TYROSINE-CONTAINING SEQUENCE MOTIFS OF THE HUMAN-IMMUNOGLOBULIN-G RECEPTORS FCRIIB1 AND FCRIIB2 ESSENTIAL FOR ENDOCYTOSIS AND REGULATION OFCALCIUM FLUX IN B-CELLS

Human B cells express two closely related immunoglobulin G receptors, FcRIIb1 and FcRIIb2, which differ by a 19 amino acid insertion in the cytoplasmic tail of FcRIIb1. The cytoplasmic tails of both isoforms co ntain a conserved sequence motif (AENTITYSLL) essential for mediating endocytosis via FcRIIb2. Truncation of this motif abolished endocytosi s, while replacement of tyrosine (Tyr-(273)) in FcRIIba by phenylalani ne had no effect on the amount and kinetics of ligand uptake. Co-cross linking of FcRIIb1 or FcRIIb2 with the antigen receptor on B cells led to an abortive calcium signal. Neither isoform interfered with the ea rly intracellular calcium mobilization, but both prevented the opening of a plasma membrane calcium channel essential for a sustained elevat ed intracellular calcium level. Modulation of calcium channel activity is mediated by the same sequence motif essential for endocytosis but requires the pres- ence of Tyr(292) in FcRIIb1 and Tyr(273) in, FcRIIb 2. Co-crosslinking of FcRIIb1 with surface IgG is associated with tyro sine phosphorylation of Tyr(292), whereas Tyr(272) in FcRIIb2 was not phosphorylated. Thus, FcRIIb phosphorylation is probably not directly involved in the modulation of the calcium signal but may be essential for further diversification of signals transduced via the coexpressed isoforms FcRIIb1 and FcRIIb2.