Multiple subfamilies of carbohydrate recognition domains in animal lectins

What contributes to an effective mannose recognition domain?

In general, carbohydrate-lectin interactions are characterized by high specificity but also low affinity. The main reason for the low affinities are desolvation costs, due to the numerous hydroxy groups present on the ligand, together with the typically polar surface of the binding sites. Nonetheless, nature has evolved strategies to overcome this hurdle, most prominently in relation to carbohydrate-lectin interactions of the innate immune system but also in bacterial adhesion, a process key for the bacterium's survival. In an effort to better understand the particular characteristics, which contribute to a successful carbohydrate recognition domain, the mannose-binding sites of six C-type lectins and of three bacterial adhesins were analyzed. One important finding is that the high enthalpic penalties caused by desolvation can only be compensated for by the number and quality of hydrogen bonds formed by each of the polar hydroxy groups engaged in the binding process. In addition, since mammalian mannose-binding sites are in general flat and solvent exposed, the half-lives of carbohydrate-lectin complexes are rather short since water molecules can easily access and displace the ligand from the binding site. In contrast, the bacterial lectin FimH benefits from a deep mannose-binding site, leading to a substantial improvement in the off-rate. Together with both a catch-bond mechanism (i.e., improvement of affinity under shear stress) and multivalency, two methods commonly utilized by pathogens, the affinity of the carbohydrate-FimH interaction can be further improved. Including those just described, the various approaches explored by nature to optimize selectivity and affinity of carbohydrate-lectin interactions offer interesting therapeutic perspectives for the development of carbohydrate-based drugs.

Prokaryotic peptides that block leukocyte adherence to selectins

Pertussis toxin binds target cells through the carbohydrate recognition properties of two subunits, S2 and S3, which share amino acid sequence similarity with the lectin domains of the eukaryotic selectin family. Selectins appear on inflamed endothelial cells and promote rolling of leukocytes by reversibly binding carbohydrates. S2, S3, and synthetic peptides representing their carbohydrate recognition domains competitively inhibited adherence of neutrophils to selectin-coated surfaces and to endothelial cells in vitro. These proteins and peptides also rapidly upregulated the function of the leukocyte integrin CD11b/CD18. These findings implicate mimicry of eukaryotic selectins by prokaryotic adhesive ligands and link the mechanisms underlying leukocyte trafficking to microbial pathogenesis.

Studies on the carbohydrate-binding characteristics of human pulmonary surfactant-associated protein A and comparison with two other collectins: mannan-binding protein and conglutinin

The surfactant-associated protein A (SP-A) belongs to the collectin family, a group of C-type lectins encompassing also surfactant-associated protein D, mannan-binding protein (MBP) and conglutinin. These proteins all have carbohydrate-recognition domains joined to collagen stalks. It seems likely that SP-A, like MBP and conglutinin, may mediate anti-microbial activity through binding to carbohydrates on the microorganisms and collectin receptors on phagocytic cells. We have studied the influence of carbohydrates on the binding of SP-A, MBP and conglutinin to mannan in an enzyme-linked lectin-binding assay. All sugars were of D-configuration, except fucose of which both L- and D-configurations were tested. The order of inhibiting potency on the binding of SP-A was: N-acetylmannosamine > L-fucose, maltose > glucose > mannose. The following sugars were non-inhibitory: galactose, D-fucose, glucosamine, mannosamine, galactosamine, N-acetylglucosamine, and N-acetylgalactosamine. The best inhibitor of MBP was N-acetylglucosamine. Otherwise MBP showed a selectivity similar to that of SP-A. Conglutinin binding was inhibited by all the sugars examined except N-acetylgalactosamine. For conglutinin, as for MBP, the best inhibitor was N-acetylglucosamine. Normal human SP-A, alveolar-proteinosis SP-A purified by ion-exchange chromatography, and alveolar-proteinosis SP-A purified by n-butanol extraction showed no difference in sugar selectivity. The influence of pH and of the calcium concentration was also examined. Organic solvent-extracted SP-A from patients suffering from alveolar proteinosis and normal SP-A showed different sensitivity profiles.

Sequence and expression of a membrane-associated C-type lectin that exhibits CD4-independent binding of human immunodeficiency virus envelope glycoprotein gp120

The binding of the human immunodeficiency virus (HIV) envelope glycoprotein gp120 to the cell surface receptor CD4 has been considered a primary determinant of viral tropism. A number of cell types, however, can be infected by the virus, or bind gp120, in the absence of CD4 expression. Human placenta was identified as a tissue that binds gp120 in a CD4-independent manner. A placental cDNA library was screened by expression cloning and a cDNA (clone 11) encoding a gp120-binding protein unrelated to CD4 was isolated. The 1.3-kilobase cDNA predicts a protein of 404 amino acids with a calculated M(r) of 45,775 and organized into three domains: an N-terminal cytoplasmic and hydrophobic region, a set of seven complete and one incomplete tandem repeat, and a C-terminal domain with homology to C-type (calcium-dependent) lectins. A type II membrane orientation (N-terminal cytoplasmic) is predicted both by the cDNA sequence and by the reactivity of C-terminal peptide-specific antiserum with the surface of clone 11 transfected cells. Native and recombinant gp120 and whole virus bind transfected cells. gp120 binding is high affinity (kd, 1.3-1.6 nM) and inhibited by mannan, D-mannose, and L-fucose; once bound, gp120 is internalized rapidly. Collectively, these data demonstrate that the gp120-binding protein is a membrane-associated mannose-binding lectin. Proteins of this type may play an important role in the CD4-independent association of HIV with cells.

Carbohydrate-recognition domains as tools for rapid purification of recombinant eukaryotic proteins

Methods have been developed for expression and purification of eukaryotic proteins by creating fusions with the carbohydrate-recognition domain (CRD) of the galactose-specific rat hepatic lectin. In order to generate the fusion proteins, vectors have been constructed so that cDNAs for passenger proteins can be inserted in any reading frame following a segment of DNA encoding the CRD. The feasibility of using this approach as an aid to protein purification has been demonstrated using human placental alkaline phosphatase. Following expression in either of two different eukaryotic expression systems, the CRD-phosphatase fusion protein can be isolated by one step of affinity chromatography on galactose-Sepharose under mild, non-denaturing conditions. Incorporation of a proteinase-sensitive linker allows cleavage of the CRD from the passenger protein. Immobilised proteinase could be rapidly separated from the cleavage products and the released, active phosphatase was purified away from the CRD by re-chromatography on galactose-Sepharose. These methods provide a means of isolating correctly folded recombinant eukaryotic proteins when cDNAs are available, but the properties of the encoded proteins are unknown.