Unmasking of CD22 Co-receptor on Germinal Center B-cells Occurs by Alternative Mechanisms in Mouse and Man

CD22 is an inhibitory B-cell co-receptor whose function is modulated by sialic acid (Sia)-bearing glycan ligands. Glycan remodeling in the germinal center (GC) alters CD22 ligands, with as yet no ascribed biological consequence. Here, we show in both mice and humans that loss of high affinity ligands on GC B-cells unmasks the binding site of CD22 relative to naive and memory B-cells, promoting recognition of trans ligands. The conserved modulation of CD22 ligands on GC B-cells is striking because high affinity glycan ligands of CD22 are species-specific. In both species, the high affinity ligand is based on the sequence Siaα2–6Galβ1–4GlcNAc, which terminates N-glycans. The human ligand has N-acetylneuraminic acid (Neu5Ac) as the sialic acid, and the high affinity ligand on naive B-cells contains 6-O-sulfate on the GlcNAc. On human GC B-cells, this sulfate modification is lost, giving rise to lower affinity CD22 ligands. Ligands of CD22 on naive murine B-cells do not contain the 6-O-sulfate modification. Instead, the high affinity ligand for mouse CD22 has N-glycolylneuraminic acid (Neu5Gc) as the sialic acid, which is replaced on GC B-cells with Neu5Ac. Human naive and memory B-cells express sulfated glycans as high affinity CD22 ligands, which are lost on GC B-cells. In mice, Neu5Gc-containing glycans serve as high affinity CD22 ligands that are replaced by Neu5Ac-containing glycans on GC B-cells. Our results demonstrate that loss of high affinity CD22 ligands on GC B-cells occurs in both mice and humans through alternative mechanisms, unmasking CD22 relative to naive and memory B-cells.     

Streptococcus pneumoniae NanC: STRUCTURAL INSIGHTS INTO THE SPECIFICITY AND MECHANISM OF A SIALIDASE THAT PRODUCES A SIALIDASE INHIBITOR*

Streptococcus pneumoniae is an important human pathogen that causes a range of disease states. Sialidases are important bacterial virulence factors. There are three pneumococcal sialidases: NanA, NanB, and NanC. NanC is an unusual sialidase in that its primary reaction product is 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en, also known as DANA), a nonspecific hydrolytic sialidase inhibitor. The production of Neu5Ac2en from α2–3-linked sialosides by the catalytic domain is confirmed within a crystal structure. A covalent complex with 3-fluoro-β-N-acetylneuraminic acid is also presented, suggesting a common mechanism with other sialidases up to the final step of product formation. A conformation change in an active site hydrophobic loop on ligand binding constricts the entrance to the active site. In addition, the distance between the catalytic acid/base (Asp-315) and the ligand anomeric carbon is unusually short. These features facilitate a novel sialidase reaction in which the final step of product formation is direct abstraction of the C3 proton by the active site aspartic acid, forming Neu5Ac2en. NanC also possesses a carbohydrate-binding module, which is shown to bind α2–3- and α2–6-linked sialosides, as well as N-acetylneuraminic acid, which is captured in the crystal structure following hydration of Neu5Ac2en by NanC. Overall, the pneumococcal sialidases show remarkable mechanistic diversity while maintaining a common structural scaffold.     

Structural implications of Siglec-5-mediated sialoglycan recognition

Sialic acid (Sia) Ig-like binding lectins are important mediators of recognition and signaling events among myeloid cells. To investigate the molecular mechanism underlying sialic acid Ig-like lectin (Siglec) functions, we determined the crystal structure of the two N-terminal extracellular domains of human myeloid cell inhibitory receptor Siglec-5 (CD170) and its complexes with two sialylated carbohydrates. The native structure revealed an unusual conformation of the CC′ ligand specificity loop and a unique interdomain disulfide bond. The α(2,3)- and α(2,6)-sialyllactose complexed structures showed a conserved Sia recognition motif that involves both Arg124 and a portion of the G-strand in the V-set domain forming β-sheet-like hydrogen bonds with the glycerol side chain of the Sia. Only few protein contacts to the subterminal sugars are observed and mediated by the highly variable GG′ linker and CC′ loop. These structural observations, in conjunction with surface plasmon resonance binding assays, provide mechanistic insights into linkage-dependent Siglec carbohydrate recognition and suggest that Siglec-5 and other CD33-related Siglec receptors are more promiscuous in sialoglycan recognition than previously understood.     

