The “lectin recognition” hypothesis was proposed to account for experimentally observed host specificity when members of the Rhizobium genus of soil bacteria infect the roots of leguminous plants, leading to the formation of nodules capable of fixing atmospheric nitrogen. According to this hypothesis, recognition between rhizobia and the legume root involves a binding of the host plant lectin to unique carbohydrates found on the bacterial symbiont. Analyses of the attachment of R. leguminosarum biovar trifolii to the white clover root surface have provided support for the hypothesis involving plant-derived lectins and bacterial surface polysaccharides. On the other hand, data accumulated on the Bradyrhizobium japonicum-soybean adhesion system are inconsistent with recognition mediated by the soybean lectin. Although this adhesion system indeed involves saccharide-specific recognition, the experimental data suggest that the lectin actually resides on the bacteria while the plant cell presents the carbohydrate ligand. This notion casts, therefore, a new view of the “lectin recognition” hypothesis.
Interaction of bFGF with heparan sulfate (HS) is prerequisite for the binding of bFGF to its high-affinity receptor and for its mitogenic activity. HS possesses a complex polymeric structure in which the majority of N-sulfated glucosamine and various O-sulfated residues are clustered in a series of short domains separated by relatively long oligosaccharide sequences with low sulfate content. The formation of highly sulfated clusters in HS has been shown to be regulated by the activity of HS-N-deacetylase/N-sulfotransferase (HS-NdAc/NST), which catalyzes both N-deacetylation and N-sulfation reactions of glucosamine residues in HS. Recent studies showed that bFGF interacts with HS through large highly sulfated clusters that are enriched in 2-O-sulfated iduronic acid-N-sulfated glucosamine disaccharide sequence.
Most secreted and cell surface proteins in eukaryotic cells are modified during their synthesis by the addition of oli-gosaccharides to specific Asn residues. This modification, N-linked glycosylation, can influence many aspects of the structure, function, and expression of glycoproteins (1-3). Studies using global inhibitors of N-linked glycosylation, such as tunicamycin, suggest that N-glycans may facilitate the transport of cell surface and secreted glycoproteins. However, these studies do not address whether individual N-glycans on glycoproteins play unique roles in their transport. Recent studies have applied molecular biological methods to investigate the role of individual N-glycans in glycoprotein transport. By this approach, individual sites for N-linked glycosylation are either deleted or introduced into the cDNA encoding a given glycoprotein. The encoded protein is then expressed, and the effects of the altered pattern of glycosylation on transport are assessed. This report will review studies investigating the role of individual N-glycans in glycoprotein transport, to consider possible mechanisms by which individual N-glycans can influence glycoprotein transport, and to address special considerations in the use of site-directed mutagenesis for studies of this type.
The process of fertilization in mammals is initiated by species-specific binding of free-swimming sperm to ovulated eggs. The search for gamete surface components that support species-specific binding of sperm to eggs in mammals has led to the conclusion that the process is mediated by protein-carbohydrate interactions. Apparently, specific oligosaccharides associated with a glycoprotein (sperm receptor) located in the egg extracellular coat (zona pellucida) are recognized by a protein (egg-binding protein) located in the sperm head plasma membrane. In this article, some of the evidence that supports this conclusion is reviewed.
Many complex processes are involved in metastasis. Of these, the most important step might be that of the tumor cell passing through tissue barrier such as endothelial cells and basement membrane. Cell adhesion molecules, selectins, which are specifically recognize tumor-associated antigens, namely sialyl Lex and sialyl Lea, are thought to be closely involved in this step. The expression of sialyl dimeric Lex and LTA receptors in primary lesions has been found to influence the metastatic potential and prognosis of transitional cell carcinoma of the human bladder. We suggest that the interaction of these carbohydrate epitopes on a 60kD glycoprotein with selectins promotes metastasis.