Longitudinal Study after Sputnik V Vaccination Shows Durable SARS-CoV-2 Neutralizing Antibodies and Reduced Viral Variant Escape to Neutralization over Time.

Recent studies have shown a temporal increase in the neutralizing antibody potency and breadth to SARS-CoV-2 variants in coronavirus disease 2019 (COVID-19) convalescent individuals. Here, we examined longitudinal antibody responses and viral neutralizing capacity to the B.1 lineage virus (Wuhan related), to variants of concern (VOC; Alpha, Beta, Gamma, and Delta), and to a local variant of interest (VOI; Lambda) in volunteers receiving the Sputnik V vaccine in Argentina. Longitudinal serum samples (N = 536) collected from 118 volunteers obtained between January and October 2021 were used. The analysis indicates that while anti-spike IgG levels significantly wane over time, the neutralizing capacity for the Wuhan-related lineages of SARS-CoV-2 and VOC is maintained within 6 months of vaccination. In addition, an improved antibody cross-neutralizing ability for circulating variants of concern (Beta and Gamma) was observed over time postvaccination. The viral variants that displayed higher escape to neutralizing antibodies with respect to the original virus (Beta and Gamma variants) were the ones showing the largest increase in susceptibility to neutralization over time after vaccination. Our observations indicate that serum neutralizing antibodies are maintained for at least 6 months and show a reduction of VOC escape to neutralizing antibodies over time after vaccination. IMPORTANCE Vaccines have been produced in record time for SARS-CoV-2, offering the possibility of halting the global pandemic. However, inequalities in vaccine accessibility in different regions of the world create a need to increase international cooperation. Sputnik V is a recombinant adenovirus-based vaccine that has been widely used in Argentina and other developing countries, but limited information is available about its elicited immune responses. Here, we examined longitudinal antibody levels and viral neutralizing capacity elicited by Sputnik V vaccination. Using a cohort of 118 volunteers, we found that while anti-spike antibodies wane over time, the neutralizing capacity to viral variants of concern and local variants of interest is maintained within 4 months of vaccination. In addition, we observed an increased cross-neutralization activity over time for the Beta and Gamma variants. This study provides valuable information about the immune response generated by a vaccine platform used in many parts of the world.

Fusion of a bacterial cadherin-like domain and green fluorescent protein as a specific probe to study biofilm matrix formation in Rhizobium spp.

Rhizobium adhering proteins or 'Raps' are secreted proteins identified in a very restricted group of rhizobial strains, specifically those belonging to R. leguminosarum and R. etli. The distinctive feature of members of the Rap family is the presence of one or two cadherin-like domains or CHDLs that are also present in numerous extracellular bacterial and archaeal proteins and were proposed to confer carbohydrate binding ability. We have previously made an in-depth characterization of RapA2, a calcium-binding lectin, composed by two CHDLs, involved in biofilm matrix remodelling in R. leguminosarum bv. viciae 3841. In this study, CHDLs derived from RapA2 were analysed in detail, finding significant structural and functional differences despite their considerable sequence similarity. Only the carboxy-terminal CHDL retained properties similar to those displayed by RapA2. Our findings were used to obtain a novel fluorescent probe to study biofilm matrix development by confocal laser scanning microscopy, and also to shed some light on the role of the ubiquitous CHDL domains in bacterial secreted proteins.

Sputnik V vaccine elicits seroconversion and neutralizing capacity to SARS-CoV-2 after a single dose.

Massive vaccination offers great promise for halting the global COVID-19 pandemic. However, the limited supply and uneven vaccine distribution create an urgent need to optimize vaccination strategies. We evaluate SARS-CoV-2-specific antibody responses after Sputnik V vaccination of healthcare workers in Argentina, measuring IgG anti-spike titers and neutralizing capacity after one and two doses in a cohort of naive or previously infected volunteers. By 21 days after receiving the first dose of the vaccine, 94% of naive participants develop spike-specific IgG antibodies. A single Sputnik V dose elicits higher antibody levels and virus-neutralizing capacity in previously infected individuals than in naive ones receiving the full two-dose schedule. The high seroconversion rate after a single dose in naive participants suggests a benefit of delaying administration of the second dose to increase the number of people vaccinated. The data presented provide information for guiding public health decisions in light of the current global health emergency.

TorsinA folding and N-linked glycosylation are sensitive to redox homeostasis.