Two doses of humanized anti-CD25 antibody in renal transplantation: A preliminary comparative study

HuCD25mAb is a humanized anti-CD25 antibody which has the same amino acid sequence as daclizumab (Zenapax, Roche). HuCD25mAb is expressed in Chinese hamster ovary (CHO) cells while daclizumab is expressed in the NSO myeloma cell line. A comparative study was performed to evaluate the pharmacokinetics and pharmacodynamics between huCD25mAb and daclizumab in a two-dose regimen incorporating triple immunosuppressant treatment regimens (MMF, CsA and steroids). Fifteen patients were enrolled and randomized to receive intravenous infusion of either huCD25mAb (n = 10) or daclizumab (n = 5) at a dosage of 1 mg·kg⁻¹ on operation day 0 and post-operation day 14. Serum concentrations of huCD25mAb and daclizumab were measured by a validated competitive ELISA. Subgroups of CD3⁺, CD25⁺, CD4⁺ and CD8⁺ lymphocytes were monitored periodically by flow cytometry. The concentration-time curves of huCD25mAb and daclizumab were found to fit well to a one-compartment model. A significant decline of proportion (%) of CD3-CD25⁺ and CD3⁺CD25⁺ lymphocytes was observed 30 min after first infusion on day 0 (3.40 ± 1.83 to 0.03 ± 0.07, 3.35 ± 2.02 to 0.37 ± 0.49), and these levels remained low for at least 70 days (0.03 ± 0.05, 0.31 ± 0.47). All pharmacokinetic parameters of huCD25mAb seemed similar to those of daclizumab. The two-dose huCD25mAb regimen was as effective as daclizumab in rapidly achieving high therapeutic concentration in the treated patients, and a significant decrease of CD3⁻CD25⁺ and CD3⁺CD25⁺ lymphocytes was demonstrated. This suggests that two-dose regimen is feasible in maintaining host immunosuppression and may provide an effective and economical strategy for reducing incidence of acute graft rejection.Key words: CD25, pharmacokinetics, kidney transplantation, enzyme immunoassay, flow cytometry, monoclonal antibody     

Sensitive and Specific Detection of the Non-Human Sialic Acid N-Glycolylneuraminic Acid In Human Tissues and Biotherapeutic Products

Background

Humans are genetically defective in synthesizing the common mammalian sialic acid N-glycolylneuraminic acid (Neu5Gc), but can metabolically incorporate it from dietary sources (particularly red meat and milk) into glycoproteins and glycolipids of human tumors, fetuses and some normal tissues. Metabolic incorporation of Neu5Gc from animal-derived cells and medium components also results in variable contamination of molecules and cells intended for human therapies. These Neu5Gc-incorporation phenomena are practically significant, because normal humans can have high levels of circulating anti-Neu5Gc antibodies. Thus, there is need for the sensitive and specific detection of Neu5Gc in human tissues and biotherapeutic products. Unlike monoclonal antibodies that recognize Neu5Gc only in the context of underlying structures, chicken immunoglobulin Y (IgY) polyclonal antibodies can recognize Neu5Gc in broader contexts. However, prior preparations of such antibodies (including our own) suffered from some non-specificity, as well as some cross-reactivity with the human sialic acid N-acetylneuraminic acid (Neu5Ac).

Methodology/Principal Findings

We have developed a novel affinity method utilizing sequential columns of immobilized human and chimpanzee serum sialoglycoproteins, followed by specific elution from the latter column by free Neu5Gc. The resulting mono-specific antibody shows no staining in tissues or cells from mice with a human-like defect in Neu5Gc production. It allows sensitive and specific detection of Neu5Gc in all underlying glycan structural contexts studied, and is applicable to immunohistochemical, enzyme-linked immunosorbent assay (ELISA), Western blot and flow cytometry analyses. Non-immune chicken IgY is used as a reliable negative control. We show that these approaches allow sensitive detection of Neu5Gc in human tissue samples and in some biotherapeutic products, and finally show an example of how Neu5Gc might be eliminated from such products, by using a human cell line grown under defined conditions.

