| Abstract: |
The structural heterogeneity of the human low affinity receptor for IgG, FcRII(CD32), has been elucidated through the isolation, characterization, and expression of cDNA clones derived from myeloid and lymphoid RNA. These clones predict amino acid sequences consistent with integral membrane glycoproteins with single membrane spanning domains. The extracellular domains display sequence homology to other FcγRs and members of the Ig supergene family. A minimum of three genes (FcγRIIa, IIa', and FcγRIIb) encode these transcripts, which demonstrate highly related extracellular and membrane spanning domains. IIa/IIa' differ substantially in the intracytoplasmic domain from IIb. Alternative splicing of the IIb gene generates further heterogeneity in both NH2- and COOH-terminal domains of the predicted proteins. Comparison to the murine homologues of these molecules reveals a high degree of conservation between the products of one of these genes, FcγRIIb, and the murine β gene in primary sequence, splicing pattern, and tissue distribution. In contrast, the sequence of IIa' indicates its relationship to the β-like genes, with mutation giving rise to a novel cytoplasmic domain, while IIa is a chimera of both α- and β-like genes. Expression of these cDNA molecules by transfection results in the appearance of IgG binding molecules that bear the epitopes defined by the FcRII(CD32) mAbs previously described. |
