THE C-TYPE LECTIN SUPERFAMILY IN THE IMMUNE-SYSTEM

Protein-carbohydrate interactions serve multiple functions in the immu ne system. Many animal lectins (sugar-binding proteins) mediate both p athogen recognition and cell-cell interactions using structurally rela ted Ca2+-dependent carbohydrate-recognition domains (C-type CRDs). Pat hogen recognition by soluble collectins such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cel l-surface mannose receptor, is effected by binding of terminal monosac charide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geom etrical arrangement of multiple CRDs in the intact lectins explains th e ability of the proteins to mediate discrimination between self and n on-self. In contrast, the much narrower binding specificity of selecti n cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface o f natural killer cells, contain C-type lectin-like domains (CTLDs) tha t are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.