Conclusions

We report a reliable antibody-based method for highly sensitive and specific detection of the non-human sialic acid Neu5Gc in human tissues and biotherapeutic products that has not been previously described.     

A uniquely human consequence of domain-specific functional adaptation in a sialic acid-binding receptor

Most mammalian cell surfaces display two major sialic acids (Sias), N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). Humans lack Neu5Gc due to a mutation in CMP-Neu5Ac hydroxylase, which occurred after evolutionary divergence from great apes. We describe an apparent consequence of human Neu5Gc loss: domain-specific functional adaptation of Siglec-9, a member of the family of sialic acid-binding receptors of innate immune cells designated the CD33-related Siglecs (CD33rSiglecs). Binding studies on recombinant human Siglec-9 show recognition of both Neu5Ac and Neu5Gc. In striking contrast, chimpanzee and gorilla Siglec-9 strongly prefer binding Neu5Gc. Simultaneous probing of multiple endogenous CD33rSiglecs on circulating blood cells of human, chimp, or gorilla suggests that the binding differences observed for Siglec-9 are representative of multiple CD33rSiglecs. We conclude that Neu5Ac-binding ability of at least some human CD33rSiglecs is a derived state selected for following loss of Neu5Gc in the hominid lineage. These data also indicate that endogenous Sias (rather than surface Sias of bacterial pathogens) are the functional ligands of CD33rSiglecs and suggest that the endogenous Sia landscape is the major factor directing evolution of CD33rSiglec binding specificity. Exon-1-encoded Sia-recognizing domains of human and ape Siglec-9 share only approximately 93-95% amino acid identity. In contrast, the immediately adjacent intron and exon 2 have the approximately 98-100% identity typically observed among these species. Together, our findings suggest ongoing adaptive evolution specific to the Sia-binding domain, possibly of an episodic nature. Such domain-specific divergences should also be considered in upcoming comparisons of human and chimpanzee genomes.     

Functional Evaluation of Activation-dependent Alterations in the Sialoglycan Composition of T Cells

Sialic acids (Sias) are often conjugated to the termini of cellular glycans and are key mediators of cellular recognition. Sias are nine-carbon acidic sugars, and, in vertebrates, the major species are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), differing in structure at the C5 position. Previously, we described a positive feedback loop involving regulation of Neu5Gc expression in mouse B cells. In this context, Neu5Gc negatively regulated B-cell proliferation, and Neu5Gc expression was suppressed upon activation. Similarly, resting mouse T cells expressed principally Neu5Gc, and Neu5Ac was induced upon activation. In the present work, we used various probes to examine sialoglycan expression by activated T cells in terms of the Sia species expressed and the linkages of Sias to glycans. Upon T-cell activation, sialoglycan expression shifted from Neu5Gc to Neu5Ac, and the linkage shifted from α2,6 to α2,3. These changes altered the expression levels of sialic acid-binding immunoglobulin-like lectin (siglec) ligands. Expression of sialoadhesin and Siglec-F ligands increased, and that of CD22 ligands decreased. Neu5Gc exerted a negative effect on T-cell activation, both in terms of the proliferative response and in the context of activation marker expression. Suppression of Neu5Gc expression in mouse T and B cells prevented the development of nonspecific CD22-mediated T cell-B cell interactions. Our results suggest that an activation-dependent shift from Neu5Gc to Neu5Ac and replacement of α2,6 by α2,3 linkages may regulate immune cell interactions at several levels.     

