Expression and function of an IgE-binding animal lectin (ε{lunate}BP) in mast cells

ε{lunate}BP (IgE-binding protein) is a 31,000 M_r protein originally identified in rat basophilic leukemia (RBL) cells. The protein is composed of two domains with the amino-terminal domain containing a highly conserved repetitive sequence and the carboxyl-terminal domain containing consensus sequences shared by other β-galactoside-binding soluble lectins. The protein has wide tissue distribution, is found on cell surfaces and in extracellular milieu. By combined efforts from several research groups including ours a multifunctional nature of this lectin began to emerge. This review emphasizes the following characteristics of ε{lunate}BP: (i) ε{lunate}BP is secreted by cells such as macrophages; (ii) like many other lectins, ε{lunate}BP functions at least bivalently; (iii) ε{lunate}BP has specificity for distinct oligosaccharide structures that have a terminal galactose not masked by sialic acids; and (iv) in addition to binding IgE, ε{lunate}BP binds to surfaces of various cell types via lectin-carbohydrate interaction. Importantly, ε{lunate}BP binds to the IgE receptor on mast cells. We propose that ε{lunate}BP can function as a modulatory protein on various cells by cross-linking critical cell surface glycoproteins. The proposed action of ε{lunate}BP on mast cells is presented as a model.