The Endoplasmic Reticulum (ER) is responsible for the folding and post-translational modification of secretory proteins, as well as for triaging misfolded proteins. During folding, there is a complex yet only partially understood interplay between disulfide bond formation, which is an enzyme catalyzed event in the oxidizing environment of the ER, along with other post-translational modifications (PTMs) and chaperone-supported protein folding. Here, we used the glycoprotein torsinA as a model substrate to explore the impact of ER redox homeostasis on PTMs and protein biogenesis. TorsinA is a AAA+ ATPase with unusual oligomeric properties and controversial functions. The deletion of a C-terminal glutamic acid residue (∆E) is associated with the development of Early-Onset Torsion Dystonia, a severe movement disorder. TorsinA differs from other AAA+ ATPases since it is an ER resident, and as a result of its entry into the ER torsinA contains two N-linked glycans and at least one disulfide bond. The role of these PTMs on torsinA biogenesis and function and the identity of the enzymes that catalyze them are poorly defined. Using a yeast torsinA expression system, we demonstrate that a specific protein disulfide isomerase, Pdi1, affects the folding and N-linked glycosylation of torsinA and torsinA∆E in a redox-dependent manner, suggesting that the acquisition of early torsinA folding intermediates is sensitive to perturbed interactions between Cys residues and the quality control machinery. We also highlight the role of specific Cys residues during torsinA biogenesis and demonstrate that torsinA∆E is more sensitive than torsinA when these Cys residues are mutated.

Emergency response for evaluating SARS-CoV-2 immune status, seroprevalence and convalescent plasma in Argentina.

We report the emergency development and application of a robust serologic test to evaluate acute and convalescent antibody responses to SARS-CoV-2 in Argentina. The assays, COVIDAR IgG and IgM, which were produced and provided for free to health authorities, private and public health institutions and nursing homes, use a combination of a trimer stabilized spike protein and the receptor binding domain (RBD) in a single enzyme-linked immunosorbent assay (ELISA) plate. Over half million tests have already been distributed to detect and quantify antibodies for multiple purposes, including assessment of immune responses in hospitalized patients and large seroprevalence studies in neighborhoods, slums and health care workers, which resulted in a powerful tool for asymptomatic detection and policy making in the country. Analysis of antibody levels and longitudinal studies of symptomatic and asymptomatic SARS-CoV-2 infections in over one thousand patient samples provided insightful information about IgM and IgG seroconversion time and kinetics, and IgM waning profiles. At least 35% of patients showed seroconversion within 7 days, and 95% within 45 days of symptoms onset, with simultaneous or close sequential IgM and IgG detection. Longitudinal studies of asymptomatic cases showed a wide range of antibody responses with median levels below those observed in symptomatic patients. Regarding convalescent plasma applications, a protocol was standardized for the assessment of end point IgG antibody titers with COVIDAR with more than 500 plasma donors. The protocol showed a positive correlation with neutralizing antibody titers, and was used for clinical trials and therapies across the country. Using this protocol, about 80% of convalescent donor plasmas were potentially suitable for therapies. Here, we demonstrate the importance of providing a robust and specific serologic assay for generating new information about antibody kinetics in infected individuals and mitigation policies to cope with pandemic needs.

Conformational plasticity of the intrinsically disordered protein ASR1 modulates its function as a drought stress-responsive gene.

Plants in arid zones are constantly exposed to drought stress. The ASR protein family (Abscisic, Stress, Ripening) -a subgroup of the late embryogenesis abundant superfamily- is involved in the water stress response and adaptation to dry environments. Tomato ASR1, as well as other members of this family, is an intrinsically disordered protein (IDP) that functions as a transcription factor and a chaperone. Here we employed different biophysical techniques to perform a deep in vitro characterization of ASR1 as an IDP and showed how both environmental factors and in vivo targets modulate its folding. We report that ASR1 adopts different conformations such as α-helix or polyproline type II in response to environmental changes. Low temperatures and low pH promote the polyproline type II conformation (PII). While NaCl increases PII content and slightly destabilizes α-helix conformation, PEG and glycerol have an important stabilizing effect of α-helix conformation. The binding of Zn2+in the low micromolar range promotes α-helix folding, while extra Zn2+ results in homo-dimerization. The ASR1-DNA binding is sequence specific and dependent on Zn2+. ASR1 chaperone activity does not change upon the structure induction triggered by the addition of Zn2+. Furthermore, trehalose, which has no effect on the ASR1 structure by itself, showed a synergistic effect on the ASR1-driven heat shock protection towards the reporter enzyme citrate synthase (CS). These observations prompted the development of a FRET reporter to sense ASR1 folding in vivo. Its performance was confirmed in Escherichia coli under saline and osmotic stress conditions, representing a promising probe to be used in plant cells. Overall, this work supports the notion that ASR1 plasticity is a key feature that facilitates its response to drought stress and its interaction with specific targets.

