Molecular structure and expression of the murine lymphocyte low-affinity receptor for IgE (Fc(ε)RII)

The cDNA encoding the murine low-affinity receptor for IgE (Fc(ε)RII) has been isolated from a cDNA library prepared from B cells activated with lipopolysaccharide and interleukin 4. It encodes a 37-kDa protein of 331 amino acids with two potential N-linked glycosylation sites. Analogues to its human counterpart, there is no signal sequence and the putative transmembrane region is close to the amino terminus, indicating an inverse membrane orientation with the carboxyl terminus at the cell exterior. The predicted murine Fc(ε)RII amino acid sequence demonstrates a 57% identity with its human counterpart. The murine sequence has an additional internal repeat motif of 21 amino acids giving four repeats as compared to three in the human sequence. Furthermore, the murine Fc(ε)RII is truncated at the carboxyl terminus and the Arg-Gly-Asp sequence, a common recognition site in integrin receptors, which is found in the reverse configuration in human Fc(ε)RII, is missing. B cells activated with interleukin 4 and lipopolysaccharide have an increased amount of Fc(ε)RII mRNA as compared with resting or lipopolysaccharide-stimulated B cells. Con A-activated normal T-cells, the TH-2 cell line D10, as well as the macrophage cell line J774 have no detectable Fc(ε)RII mRNA. Expression analysis using transiently transfected COS cells revealed that recombinant murine Fc(ε)RII binds anti-Fc(ε)RII as well as mouse and rat IgE but does not bind human IgE or mouse IgE. Fc(ε)RII expressed in COS cells has a molecular mass of 45 kDa whereas the Fc(ε)RII from B-cell lines ia a 49-kDa protein.