant reduces affinity by approximately 1000-fold, whereas Y86F is virtually without effect, underscoring the importance of the phenyl ring rather than the hydroxyl moiety. The CD2-CD58 crystal structure offers a detailed view of this key functional epitope: CD2 D31 and D32 orient the side-chain of CD58 K34 such that CD2 Y86 makes hydrophobic contact with the extended aliphatic component of CD58 K34 between CD2 Y86 and CD58 F46. The elucidation of this hot spot provides a new target for rational design of immunosuppressive compounds and suggests a general approach for other receptors.

Distinct interactions of the X-linked lymphoproliferative syndrome gene product SAP with cytoplasmic domains of members of the CD2 receptor family

X-linked lymphoproliferative syndrome (XLP; Duncan's disease) is a primary immunodeficiency disease that manifests as an inability to regulate the immune response to Epstein-Barr virus (EBV) infection. Here we examine the ability of the product of the gene defective in XLP, SAP (DSHP/SH2D1A), to associate with the cytoplasmic domains of several members of the CD2 subfamily of cell surface receptors, including SLAM, 2B4, and CD84. While recruitment of SAP to SLAM occurred in a phosphorylation-independent manner, SAP was found to bind preferentially to tyrosine-phosphorylated cytoplasmic domains within 2B4 and CD84. Missense or nonsense mutations in the SAP open reading frame were identified in five of seven clinically diagnosed XLP patients from different kindreds. Four of these variants retained the ability to bind to the cytoplasmic tails of SLAM and CD84. While ectopic expression of wild-type SAP was observed to block the binding of SHP-2 to SLAM, mutant SAP derivatives that retained the ability to bind SLAM did not inhibit recruitment of SHP-2 to SLAM. In contrast, SAP binding to CD84 had no effect on the ability of CD84 to recruit SHP-2, but instead displaced SHP-1 from the cytoplasmic tail of CD84. These results suggest that mutations in the gene encoding the XLP protein SAP lead to functional defects in the protein that include receptor binding and SHP-1 and SHP-2 displacement and that SAP utilizes different mechanisms to regulate signaling through the CD2 family of receptors.

Nonnative intermediate state of acid-stable beta-sheet protein

Cell adhesion molecule, CD2, from the immunoglobulin superfamily, is comprised of antibodies and Ig-like domains and plays a fundamental role, not only in the immune system, but also in the interactions between cells, specifically in cell-cell adhesion. This study examines the N-terminal domain 1 of CD2 (CD2-1) at different pHs, and in 2,2,2-trifluoroethanol (TFE), using nears- and far-UV circular dichroism (CD), fluorescence, and 1H nuclear magnetic resonance to elucidate factors contributing to the Ig beta-structure. Contrary to the complete unfolding induced by guanidinehydrochloride, CD2-1 retains its native tertiary structure at pHs from 1.0 to 10.0. Like the effects of high temperatures that have previously been observed, TFE reduces the integrity of the tertiary structure, while reorganizing the secondary structure from a native all-beta-sheet to a significantly alpha-helical conformation. The induced helicity of CD2-1 correlates with the helicity inherent in its primary sequence. Our results suggest that electrostatic interactions are less important for the formation of the native secondary and tertiary structure of CD2-1, although they are crucial for CD2's adhesion function. Interference with the protein's hydrophobic interactions and hydrogen-bonding networks, however, causes significant changes in its conformation. Residues of CD2-1, with high conformational flexibility, may contribute for the formation of a metastable dimer by domain-swapping.

2B4 (CD244) and CS1: novel members of the CD2 subset of the immunoglobulin superfamily molecules expressed on natural killer cells and other leukocytes

2B4 is a member of the CD2 subset of the immunoglobulin superfamily molecules expressed on natural killer (NK) cells and other leukocytes. It is the high affinity ligand for CD48. Engagement of 2B4 on NK-cell surfaces with specific antibodies or CD48 can trigger cell-mediated cytotoxicity, interferon-gamma secretion, phosphoinositol turnover and NK-cell invasiveness. The function of 2B4 in CD8+ T cells and myeloid cells remains unknown. The cytoplasmic domain of 2B4 contains unique tyrosine motifs (TxYxxV/I) that associate with src homology 2 domain-containing protein or signaling lymphocyte activation molecule (SLAM)-associated protein, whose mutation is the underlying genetic defect in the X-linked lymphoproliferative disease (XLPD). Impaired signaling via 2B4 and SLAM is implicated in the immunopathogenesis of XLPD. CS1 is a novel member of the CD2 subset that contains two of the unique tyrosine motifs present in 2B4 and SLAM. Signaling through 2B4, CS1 and other members of the CD2 subset may play a major role in the regulation of NK cells and other leukocyte functions.