N-glycosylation Triggers a Dual Selection Pressure in Eukaryotic Secretory Proteins.

Nearly one third of the eukaryotic proteome traverses the secretory pathway and most of these proteins are N-glycosylated in the lumen of the endoplasmic reticulum. N-glycans fulfill multiple structural and biological functions, and are crucial for productive folding of many glycoproteins. N-glycosylation involves the attachment of an oligosaccharide to selected asparagine residues in the sequence N-X-S/T (X ≠ P), a motif known as an N-glycosylation'sequon'. Mutations that create novel sequons can cause disease due to the destabilizing effect of a bulky N-glycan. Thus, an analogous process must have occurred during evolution, whenever ancestrally cytosolic proteins were recruited to the secretory pathway. Here, we show that during evolution N-glycosylation triggered a dual selection pressure on secretory pathway proteins: while sequons were positively selected in solvent exposed regions, they were almost completely eliminated from buried sites. This process is one of the sharpest evolutionary signatures of secretory pathway proteins, and was therefore critical for the evolution of an efficient secretory pathway.

Structural coalescence underlies the aggregation propensity of a ß-barrel protein motif.

A clear understanding of the structural foundations underlying protein aggregation is an elusive goal of central biomedical importance. A step toward this aim is exemplified by the ß-barrel motif represented by the intestinal fatty acid binding protein (IFABP) and two abridged all-ß sheet forms (Δ98Δ and Δ78Δ). At odds with the established notion that a perturbation of the native fold should necessarily favor a buildup of intermediate forms with an enhanced tendency to aggregate, the intrinsic stability (ΔG°H2O) of these proteins does not bear a straightforward correlation with their trifluoroethanol (TFE)-induced aggregation propensity. In view of this fact, we found it more insightful to delve into the connection between structure and stability under sub-aggregating conditions (10% TFE). In the absence of the co-solvent, the abridged variants display a common native-like region decorated with a disordered C-terminal stretch. Upon TFE addition, an increase in secondary structure content is observed, assimilating them to the parent protein. In this sense, TFE perturbs a common native like region while exerting a global compaction effect. Importantly, in all cases, fatty acid binding function is preserved. Interestingly, energetic as well as structural diversity in aqueous solution evolves into a common conformational ensemble more akin in stability. These facts reconcile apparent paradoxical findings related to stability and rates of aggregation. This scenario likely mimics the accrual of aggregation-prone species in the population, an early critical event for the development of fibrillation.

Alternative Splicing of G9a Regulates Neuronal Differentiation.

Chromatin modifications are critical for the establishment and maintenance of differentiation programs. G9a, the enzyme responsible for histone H3 lysine 9 dimethylation in mammalian euchromatin, exists as two isoforms with differential inclusion of exon 10 (E10) through alternative splicing. We find that the G9a methyltransferase is required for differentiation of the mouse neuronal cell line N2a and that E10 inclusion increases during neuronal differentiation of cultured cells, as well as in the developing mouse brain. Although E10 inclusion greatly stimulates overall H3K9me2 levels, it does not affect G9a catalytic activity. Instead, E10 increases G9a nuclear localization. We show that the G9a E10(+) isoform is necessary for neuron differentiation and regulates the alternative splicing pattern of its own pre-mRNA, enhancing E10 inclusion. Overall, our findings indicate that by regulating its own alternative splicing, G9a promotes neuron differentiation and creates a positive feedback loop that reinforces cellular commitment to differentiation.

The PKA regulatory subunit from yeast forms a homotetramer: Low-resolution structure of the N-terminal oligomerization domain.

