Stimulatory function of gp49A, a murine Ig-like receptor, in rat basophilic leukemia cells

Murine gp49, a 49-kDa type I transmembrane glycoprotein, is a member of the Ig-like receptors expressed on the surface of cells involved in natural im munity such as mast cells, NK cells, and macrophages. The two major subtype s, gp49A and gp49B, are encoded by two different genes adjacent to each oth er. gp49B contains an immunoreceptor tyrosine-based inhibitory motif in its cytoplasmic region and is known to function as an inhibitory molecule. In contrast, gp49A does not harbor any specific motif for signal transduction, nor has its physiological role been determined. Here we report on the stim ulatory nature of gp49A by analyzing biochemical characteristics of chimeri c molecules consisting of an ectodomain of Fc receptor and a C-terminal hal f of gp49A, namely the pretransmembrane, transmembrane, and cytoplasmic por tions, expressed on the rat basophilic leukemia mast cell line. Cross-linki ng of the chimeric receptors evoked cytoplasmic calcium mobilization, PGD(2 ) release, and transcription of IL-3 and IL-4 genes, but did not elicit deg ranulation of the cells. The chimeric molecule could be expressed as a sing let and a homodimeric form on the cell surface. A pretransmembrane cysteine residue of gp49A was necessary for dimer formation. Dimerization was be ne cessary for their incorporation into glycolipid-enriched membrane fraction (GEM) upon cross-linking stimuli. The calcium mobilization response was inh ibited by treatment of cells with methyl-beta -cyclodextrin, an inhibitor o f GEM formation. Together with these results, it was strongly suggested tha t gp49A could be expressed as a homodimer and elicit activation signals tha t lead to calcium mobilization, eicosanoid production, and cytokine gene tr anscription through its incorporation into GEM.