Cloning and characterization of Fc alpha Rb, a novel Fc alpha receptor (CD89) isoform expressed in eosinophils and neutrophils

The Fc receptor for IgA (Fc alpha R, CD89) is a transmembrane glycoprotein found on monocytes, macrophages, neutrophils, and eosinophils. Here we describe the characterization of a novel isoform of the Fc alpha R cloned from a human eosinophil cDNA library. This clone, Fc alpha Rb, lacks the exon encoding the transmembrane/intracellular region of wild type Fc alpha R, which is replaced by 23 new amino acids. Expression of Fc alpha Rb mRNA could be detected in eosinophils and neutrophils. IIA1.6 murine pro-B cells transfected with Fc alpha Rb cDNA secrete high levels of the protein, but also a substantial amount of Fc alpha Rb is expressed at the cell membrane. Membrane-bound Fc alpha Rb binds IgA-coated beads equally well as wild type Fc alpha R. Surface expression is not affected by phosphatidyl inositol phospholipase C, indicating that glycosyl phosphatidyl inositol-linkage of Fc alpha Rb is not likely. In IIA1.6 cells expressing Fc alpha Rb and FcR gamma, which is necessary for signal transduction by wild type Fc alpha R, no tyrosine phosphorylation or Ca(2+)-mobilization could be observed after receptor cross-linking. These results indicate that Fc alpha Rb is likely to have a different function than wild-type Fc alpha R receptor.