The cAMP dependent protein kinase (PKA) is a key enzyme involved in many cellular processes in eukaryotes. In mammals, the regulatory (R) subunit localises the catalytic (C) subunit to specific subcellular sites through the interaction of its N-terminal homodimeric docking and dimerization (D/D) domain with specific scaffold proteins. The structure of the D/D domain has been extensively studied in mammals, but there is little information from non-mammalian species. In this work, we present the structural analysis of the D/D domain of Bcy1, the R subunit of PKA from Saccharomyces cerevisiae. Using chemical crosslinking experiments and static light scattering measurements we found that this R subunit forms a tetramer in solution, unlike its dimeric mammalian counterparts. We determined that the D/D domain is responsible for this unusual oligomeric state. Using biophysical techniques including size-exclusion chromatography, sucrose gradient sedimentation, small angle X-ray scattering (SAXS), and circular dichroism, we performed a detailed structural characterization of the tetrameric D/D domain of Bcy1. We used homology modelling in combination with computer-aided docking methods and ab initio SAXS modelling methods to develop structural models for the D/D domain tetramer. The models consist of two homodimers with a canonical D/D domain fold that generate a dimer of dimers with novel putative interaction surfaces. These findings indicate that the oligomerization states of PKA R subunits is more diverse than previously thought, and suggest that this might allow some forms of PKA to interact with a wide range of intracellular partners.

A sweet code for glycoprotein folding.

Glycoprotein synthesis is initiated in the endoplasmic reticulum (ER) lumen upon transfer of a glycan (Glc3Man9GlcNAc2) from a lipid derivative to Asn residues (N-glycosylation). N-Glycan-dependent quality control of glycoprotein folding in the ER prevents exit to Golgi of folding intermediates, irreparably misfolded glycoproteins and incompletely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones (calnexin and calreticulin) that recognize monoglucosylated polymannose protein-linked glycans, lectin-associated oxidoreductase acting on monoglucosylated glycoproteins (ERp57), a glucosyltransferase that creates monoglucosylated epitopes in protein-linked glycans (UGGT) and a glucosidase (GII) that removes the glucose units added by UGGT. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded glycoproteins or in not completely assembled multimeric glycoprotein complexes. Glycoproteins that fail to properly fold are eventually driven to proteasomal degradation in the cytosol following the ER-associated degradation pathway, in which the extent of N-glycan demannosylation by ER mannosidases play a relevant role in the identification of irreparably misfolded glycoproteins.

Protein fibrillation lag times during kinetic inhibition.

Protein aggregation is linked to more than 30 human pathologies, including Alzheimer's and Parkinson's diseases. Since small oligomers that form at the beginning of the fibrillation process probably are the most toxic elements, therapeutic strategies involving fibril fragmentation could be detrimental. An alternative approach, named kinetic inhibition, aims to prevent fibril formation by using small ligands that stabilize the parent protein. The factors that govern fibrillation lag times during kinetic inhibition are largely unknown, notwithstanding their importance for designing effective long-term therapies. Inhibitor-bound species are not likely to be incorporated into the core of mature fibrils, although their presence could alter the kinetics of the fibrillation process. For instance, inhibitor-bound species may act as capping elements that impair the nucleation process and/or fibril growth. Here, we address this issue by studying the effect of two natural inhibitors on the fibrillation behavior of lysozyme at neutral pH. We analyzed a set of 79 fibrillation curves obtained in lysozyme alone and a set of 37 obtained in the presence of inhibitors. We calculated the concentrations of the relevant species at the beginning of the curves using the inhibitor-binding constants measured under the same experimental conditions. We found that inhibitor-bound protein species do not affect fibrillation onset times, which are mainly determined by the concentration of unbound protein species present in equilibrium. In this system, knowledge of the fibrillation kinetics and inhibitor affinities suffices to predict the effect of kinetic inhibitors on fibrillation lag times. In addition, we developed a new methodology to better estimate fibrillation lag times from experimental curves.

Toward a common aggregation mechanism for a ß-barrel protein family: insights derived from a stable dimeric species.

Δ78Δ is a second generation functional all-ß sheet variant of IFABP (intestinal fatty acid binding protein) corresponding to the fragment 29-106 of the parent protein. This protein and its predecessor, Δ98Δ (segment 29-126 of IFABP), were initially uncovered by controlled proteolysis. Remarkably, although IFABP and Δ98Δ are monomers in solution, Δ78Δ adopts a stable dimeric structure. With the aim of identifying key structural features that modulate the aggregation of ß-proteins, we evaluate here the structure and aggregation propensity of Δ78Δ. The 2,2,2-trifluoroethanol (TFE) induced aggregation of this protein shows a primary nucleation-elongation mechanism, characterized by the stabilization of a dimeric nucleus. Its rate of production from the co-solvent induced aggregation prone state governs the kinetics of polymerization. In this context, the value of Δ78Δ lies in the fact that - being a stable dimeric species - it reduces an otherwise bimolecular reaction to a unimolecular one. Interestingly, even though Δ78Δ and IFABP display similar conformational stability, the abrogated form of IFABP shows an enhanced aggregation rate, revealing the ancillary role played on this process by the free energy of the native proteins. Δ78Δ share with IFABP and Δ98Δ a common putative aggregation-prone central peptide. Differences in the exposure/accessibility of this segment dictated by the environment around this region might underlie the observed variations in the speed of aggregation. Lessons learnt from this natural dimeric protein might shed light on the early conformational events leading to ß-conversion from barrels to amyloid aggregates.

