The genome of Drosophila melanogaster encodes several proteins that are predicted to contain Ca 2؉ -dependent, C-type carbohydrate-recognition domains. The CG2958 gene encodes a protein containing 359 amino acid residues. Analysis of the CG2958 sequence suggests that it consists of an N-terminal domain found in other Drosophila proteins, a middle segment that is unique, and a C-terminal C-type carbohydrate-recognition domain. Expression studies show that the full-length protein is a tetramer formed by noncovalent association of disulfide-linked dimers that are linked through cysteine residues in the N-terminal domain. The expressed protein binds to immobilized yeast invertase through the C-terminal carbohydrate-recognition domain. Competition binding studies using monosaccharides demonstrate that CG2958 interacts specifically with fucose and mannose. Fucose binds ϳ5-fold better than mannose. Blotting studies reveal that the best glycoprotein ligands are those that contain N-linked glycans bearing ␣1,3-linked fucose residues. Binding is enhanced by the additional presence of ␣1,6-linked fucose. It has previously been proposed that labeling of the Drosophila neural system by anti-horseradish peroxidase antibodies is a result of the presence of difucosylated N-linked glycans. CG2958 is a potential endogenous receptor for such neural-specific carbohydrate epitopes.Animal lectins provide a mechanism for recognition of protein-and lipid-linked glycans. Recognition of endogenous glycoconjugates at the cell surface, in the extracellular matrix, and in serum can lead to intercellular adhesion and signaling as well as uptake and degradation. Animal lectins are diverse in structure, but they usually contain modular carbohydraterecognition domains (CRDs) 1 (1, 2). The C-type CRDs are the largest and most diverse class of CRDs. These domains share a common fold and show Ca 2ϩ -dependent sugar binding activity, although their selectivity for different carbohydrate ligands varies. Binding of sugars involves formation of a ternary complex between the protein, a bound Ca 2ϩ , and the sugar (3). Selectivity for particular sugar ligands is determined in large part by the disposition of Ca 2ϩ -ligating residues in the protein, but can also reflect additional interactions with nearby regions of the protein surface. The C-type CRDs are a subset of a larger family of protein modules, the C-type lectin-like domains (CTLDs) (4). Although the CTLDs share a common fold, many do not bind Ca 2ϩ and they often interact with ligands others than sugars.Profile analysis provides a powerful method for identifying CTLDs in proteins identified by genome sequence analysis. Using information about the structures and sugar binding activities of known C-type CRDs, it is possible to identify CTLDs that are likely to display carbohydrate binding activity. This approach has been applied to the complete genomic sequences of model organisms such as Caenorhabditis elegans (5) and Drosophila melanogaster (2) as well as to the human genome (see ctld.glycob.ox...
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