Molecular dissection of the FcRβ signaling amplifier

Abstract

Human high affinity IgE receptors are expressed as two different isoforms: the tetrameric isoform, αβγ2, or the trimeric isoform, αγ2. The α chain is the IgE binding subunit, whereas the FcRβ and FcRγ chains are the signaling modules. Both FcRβ and FcRγ contain immunoreceptor tyrosine-based activation motifs (ITAM), but the β ITAM differs from canonical ITAMs in two ways; the spacing between the two canonical tyrosines harbors a third tyrosine, and it is one amino acid shorter than in canonical ITAMs, making it unfit to bind the tandem SH2 of Syk. We have shown that FcRβ functions as an amplifier of the FcRγ signaling function. However, the molecular mechanism of this amplification remains unclear. Here we show that mutation of the three tyrosines (Tyr-219, Tyr-225, and Tyr-229) in the β ITAM essentially converts αβγ2 into an αγ2 complex in terms of Lyn recruitment, FcRγ phosphorylation, Syk activation, and calcium mobilization. Tyr-219 is the most critical residue in this regard. In addition, a detailed analysis of the dynamics of calcium mobilization suggests a possible inhibitory role for Tyr-225, which becomes apparent when Tyr-219 is mutated. Thus, the signaling amplification function of FcRβ is mainly encoded in Tyr-219 and in its capacity to recruit Lyn. In turn, this Tyr-219-mediated Lyn recruitment enhances γ chain phosphorylation, Syk activation, and calcium mobilization. The two other tyrosines appear to have a modulating function that remains to be fully assessed.