Glycosylation-dependent lectin-receptor interactions preserve angiogenesis in anti-VEGF refractory tumors.

The clinical benefit conferred by vascular endothelial growth factors (VEGF)-targeted therapies is variable, and tumors from treated patients eventually reinitiate growth. Here, we identify a glycosylation-dependent pathway that compensates for the absence of cognate ligand and preserves angiogenesis in response to VEGF blockade. Remodeling of the endothelial cell (EC) surface glycome selectively regulated binding of galectin-1 (Gal1), which upon recognition of complex N-glycans on VEGFR2, activated VEGF-like signaling. Vessels within anti-VEGF-sensitive tumors exhibited high levels of α2-6-linked sialic acid, which prevented Gal1 binding. In contrast, anti-VEGF refractory tumors secreted increased Gal1 and their associated vasculature displayed glycosylation patterns that facilitated Gal1-EC interactions. Interruption of ß1-6GlcNAc branching in ECs or silencing of tumor-derived Gal1 converted refractory into anti-VEGF-sensitive tumors, whereas elimination of α2-6-linked sialic acid conferred resistance to anti-VEGF. Disruption of the Gal1-N-glycan axis promoted vascular remodeling, immune cell influx and tumor growth inhibition. Thus, targeting glycosylation-dependent lectin-receptor interactions may increase the efficacy of anti-VEGF treatment.

Structural basis of redox-dependent modulation of galectin-1 dynamics and function.

Galectin-1 (Gal-1), a member of a family of multifunctional lectins, plays key roles in diverse biological processes including cell signaling, immunomodulation, neuroprotection and angiogenesis. The presence of an unusual number of six cysteine residues within Gal-1 sequence prompted a detailed analysis of the impact of the redox environment on the functional activity of this lectin. We examined the role of each cysteine residue in the structure and function of Gal-1 using both experimental and computational approaches. Our results show that: (i) only three cysteine residues present in each carbohydrate recognition domain (CRD) (Cys2, Cys16 and Cys88) were important in protein oxidation, (ii) oxidation promoted the formation of the Cys16-Cys88 disulfide bond, as well as multimers through Cys2, (iii) the oxidized protein did not bind to lactose, probably due to poor interactions with Arg48 and Glu71, (iv) in vitro oxidation by air was completely reversible and (v) oxidation by hydrogen peroxide was relatively slow (1.7 ± 0.2 M(-1) s(-1) at pH 7.4 and 25°C). Finally, an analysis of key cysteines in other human galectins is also provided in order to predict their behaviour in response to redox variations. Collectively, our data provide new insights into the structural basis of Gal-1 redox regulation with critical implications in physiology and pathology.

Progesterone regulates the expression and activity of two mouse isoforms of the glycoprotein folding sensor UDP-Glc: glycoprotein glucosyltransferase (UGGT).

UDP-Glucose:glycoprotein glucosyltransferase (UGGT) is a central component of the endoplasmic reticulum (ER) glycoprotein-folding quality control system, which prevents the exit of partially folded species. UGGT activity can be regulated by the accumulation of misfolded proteins in the ER, a stimulus that triggers a complex signaling pathway known as unfolded protein response (UPR) which is closely associated with inflammation and disease. In this work, we investigated the effect of progesterone (P4) on the expression and activity of UGGT in a mouse hybridoma. We detected the expression of two UGGT isoforms, UGGT1 and UGGT2, and demonstrated that both isoforms are active in these cells. Interestingly, the expression of each isoform is regulated by high physiological P4 concentrations. This work provides the first evidence of a hormonal regulation of UGGT isoform expression and activity, which might influence the glycoprotein quality control mechanism. These findings could contribute to the study of pathologies triggered by the accumulation of misfolded proteins.

Identification and characterization of a high-affinity choline uptake system of Brucella abortus.

