The mast cell function-associated antigen exhibits saccharide binding capacity

The mast cell function-associated antigen (MAFA) is a membrane glycoprotein first identified on rat mucosal type mast cells (line RBL-2H3) and known to inhibit the Fc epsilon RI-mediated secretory response. In its extracellular domain, an amino acid stretch homologous to the carbohydrate binding domain of calcium dependent animal lectins has been found. To investigate its carbohydrate binding capacity, the MAFA has been expressed in the Spodoptera frugiperda insect cell line (Sf9) using the baculovirus expression system. Analysis by flow cytometry and surface labeling with I-125 showed that the recombinant MAFA (rMAFA) was expressed as a monomeric and disulfide-linked homodimeric glycoprotein in the membrane of the insect cells, and both forms exhibited the same epitopes as the protein isolated from RBL-2H3 cells. Immunoaffinity-purified rMAFA was then employed for studies of its saccharide binding capacity by using different neoglycans and glycoproteins. The rMAFA was found to bind specifically terminal mannose residues in a Ca2+-dependent manner. These results support the notion that the extracellular domain of the MAFA is indeed able to bind ligands, which may be modulatory for the mast cell response.