A Simple Method for Assessment of Human Anti-Neu5Gc Antibodies Applied to Kawasaki Disease

N-Glycolylneuraminic acid (Neu5Gc) is an immunogenic sugar of dietary origin that metabolically incorporates into diverse native glycoconjugates in humans. Anti-Neu5Gc antibodies are detected in all human sera, though with variable levels and epitope-recognition profiles. These antibodies likely play a role in several inflammation-mediated pathologies including cardiovascular diseases and cancer. In cancer, they have dualistic and opposing roles, either stimulating or repressing disease, as a function of their dose, and some of these antibodies serve as carcinoma biomarkers. Thus, anti-Neu5Gc antibodies may signify risk of inflammation-mediated diseases, and changes in their levels could potentially be used to monitor disease progression and/or response to therapy. Currently, it is difficult to determine levels of anti-Neu5Gc antibodies in individual human samples because these antibodies recognize multiple Neu5Gc-epitopes. Here we describe a simple and specific method for detection and overall estimation of human anti-Neu5Gc antibodies. We exploit the difference between two mouse models that differ only by Neu5Gc-presence (wild-type) or Neu5Gc-absence (Cmah^−/− knockout). We characterize mouse serum from both strains by HPLC, lectin and mass-spectrometry analysis and show the target Neu5Gc-epitopes. We then use Cmah^−/− knockout sera to inhibit all non-Neu5Gc-reactivity followed by binding to wild-type sera to detect overall anti-Neu5Gc response in a single assay. We applied this methodology to characterize and quantify anti-Neu5Gc IgG and IgA in sera of patients with Kawasaki disease (KD) at various stages compared to controls. KD is an acute childhood febrile disease characterized by inflammation of coronary arteries that untreated may lead to coronary artery aneurysms with risk of thrombosis and myocardial infarction. This estimated response is comparable to the average of detailed anti-Neu5Gc IgG profile analyzed by a sialoglycan microarray. Both assays revealed an elevated response in acute KD patients with normal coronaries compared to patients with aneurysm or dilated coronaries. Implications of these findings are discussed.     

Sialoglycan-binding patterns of bacterial AB5 toxin B subunits correlate with host range and toxicity,indicating evolution independent of A subunits

Many pathogenic bacteria secrete AB₅ toxins that can be virulence factors. Cytotoxic A subunits are delivered to the cytosol following B subunit binding to specific host cell surface glycans. Some B subunits are not associated with A subunits, for example, YpeB of Yersinia pestis, the etiologic agent of plague. Plague cannot be eradicated because of Y. pestis'' adaptability to numerous hosts. We previously showed selective binding of other B₅ pentamers to a sialoglycan microarray, with sialic acid (Sia) preferences corresponding to those prominently expressed by various hosts, for example, N-acetylneuraminic acid (Neu5Ac; prominent in humans) or N-glycolylneuraminic acid (Neu5Gc; prominent in ruminant mammals and rodents). Here, we report that A subunit phylogeny evolved independently of B subunits and suggest a future B subunit nomenclature based on bacterial species names. We also found via phylogenetic analysis of B subunits, which bind Sias, that homologous molecules show poor correlation with species phylogeny. These data indicate ongoing lateral gene transfers between species, including mixing of A and B subunits. Consistent with much broader host range of Y. pestis, we show that YpeB recognizes all mammalian Sia types, except for 4-O-acetylated ones. Notably, YpeB alone causes dose-dependent cytotoxicity, which is abolished by a mutation (Y77F) eliminating Sia recognition, suggesting that cell proliferation and death are promoted via lectin-like crosslinking of cell surface sialoglycoconjugates. These findings help explain the host range of Y. pestis and could be important for pathogenesis. Overall, our data indicate ongoing rapid evolution of both host Sias and pathogen toxin-binding properties.     

CD38 Structure-Based Inhibitor Design Using the N1-Cyclic Inosine 5′-Diphosphate Ribose Template