Phosphatidylcholine (PC), a common phospholipid of the eukaryotic cell membrane, is present in the cell envelope of the intracellular pathogen Brucella abortus, the etiological agent of bovine brucellosis. In this pathogen, the biosynthesis of PC proceeds mainly through the phosphatidylcholine synthase pathway; hence, it relies on the presence of choline in the milieu. These observations imply that B. abortus encodes an as-yet-unknown choline uptake system. Taking advantage of the requirement of choline uptake for PC synthesis, we devised a method that allowed us to identify a homologue of ChoX, the high-affinity periplasmic binding protein of the ABC transporter ChoXWV. Disruption of the choX gene completely abrogated PC synthesis at low choline concentrations in the medium, thus indicating that it is a high-affinity transporter needed for PC synthesis via the PC synthase (PCS) pathway. However, the synthesis of PC was restored when the mutant was incubated in media with higher choline concentrations, suggesting the presence of an alternative low-affinity choline uptake activity. By means of a fluorescence-based equilibrium-binding assay and using the kinetics of radiolabeled choline uptake, we show that ChoX binds choline with an extremely high affinity, and we also demonstrate that its activity is inhibited by increasing choline concentrations. Cell infection assays indicate that ChoX activity is required during the first phase of B. abortus intracellular traffic, suggesting that choline concentrations in the early and intermediate Brucella-containing vacuoles are limited. Altogether, these results suggest that choline transport and PC synthesis are strictly regulated in B. abortus.

Anticoagulant activity of a unique sulfated pyranosic (1->3)-ß-L-arabinan through direct interaction with thrombin.

A highly sulfated 3-linked ß-arabinan (Ab1) with arabinose in the pyranose form was obtained from green seaweed Codium vermilara (Bryopsidales). It comprised major amounts of units sulfated on C-2 and C-4 and constitutes the first polysaccharide of this type isolated in the pure form and fully characterized. Ab1 showed anticoagulant activity by global coagulation tests. Less sulfated arabinans obtained from the same seaweed have less or no activity. Ab1 exerts its activity through direct and indirect (antithrombin- and heparin cofactor II-mediated) inhibition of thrombin. Direct thrombin inhibition was studied in detail. By native PAGE, it was possible to detect formation of a complex between Ab1 and human thrombin (HT). Ab1 binding to HT was measured by fluorescence spectroscopy. CD spectra of the Ab1 complex suggested that ligand binding induced a small conformational change on HT. Ab1-thrombin interactions were studied by molecular dynamic simulations using the persulfated octasaccharide as model compound. Most carbohydrate-protein contacts would occur by interaction of sulfate groups with basic amino acid residues on the surface of the enzyme, more than 60% of them being performed by the exosite 2-composing residues. In these interactions, the sulfate groups on C-2 were shown to interact more intensely with the thrombin structure. In contrast, the disulfated oligosaccharide does not promote major conformational modifications at the catalytic site when complexed to exosite 1. These results show that this novel pyranosic sulfated arabinan Ab1 exerts its anticoagulant activity by a mechanism different from those found previously for other sulfated polysaccharides and glycosaminoglycans.

RapA2 is a calcium-binding lectin composed of two highly conserved cadherin-like domains that specifically recognize Rhizobium leguminosarum acidic exopolysaccharides.

In silico analyses have revealed a conserved protein domain (CHDL) widely present in bacteria that has significant structural similarity to eukaryotic cadherins. A CHDL domain was shown to be present in RapA, a protein that is involved in autoaggregation of Rhizobium cells, biofilm formation, and adhesion to plant roots as shown by us and others. Structural similarity to cadherins suggested calcium-dependent oligomerization of CHDL domains as a mechanistic basis for RapA action. Here we show by circular dichroism spectroscopy, light scattering, isothermal titration calorimetry, and other methods that RapA2 from Rhizobium leguminosarum indeed exhibits a cadherin-like ß-sheet conformation and that its proper folding and stability are dependent on the binding of one calcium ion per protein molecule. By further in silico analysis we also reveal that RapA2 consists of two CHDL domains and expand the range of CHDL-containing proteins in bacteria and archaea. However, light scattering assays at various concentrations of added calcium revealed that RapA2 formed neither homo-oligomers nor hetero-oligomers with RapB (a distinct CHDL protein), indicating that RapA2 does not mediate cellular interactions through a cadherin-like mechanism. Instead, we demonstrate that RapA2 interacts specifically with the acidic exopolysaccharides (EPSs) produced by R. leguminosarum in a calcium-dependent manner, sustaining a role of these proteins in the development of the biofilm matrix made of EPS. Because EPS binding by RapA2 can only be attributed to its two CHDL domains, we propose that RapA2 is a calcium-dependent lectin and that CHDL domains in various bacterial and archaeal proteins confer carbohydrate binding activity to these proteins.