Revised Manuscript Received May 10, 1993 abstract: IgE-binding protein (eBP) was originally identified in rat basophilic leukemia (RBL) cells by virtue of its affinity for IgE.«BP is now known to be a/8-galactoside-binding lectin containing an S-type carbohydrate recognition domain. It is identical to a macrophage surface antigen, Mac-2, and lectins designated as CBP35, L-34, and RL-29, for which various functions have been suggested. Studies from other groups as well as ours have indicated that eBP is secretedby cells such as macrophages and is present in extracellular fluids. We demonstrated previously that binding sites for eBP are present on the surface of RBL cells. In this report, we show that eBP binds to a small number of glycoprotein species on the surface of RBL cells. Significantly, one of these glycoproteins is the high-affinity IgE receptor (FceRI). Preliminary studies showed that eBP causes mediator release from RBL cells, possibly through cross-linking of FceRI. The results suggest a function of eBP as an activator of mast cells.

IgE receptor (FceRI) 1 present on mast cells and basophils is a key component responsible for IgE-mediated allergic reactions. Multivalent allergens bind to the receptor-bound IgE and cause cross-linking of the receptor, resulting in mediator release from mast cells and basophils (Metzger et al., 1986). FceRI is a glycoprotein with a tetrameric structure: one IgE-binding a-subunit, one/8-subunit, and two 7-subunits (Metzger, 1991; Ravetch & Kinet, 1991). The primary structure of this receptor has been elucidated. However, the structure and functional significance of the oligosaccharides linked to the protein core are largely unknown. It is likely that the function of oligosaccharide components of