Few inhibitors exist for CD38, a multifunctional enzyme catalyzing the formation and metabolism of the Ca²⁺-mobilizing second messenger cyclic adenosine 5′-diphosphoribose (cADPR). Synthetic, non-hydrolyzable ligands can facilitate structure-based inhibitor design. Molecular docking was used to reproduce the crystallographic binding mode of cyclic inosine 5′-diphosphoribose (N1-cIDPR) with CD38, revealing an exploitable pocket and predicting the potential to introduce an extra hydrogen bond interaction with Asp-155. The purine C-8 position of N1-cIDPR (IC₅₀ 276 µM) was extended with an amino or diaminobutane group and the 8-modified compounds were evaluated against CD38-catalyzed cADPR hydrolysis. Crystallography of an 8-amino N1-cIDPR:CD38 complex confirmed the predicted interaction with Asp-155, together with a second H-bond from a realigned Glu-146, rationalizing the improved inhibition (IC₅₀ 56 µM). Crystallography of a complex of cyclic ADP-carbocyclic ribose (cADPcR, IC₅₀ 129 µM) with CD38 illustrated that Glu-146 hydrogen bonds with the ligand N6-amino group. Both 8-amino N1-cIDPR and cADPcR bind deep in the active site reaching the catalytic residue Glu-226, and mimicking the likely location of cADPR during catalysis. Substantial overlap of the N1-cIDPR “northern” ribose monophosphate and the cADPcR carbocyclic ribose monophosphate regions suggests that this area is crucial for inhibitor design, leading to a new compound series of N1-inosine 5′-monophosphates (N1-IMPs). These small fragments inhibit hydrolysis of cADPR more efficiently than the parent cyclic compounds, with the best in the series demonstrating potent inhibition (IC₅₀ = 7.6 µM). The lower molecular weight and relative simplicity of these compounds compared to cADPR make them attractive as a starting point for further inhibitor design.     

Homo floresiensis Contextualized: A Geometric Morphometric Comparative Analysis of Fossil and Pathological Human Samples

The origin of hominins found on the remote Indonesian island of Flores remains highly contentious. These specimens may represent a new hominin species, Homo floresiensis, descended from a local population of Homo erectus or from an earlier (pre-H. erectus) migration of a small-bodied and small-brained hominin out of Africa. Alternatively, some workers suggest that some or all of the specimens recovered from Liang Bua are pathological members of a small-bodied modern human population. Pathological conditions proposed to explain their documented anatomical features include microcephaly, myxoedematous endemic hypothyroidism (“cretinism”) and Laron syndrome (primary growth hormone insensitivity). This study evaluates evolutionary and pathological hypotheses through comparative analysis of cranial morphology. Geometric morphometric analyses of landmark data show that the sole Flores cranium (LB1) is clearly distinct from healthy modern humans and from those exhibiting hypothyroidism and Laron syndrome. Modern human microcephalic specimens converge, to some extent, on crania of extinct species of Homo. However in the features that distinguish these two groups, LB1 consistently groups with fossil hominins and is most similar to H. erectus. Our study provides further support for recognizing the Flores hominins as a distinct species, H. floresiensis, whose affinities lie with archaic Homo.     

N-glycolylneuraminic acid as a carbohydrate cancer biomarker

One of the forms of aberrant glycosylation in human tumors is the expression of N-glycolylneuraminic acid (Neu5Gc). The only known enzyme to biosynthesize Neu5Gc in mammals, cytidine-5′-monophosphate-N-acetylneuraminic acid (CMAH), appears to be genetically inactivated in humans. Regardless, low levels of Neu5Gc have been detected in healthy humans. Therefore, it is proposed that the presence of Neu5Gc in humans is from dietary acquisition, such as red meat. Notably, detection of elevated Neu5Gc levels has been repeatedly found in cancer tissues, cells and serum samples, thereby Neu5Gc-containing antigens may be exploited as a class of cancer biomarkers. Here we review the findings to date on using Neu5Gc-containing tumor glycoconjugates as a class of cancer biomarkers for cancer detection, surveillance, prognosis and therapeutic targets. We review the evidence that supports an emerging hypothesis of de novo Neu5Gc biosynthesis in human cancer cells as a source of Neu5Gc in human tumors, generated under certain metabolic conditions.     

Loss of N-glycolylneuraminic acid in human evolution. Implications for sialic acid recognition by siglecs

