ASSOCIATION OF IGA-FC RECEPTORS (FC-ALPHA-R) WITH FC-EPSILON-RL-GAMMA-2 SUBUNITS IN U937 CELLS - AGGREGATION INDUCES THE TYROSINE PHOSPHORYLATION OF GAMMA-2

We investigated the possibility that IgA-binding chains of Fc alpha R on monocytic cells are physically associated with gamma 2 subunits of Fc epsilon RI (Fc epsilon Rl gamma 2 or gamma 2). Fc alpha R was preci pitated from lysates of IFN gamma-treated U937 cells, subclone 10.6, a nd probed by immunoblotting with Ab against human gamma 2. Fc alpha R was precipitated through anti-Fc alpha R mAbs A59 or A62, through A62 from lysates that had been exhaustively precleared of high affinity Ig G-Fc receptors (Fc gamma RI) and of low affinity Fc gamma RII, and thr ough anti-Fc alpha R mAb A77 from Fc gamma RI-precleared lysates of un treated 10.6 cells. Precipitation was also performed through F(ab')(2) A77 and through the native ligand of the receptor, hlgA. In all cases , Fc alpha R precipitates contained co-isolated 22-kDa gamma 2 (unredu ced). The Fc alpha R alpha-chain/gamma 2 complex did not readily disso ciate in 1% Nonidet P-40 as did Fc gamma RI alpha-chain/gamma 2, sugge sting a novel aspect to the Fc alpha R subunit interaction. Specific F c alpha R aggregation on cells triggered a robust respiratory burst an d the tyrosine phosphorylation of several proteins. Among them was pho spho-gamma 2, which migrated as a 24- to 28-kDa gamma 2 phosphoprotein on gels and was detected as a 28-kDa phosphoprotein by anti-phosphoty rosine immunoblot. Aggregated Fc alpha Rs that were precipitated from Fc alpha R-triggered cells also contained a phosphoprotein of the same mobility and immunoreactivity, as did aggregated Fc gamma RI from whi ch the 28-kDa phosphoprotein could be more readily eluted and identifi ed (as phospho-gamma 2). We conclude that myelocytic Fc alpha Rs are m ultichain complexes containing gamma 2 subunits that are tyrosine phos phorylated upon Fc alpha R aggregation. As IgA is the predominant Ig o n mucosal surfaces, gamma-subunits may play an important role in mucos al immunity involving leukocytic Fc alpha R.