Identification of Residues within the Extracellular Domain 1 of Bovine Fcγ2R Essential for Binding Bovine IgG2 *

Neutrophils and monocytes in cattle express a novel class of immunoglobulin Fc receptor, specific for bovine IgG2 (bIgG2), termed bFcγ2R. In cows, the ability of neutrophils to kill immunoglobulin-opsonized microorganisms appears to depend largely on this subclass, whose interaction with bFcγ2R initiates the killing process. bFcγ2R is a transmembrane glycoprotein consisting of two extracellular immunoglobulin-like domains, followed by a 19-amino acid membrane-spanning region and a short cytoplasmic tail. Although related to other mammalian FcγRs, bFcγ2R belongs to a novel gene family that includes the human killer cell inhibitory receptor and FcαRI (CD89) proteins. We have shown previously (Morton, H. C., van Zandbergen, G., van Kooten, C., Howard, C. J., van de Winkel, J. G., and Brandtzaeg, P. (1999) J. Exp. Med. 189, 1715–1722) that like these proteins (and unlike other FcγRs), bFcγ2R binds bIgG2 via the membrane-distal extracellular domain 1 (EC1). In this present study, we introduced mutations into the predicted loop regions of the EC1 domain and assayed the resulting bFcγ2R mutants for their ability to bind bIgG2. Our results indicated that the bIgG2 binding site lies within the predicted F–G loop region of the EC1 domain. Furthermore, single amino acid mutational analysis of this region identified Phe-82 and Trp-87 as being critical for bIgG2 binding.