Identification of multiple isoforms of the low-affinity human IgG Fc receptor

Two varieties of similar, but structurally distinct, cDNA clones for the human low-affinity receptors for the Fc portion of immunoglobulin G (FcγRII) have been isolated. One type of clone was obtained from human B lymphocytes, and the other from PHA-activated peripheral T cells and monocytes. Transfection of both prototype clones into Cos-7 cells and subsequent specific staining with monoclonal antibodies of the CDw32 group confirmed the identification of the gene products. The nucleotide sequence of the cDNA clone from B lymphocytes contains an open reading frame that encodes a protein of relative mass (M _r) 27000 with an extracellular domain of 179 amino acids containing three potential N-glycosylation sites, a 26 amino acid transmembrane domain, and a 44 amino acid cytoplasmic domain. The clones from peripheral T cells and monocytes both encoded a protein ofM _r 31000 with a 179 amino acid extracellular domain containing two potential N-glycosylation sites and a 26 amino acid transmembrane domain. The two types of clones had similar sequences in their immunoglobulin-like extracellular and transmembrane domains, but differed in their leader sequences and 3′-untranslated regions. The most notable difference between the clones was the presence of a distinctive 76 amino acid cytoplasmic domain in those isolated from T cells and monocytes.