The common sialic acids of mammalian cells are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). Humans are an exception, because of a mutation in CMP-sialic acid hydroxylase, which occurred after our common ancestor with great apes. We asked if the resulting loss of Neu5Gc and increase in Neu5Ac in humans alters the biology of the siglecs, which are Ig superfamily members that recognize sialic acids. Human siglec-1 (sialoadhesin) strongly prefers Neu5Ac over Neu5Gc. Thus, humans have a higher density of siglec-1 ligands than great apes. Siglec-1-positive macrophages in humans are found primarily in the perifollicular zone, whereas in chimpanzees they also occur in the marginal zone and surrounding the periarteriolar lymphocyte sheaths. Although only a subset of chimpanzee macrophages express siglec-1, most human macrophages are positive. A known evolutionary difference is the strong preference of mouse siglec-2 (CD22) for Neu5Gc, contrasting with human siglec-2, which binds Neu5Ac equally well. To ask when the preference for Neu5Gc was adjusted in the human lineage, we cloned the first three extracellular domains of siglec-2 from all of the great apes and examined their preference. In fact, siglec-2 had evolved a higher degree of recognition flexibility before Neu5Gc was lost in humans. Human siglec-3 (CD33) and siglec-6 (obesity-binding protein 1) also recognize both Neu5Ac and Neu5Gc, and siglec-5 may have some preference for Neu5Gc. Others showed that siglec-4a (myelin-associated glycoprotein) prefers Neu5Ac over Neu5Gc. Thus, the human loss of Neu5Gc may alter biological processes involving siglec-1, and possibly, siglec-4a or -5.     

Structural Basis for Substrate Specificity of Mammalian Neuraminidases

The removal of sialic acid (Sia) residues from glycoconjugates in vertebrates is mediated by a family of neuraminidases (sialidases) consisting of Neu1, Neu2, Neu3 and Neu4 enzymes. The enzymes play distinct physiological roles, but their ability to discriminate between the types of linkages connecting Sia and adjacent residues and between the identity and arrangement of the underlying sugars has never been systematically studied. Here we analyzed the specificity of neuraminidases by studying the kinetics of hydrolysis of BODIPY-labeled substrates containing common mammalian sialylated oligosaccharides: 3′Sia-LacNAc, 3′SiaLac, SiaLe_x, SiaLe_a, SiaLe_c, 6′SiaLac, and 6′SiaLacNAc. We found significant differences in substrate specificity of the enzymes towards the substrates containing α2,6-linked Sia, which were readily cleaved by Neu3 and Neu1 but not by Neu4 and Neu2. The presence of a branching 2-Fuc inhibited Neu2 and Neu4, but had almost no effect on Neu1 or Neu3. The nature of the sugar residue at the reducing end, either glucose (Glc) or N-acetyl-D-glucosamine (GlcNAc) had only a minor effect on all neuraminidases, whereas core structure (1,3 or 1,4 bond between D-galactose (Gal) and GlcNAc) was found to be important for Neu4 strongly preferring β3 (core 1) to β4 (core 2) isomer. Neu3 and Neu4 were in general more active than Neu1 and Neu2, likely due to their preference for hydrophobic substrates. Neu2 and Neu3 were examined by molecular dynamics to identify favorable substrate orientations in the binding sites and interpret the differences in their specificities. Finally, using knockout mouse models, we confirmed that the substrate specificities observed in vitro were recapitulated in enzymes found in mouse brain tissues. Our data for the first time provide evidence for the characteristic substrate preferences of neuraminidases and their ability to discriminate between distinct sialoside targets.     

Solution structure and activation mechanism of ubiquitin-like small archaeal modifier proteins

In archaea, two ubiquitin-like small archaeal modifier protein (SAMPs) were recently shown to be conjugated to proteins in vivo. SAMPs display homology to bacterial MoaD sulfur transfer proteins and eukaryotic ubiquitin-like proteins, and they share with them the conserved C-terminal glycine-glycine motif. Here, we report the solution structure of SAMP1 from Methanosarcina acetivorans and the activation of SAMPs by an archaeal protein with homology to eukaryotic E1 enzymes. Our results show that SAMP1 possesses a β-grasp fold and that its hydrophobic and electrostatic surface features are similar to those of MoaD. M. acetivorans SAMP1 exhibits an extensive flexible surface loop between helix-2 and the third strand of the β-sheet, which contributes to an elongated surface groove that is not observed in bacterial ubiquitin homologues and many other SAMPs. We provide in vitro biochemical evidence that SAMPs are activated in an ATP-dependent manner by an E1-like enzyme that we have termed E1-like SAMP activator (ELSA). We show that activation occurs by formation of a mixed anhydride (adenylate) at the SAMP C-terminus and is detectable by SDS-PAGE and electrospray ionization mass spectrometry.     

