A SECRETION INHIBITORY SIGNAL-TRANSDUCTION MOLECULE ON MAST-CELLS IS ANOTHER C-TYPE LECTIN

Secretion of inflammatory mediators by rat mast cells (line RBL-2H3) was earlier shown to be inhibited upon clustering a membrane glycoprotein by monoclonal antibody G63. This glycoprotein, named mast cell function-associated antigen (MAFA), was also shown to interfere with the coupling cascade of the type 1 Fc epsilon receptor upstream to phospholipase C-gamma 1 activation by protein-tyrosine kinases. Here we report that the MAFA is expressed as both a monomer and a homodimer, Expression cloning of its cDNA shows that it contains a single open reading frame, encoding a 188-amino acid-long type II integral membrane protein. The 114 C-terminal amino acids display sequence homology with the carbohydrate-binding domain of calcium-dependent animal lectins, many of which have immunological functions. The cytoplasmic tail of MAFA contains a YXXL (YSTL) motif, which is conserved among related C-type lectins and is an essential element in the immunoreceptor tyrosine-based activation motifs, Finally, changes in the MAFA tyrosyl- and seryl-phosphorylation levels are observed in response to monoclonal antibody G63 binding, antigenic stimulation, and a combination of both treatments.