Porcine CD58: cDNA cloning and molecular dissection of the porcine CD58-human CD2 interface

The porcine ligands of human CD2 remain unknown in xenotransplantation despite being an important pathway of T cell costimulation. Of the two main candidates, i.e., CD48 and CD58, the cDNA of the most likely ligand poCD58 was cloned from CD48-negative endothelial cells costimulating human CD4(+) T cells through the CD2 pathway. The deduced protein sequence is 244 residues long and is 43% homologous to the human sequence. Based on similarity between porcine and human CD58 external V-set Ig-type domains, a structural model of poCD58-huCD2 interaction was built. Most of the charged residues located at the interface with huCD2 are highly conserved. Six putative hydrogen bonds between poCD58 and huCD2 were identified; five involve the same residues as in the syngeneic combination while the sixth is formed between an additional tyrosine in poCD58 and Arg48 in huCD2, increasing the complementarity between the two molecules. These structural data will help us to develop poCD58 blocking agents for xenotransplantation.     

Peptide Bond Synthesis by a Mechanism Involving an Enzymatic Reaction and a Subsequent Chemical Reaction

We recently reported that an amide bond is unexpectedly formed by an acyl-CoA synthetase (which catalyzes the formation of a carbon-sulfur bond) when a suitable acid and l-cysteine are used as substrates. DltA, which is homologous to the adenylation domain of nonribosomal peptide synthetase, belongs to the same superfamily of adenylate-forming enzymes, which includes many kinds of enzymes, including the acyl-CoA synthetases. Here, we demonstrate that DltA synthesizes not only N-(d-alanyl)-l-cysteine (a dipeptide) but also various oligopeptides. We propose that this enzyme catalyzes peptide synthesis by the following unprecedented mechanism: (i) the formation of S-acyl-l-cysteine as an intermediate via its “enzymatic activity” and (ii) subsequent “chemical” S → N acyl transfer in the intermediate, resulting in peptide formation. Step ii is identical to the corresponding reaction in native chemical ligation, a method of chemical peptide synthesis, whereas step i is not. To the best of our knowledge, our discovery of this peptide synthesis mechanism involving an enzymatic reaction and a subsequent chemical reaction is the first such one to be reported. This new process yields peptides without the use of a thioesterified fragment, which is required in native chemical ligation. Together with these findings, the same mechanism-dependent formation of N-acyl compounds by other members of the above-mentioned superfamily demonstrated that all members most likely form peptide/amide compounds by using this novel mechanism. Each member enzyme acts on a specific substrate; thus, not only the corresponding peptides but also new types of amide compounds can be formed.     

Siglec-7: a sialic acid-binding lectin of the immunoglobulin superfamily

The Siglecs are a recently discovered family of sialic acid-binding lectins of the immunoglobulin (Ig) superfamily. We report a molecule showing homology to the six first reported Siglecs, with the closest relationship to Siglec-3(CD33), Siglec-5, and Siglec-6(OBBP-1). The extracellular portion has two Ig-like domains, with the amino-terminal V-set Ig domain including amino acid residues known to be involved in sialic acid recognition by other Siglecs. The cytoplasmic domain has putative sites of tyrosine phosphorylation shared with some Siglecs, including an Immuno-receptor Tyrosine-based Inhibitory Motif (ITIM). Expression of the full-length cDNA induces sialic acid-dependent binding to human erythrocytes. A recombinant chimeric form containing the extracellular Ig domains selectively recognizes the sequence Neu5Acalpha2-6Galbeta1-4Glc, and binding requires the side chain of sialic acid. Mutation of an arginine residue predicted to be critical for sialic acid binding abolishes both interactions. Taken together, our findings justify designation of the molecule as Siglec-7. Analysis of bacterial artificial chromosome (BAC) clones spanning the known human genomic location of Siglec-3 indicates that the Siglec-7 gene is also located on chromosome 19q13.3-13.4. Human tissues show strong expression of Siglec-7 mRNA in spleen, peripheral blood leukocytes, and liver. The combination of an extracellular sialic acid binding site and an intracellular ITIM motif suggests that this molecule is involved in trans-membrane regulatory signaling reactions.